Q: Which presidential Inauguration occurred during the worst weather conditions?

A: Extreme cold -- including a noontime temperature of just 7 degrees and afternoon wind chills between 10 and 20 below zero -- forced Ronald Reagan's second swearing-in ceremony indoors and canceled the parade on Jan. 21, 1985. President Taft's ceremony on March 4, 1909, was also forced indoors due to a crippling snowstorm that dumped 9.8 inches of snow on Washington, D.C.  This page from the National Weather Service in Washington has more about historic presidential inaugural weather.

The Northeast Regional Climate Center also reports that the windiest Inauguration Day occurred for Ulysses S. Grant on March 4, 1873, when gusts reached 24 mph, from the northwest, at noon. -- Bob Swanson

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Q: Three questions about hail

Three questions about hail: Can you tell when and where the hailstorms occur in Texas? Are there more hailstorms in the winter or in the summer? Are there more hailstorms in one city more than any other?

A: To determine when the last hailstorm was in Texas, you can go back through the storm reports from the Storm Prediction Center. As of the time of writing, the last Texas hailstorm was on Dec. 27, 2008, when quarter- and nickel-sized hail was reported near Bridgeport.

Keep in mind that only hail 0.75 inch in diameter (penny-size) or greater is considered "severe" hail and kept in meteorological records. There is a possibility that smaller hail (say pea-size) has occurred since Dec. 27 somewhere in Texas, but there is really no way of proving it.

To answer your question regarding the number of hailstorms in winter compared to summer, there are far more hailstorms during the spring and summer. Prove this to yourself by visiting this National Severe Storms Laboratory web page -- selecting "hail" and clicking a point in Texas to see the annual hail cycle and probabilities. You can also see the nationwide annual trend, just by checking out the Storm Prediction Center's 2008 annual summary and clicking "large hail."

Hail is typically only a warm-weather phenomenon, as its formation requires the strong updrafts of a thunderstorm. However, on rare occasions, sufficient updrafts can develop in a winter storm to produce hail as well as thundersnow. It is important to note that hail forms differently than sleet. Whereas hail are chunks of ice that form in the storm cloud and fall to Earth before melting, sleet starts as snow or ice in the cloud, melts during its descent, then refreezes as an ice pellet before hitting the ground.

As for the most hail-prone city, that distinction goes to Cheyenne, Wyoming, where 9 to 10 hailstorms occur each year. -- Bob Swanson

This question was submitted by Vicky Preston.

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Q: Why do surface winds and jet-stream winds sometimes blow from different directions?

A: The wind's speed and direction result from a balancing act between differences in air pressure, the effect of the Earth's rotation, and frictional effects. While the effect of the Earth's rotation at a given latitude is nearly the same between ground and jet-stream levels, the pressure gradient can be dramatically different and frictional effects are often much smaller high above the ground.

These USA TODAY graphics show more about jet streams, how they stir up storms, and why their locations change with the seasons.

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Q: Where are tornadoes most common in January?

A: Overall, January is typically a quiet month for tornadoes across the USA, with a historical average of about 20 each year, according to data from the Storm Prediction Center. The three winter months -- December, January, and February -- are the three months when tornadoes are the most rare in the USA.

When January tornadoes do occur, they tend to form in the Southeast, especially in central Florida and southern Mississippi, according to a severe thunderstorm climatology study by Harold Brooks of the National Severe Storms Laboratory. -- Doyle Rice

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Q: Do plants freeze when the wind chill is below freezing?

A: If the air temperature is above freezing (32 degrees), plants won't freeze, even if the wind chill temperature is below freezing. Strong winds can actually help keep plants and vegetation from freezing on cool nights, by stirring up layers of air and bringing warmer layers down to the surface.

The wind chill index applies to humans and animals. Revamped in 2001, this newer wind chill index:

--Calculates wind speed at an average height of five feet (typical height of an adult human face) based on readings from the national standard height of 33 feet (typical height of an anemometer).
--Is based on a human face model.
--Incorporates modern heat transfer theory (heat loss from the body to its surroundings, during cold and breezy/windy days).
--Lowers the calm wind threshold to 3 mph.
--Uses a consistent standard for skin tissue resistance.
--Assumes no impact from the sun (i.e., clear night sky). -- Doyle Rice

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Q: What's the difference between rain and showers?

A: In forecasts, "rain" usually implies precipitation that falls steadily from the sky for several hours.  “Showers” are rain that falls intermittently over a small area, can be heavy or light, and usually don't last more than an hour or so.

And what about drizzle? Well, rain indicates falling drops of water larger than 0.02 inch in diameter, whereas "drizzle" is falling drops of water smaller than 0.02 inch in diameter. Although drizzle does appear to float in air currents, it does fall to the ground, unlike fog. -- Doyle Rice

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Q: Which U.S. city has the steadiest barometric pressure?

A: Honolulu has the steadiest barometric pressure in the USA, when measured by the difference in record extremes of high and low pressure, as noted in the book Extreme Weather. The city's record high of 30.32 inches of mercury and record low of 29.34 inches is the smallest difference in the country. This is a difference of 0.98 inch of mercury. In the continental USA, San Diego has the steadiest pressure, with a record high of 30.53 inches and a record low of 29.37 inches, a difference of 1.16 inches.

As for largest fluctuations between record high pressure and record low pressure, St. Paul Island, Alaska, has a record high pressure of 30.86 inches of mercury and a record low of 27.35 inches, a whopping 3.51 inches difference. In the continental U.S., Charleston, S.C. has a record high of 30.85 inches and a record low of 27.64, with the difference being 3.21 inches. -- Doyle Rice

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Q: What’s the forecast for January across the USA?

A: The federal government's Climate Prediction Center's forecast for January is for above-average temperatures across nearly the entire continental USA, with the exception of the West Coast and the far northern tier. As for precipitation, the Pacific Northwest is expected to continue to be extremely wet, as should the Great Lakes and Ohio Valley regions. Only the Southeast coast and Florida is forecast to be drier than average.

In their online discussion about January's forecast, climate scientists at the CPC wrote: "As far as tropical sea-surface temperature is concerned, we appear to be witnessing a very late and very rapid transition to more definitive La Nina conditions. This happened just in the last two weeks and even the monthly mean for December does not do much justice to the present condition. We thus feel more comfortable adjusting the January forecast in the direction of a La Nina composite."

As for what La Nina typically means for our weather across the USA, check out this USA TODAY interactive graphic. -- Doyle Rice

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Q: What is the coldest wind chill on record in New York City?

A: Wind chill is not one of the official weather measurements kept by the National Weather Service, so no precise wind chill records exist. As for temperature, the lowest reading ever officially recorded in Central Park was -15 degrees, set on the morning of February 9, 1934, according to the weather service forecast office in New York City.  This page from the NWS in NYC has much more historical climate information about the Big Apple.

In his excellent weather record book, Extreme Weather, author Christopher Burt reports that the coldest day ever in New York City may well have been Jan. 10, 1859. On that day, the high temperature never rose above 0 degrees, according to several thermometers around the city. This is probably the only day in history when the temperature failed to rise above zero in New York City, says Burt. However, this isn't considered an "official" measurement as the period of record at the NWS in New York only goes back to 1869. -- Doyle Rice

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Q: Is snow really necessary in winter?

A: For many agricultural areas, snow is indeed necessary for better crops. Not only does it play a part in storing moisture and slowly releasing it to the soil, but it also has an insulating effect due to the air in the pore spaces of the snow. Without this insulating protection, some winter crops would be damaged or destroyed. -- Bob Swanson

This question was submitted by Angie Bridgman.

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Q: What is the coldest city, on average, in the USA?

A: The coldest city in the USA is Barrow, Alaska, with an annual average temperature of 10.4 degrees. In the contiguous 48 states, Mt. Washington, N.H., is the coolest location at 27.2 degrees. The coldest inhabited city is International Falls, Minn., with an annual average of 37.4 degrees, followed by Marquette, Mich., at 38.7 degrees.

This data is from the National Climatic Data Center. Feel free to play around with the data to see if you get the same results when looking only at daily highs and daily lows.

This question was submitted by Larisa.

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Q: Is there less oxygen in winter air?

A: There is a difference in the amount of oxygen in the air in the winter compared to the summer (especially in the Northern Hemisphere). The amount of land (and thus trees and plants) in the Northern Hemisphere produces a summertime minimum in the CO2 in the atmosphere as well as a O2 maximum (due to photosynthesis). A reversal occurs in the winter, both due to a lack of photosynthesis as well as an uptake in O2 as bacteria decompose organic matter (such as fallen leaves). Ralph Keeling – son of Charles David Keeling who was the father of atmospheric CO2 measurement and gave rise to the infamous Keeling curve – has long studied atmospheric O2 concentrations. Here is a link to one of his papers on the topic.

Despite the annual signal, the change in concentration is not significant enough to have much impact on human health. However, lack of moisture in winter air, especially indoor air, can have an effect on the body. Outside air dries out further as it is brought inside and warmed. Ordinary heaters don’t add moisture. Water from skin, noses, and throats evaporates into the air. To reduce evaporation, use a humidifier and keep it clean. Ideal indoor humidity is between 30 and 50 percent. -- Bob Swanson

This question was submitted by Frank K.

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Q: Why isn't the earliest sunrise on the longest day and the latest sunrise on the shortest?

A: During the Northern Hemisphere’s winter solstice, the direct rays of the sun are over the Tropic of Capricorn, 23.5° south of the equator. For residents in the Northern Hemisphere, this means the shortest amount of time between sunrise and sunset. However, it is not the day with the latest sunrise or earliest sunset. In fact, the latest sunrise tends to occur several weeks before the solstice and the earliest sunset several weeks after. This is largely due to the tilt of the Earth’s axis and, to a lesser extent, the eccentricity of the Earth’s orbit around the sun, which is elliptical rather than circular.

In order to understand why, one must first understand a little bit about the solar day versus the day measured on our clocks. Solar noon is the time when the sun is highest in the sky (it is usually before or after clock noon – only rarely does solar noon coincide with clock noon) and a solar day is the amount of time that elapses between one solar noon and the next. Near the summer and winters solstices, the solar day is more than 24 hours long and, therefore, solar noon occurs at a slightly later time each day. The time between solar noon and sunset does not change very much near the winter solstice. Therefore, at the winter solstice, a later solar noon means a later sunset, which implies that the day with the earliest sunset has already occurred. Likewise, a later solar noon implies a later sunrise which means that, at the winter solstice, the latest sunrise has yet to come. The actual dates of earliest sunset and latest sunrise depend on latitude---near the equator, the earliest sunset occurs in November and the latest sunrise occurs in February.

A more complete explanation by Larry Denenberg can be found here. -- Bob Swanson

This question was submitted by Dale Holzen.

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Q: The sky is blue, but I just saw swirling sand and leaves -- can there be a tornado without a storm?

A: What you experienced was probably not a tornado, but rather a whirlwind (when dust is picked up, it is called a dust devil). On a cold weather day (as you describe), you can still get whirlwinds. This can sometimes happen on windy days around buildings as wind forms eddies as it moves around the corners of the building. On calm, sunny days (even when it feels relatively cool), you can still get whirlwinds, especially at interfaces of various ground cover. That is, a pavement playground will heat up much more rapidly than a nearby grassy field. The air over the playground rises rapidly and the air from over the grassy surface moves toward the playground to take the place of the rising air.

To answer your follow-up question (“can there be a tornado without a storm?” ), most tornadoes form as a result of a supercell thunderstorm. On occasion, gustnadoes (not technically tornadoes) form that are not attached to a parent thunderstorm. Waterspouts sometimes form from a towering cumulus cloud that may or may not be producing lightning and thunder (if no lightning, then not technically a thunderstorm). As to your second question about a “tornado without rain,” I would venture to say that the majority of tornadoes occur with some sort of rain (if only due to the vigorous nature of supercell thunderstorms). -- Bob Swanson

This question was submitted by Sheila Menning.

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Q: Are clouds necessary for a colorful sunrise or sunset?

A: Certainly a layer of clouds can enhance a sunrise, but the clouds themselves are not responsible for the color of the light. Rather, clouds act like mirrors (spectrally nonselective mirrors), in that they are white when illuminated by white light and are red when they are illuminated by red light. With or without clouds, scattering of light by small particles in the atmosphere scatters out more of the blue end of the spectrum, thereby transmitting orange- and red-enhanced light to your eye as an observer, at sunrise and sunset. Therefore in a cloud-free atmosphere, the setting or rising sun will appear to be more of a yellowish-orange compared to the yellowish-white of the midday sun. It is also far less intense, and can be viewed more safely, than the midday sun due to the greater amount of atmosphere it passes through on its way to your eye. When you put add clouds to the equation, the reddened hues from the sun not only reach your eyes directly, but are also reflected by clouds – making for even prettier sunrises and sunsets.

To go one step further, add in stratospheric clouds resulting from volcanic eruptions (this was widely observed after the eruption of Mount Pinatubo in 1991) and you can get absolutely breathtaking sunrises and sunsets.

Learn more about the colors of clouds in this graphic, “What color is your cloud?” To learn more about atmospheric optics, check out this graphic about why the sky is blue. Another excellent discussion about the color of sunrises and sunsets can be found here. -- Bob Swanson

This question was submitted by Liz Filippelli.

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Q: Why does the moisture not freeze and drop from the air at temperatures below freezing?

A: Most of us in school learned that 32°F is the freezing point of water. However, it is more proper to say that 32°F is the melting point of ice. While ice always melts at 32°F, liquid water can, but does not have to, freeze at 32°F (for reasons I’ll get into below). In fact, if you want to get into the chemical technicalities, microscopic ice crystals start to form in liquid water when the temperature drops below 39.2°F (which would lead us tangentially into why ice floats, but we won’t go down that path).

When we typically think of water freezing into ice, we might think of an ice cube tray in the freezer. There are two main reasons why ice cubes begin to form at or slightly below 32°F – a surface (the tray) where ice crystals can attach and impurities within the water itself that serve as nucleation sites for ice crystal formation. Remove the impurities (using distilled water) and you’ll find that the water will have to cool more in order for freezing to begin. Remove the tray (levitate water in the laboratory or suspend it as tiny cloud droplets in the atmosphere) and you’ll see that the required temperature for freezing is even lower. In fact, sufficiently pure (no impurities serving as nucleation sites) cloud droplets can remain liquid to temperatures as low as -40°F.

Therefore, in the case you describe, you can have subfreezing temperatures, fog (liquid water droplets) in the air, as well as ice forming on the railing. The fog droplets are actually supercooled (below 32°F) and freeze upon contact with the railing as it provides a surface upon which ice crystals can form. Freezing fog and freezing drizzle can both be extremely hazardous, not only forming a treacherous coating of ice on elevated surfaces such as decks and stairs, but also “black ice” on roadways. -- Bob Swanson

This question was submitted by John Semple.

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Q: How can the day start out sunny, yet be cloudy by the afternoon?

A: Those out-of-the-blue clouds are often referred to as fair-weather cumulus. You are correct in your observation that the day typically begins clear, but those clouds start to pop up around noon and thereafter. The reason being that the clear skies result in plenty of sunshine. The earth's surface heats unevenly depending on what is being heated (a dry dirt field will heat faster than a recently watered lawn, a parking lot will heat quicker than a grove of trees). As parcels of air are heated above these surfaces, they rise. The water vapor contained within cools as it rises, eventually condensing and forming clouds. I like to refer to this process as "self-destructive sunshine," as it is the sunshine that gives rise to the clouds that can eventually block the sunshine from reaching the earth.

Fair weather cumulus are not easy to forecast. As mentioned, the rising air parcels have water vapor that condenses. The less water vapor (determined either by relative humidity, or, even better, dew point readings), the less likely the formation of fair-weather cumulus. Watching for relatively high pressure readings, indicative of sinking air in the atmosphere, is also a good sign for clear skies. Remember that rising air breeds condensation, and sinking air breeds evaporation.

If you're handy at reading SkewT-logP diagrams, you can check out the forecast surface temperature and the convective temperature. If the forecast temperature falls shy of the convective temperature, fair-weather cumulus clouds are not likely to form. -- Bob Swanson

This question was submitted by Myron Shulman.

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Q: Is there a source where I can obtain statistics for the odds of snow in a particular city on any given day?

A: Yes, the data is available on this National Climatic Data Center website.

This is the starting page that will allow you to select a state (note that Washington, D.C., is not listed separately). Click on the state of interest, then select “Station Snow Climatology” on the following page. On the next page, click the “Select a station” dropdown and drag until you see the station you're interested in. For example, I chose Virginia as my state and then “WASHINGTON NATL WSCMO AP” (that’s Reagan-National Airport). Clicking “Probabilities” and then selecting your month of choice will get you where you need to be. For example, the probability of receiving snowfall greater than or equal to a certain amount (0.1”, 1”, 2”, 5”, 10”, 12”) on a specific date are as follows (again, this is for D.C.'s Reagan-National Airport):

Jan. 22: 10% 5% 5% 3% 1% 0%
Feb. 18: 8% 3% 1% 0% 0% 0%

Keep in mind that this is completely statistically driven and takes into no account present weather patterns, but this should give you some ballpark figures to work with. -- Bob Swanson

This question was submitted by Jeffrey Adler.

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Q: Is 318 mph the strongest tornado wind ever measured?

A: Yes, 318 mph is indeed the mark to beat. The devastating F5 tornado on May 3, 1999 that ripped through Moore, Okla., is believed to be the strongest tornado ever recorded.

It is important to remember that wind speeds associated with tornadoes are typically not measured directly. Rather, they are estimated wind speeds based on surveys and assessments done hours, if not days, after the storm. Forensic meteorologists look at the level of damage and estimate (with the help of information provided by wind and structural engineers) the winds necessary to produce varying levels of damage.

In the case of the Moore tornado, it was one of the rare occasions when a Doppler-on-Wheels (DOW) was able to intercept the storm and take a direct measurement (or at least a Doppler radar estimate) of the winds. A direct measurement of the wind would require that an anemometer be placed directly in the path of the storm – which is nearly impossible to do and, even if successfully placed, the anemometer would not be able to operate at such wind speeds. Anyway, the DOW estimated winds of 318 mph several hundred feet above the ground. You can find out more about this historic measurement on this USA TODAY web page. -- Bob Swanson

This question was submitted by Jill.

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Q: What were the wind speeds in these tornadoes -- Tupelo, Gainesville, Woodward, Flint and Waco?

A: The tornadoes you are interested in are among the top 10 deadliest tornadoes in U.S. history. You can get more details about each at this Tornado Project webpage.

You’ll notice that exact wind speeds are not included in the data for these tornadoes. It is important to remember that wind speeds associated with tornadoes are not directly-measured wind speeds. Rather, they are estimated wind speeds based on surveys and assessments done hours, if not days, after the storm. Forensic meteorologists look at the level of damage and estimate (with the help of information provided by wind and structural engineers) the winds necessary to produce varying levels of damage.

The storms in your list were rated either F4 or F5. Under the original Fujita Intensity Scale (which has been replaced by the Enhanced Fujita Scale as of Feb. 2007), an F4 tornado carried with it winds ranging from 210 to 261 mph (3 second gusts). An F5 tornado was estimated to have winds ranging from 262 to 317 mph. -- Bob Swanson

This question was submitted by Jessica Loehr.

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Q: I'm cruising to Hawaii -- what kind of weather can I expect enroute?

A: Forecasting for marine conditions is a difficult business. Unlike over land where there are hundreds of airports making hourly observations and National Weather Service offices launching weather balloons twice a day to get information throughout the atmosphere, there is precious little information (apart from a few buoy networks and occasional ship observations) over the water. Most assessments of the weather over the oceans come from analysis of satellite images and remote sensing from satellites.

That said, one resource for information over the Pacific is the Ocean Prediction Center. Check out the latest surface analysis and 24, 48 and 96 hour forecasts. If you are planning to travel during hurricane season (May 15 through November 30 in the eastern Pacific Basin), it is also worth taking a look at the National Hurricane Center's website. The Central Pacific Hurricane Center is another useful resource. If you have access to internet during your trip, you might want to revisit these websites.

As for local conditions as you approach Hawaii, make sure to visit the website of the National Weather Service office in Honolulu. -- Bob Swanson

This question was submitted by Cathy Kerley.

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Q: Santa Ana winds result from high pressure -- what creates the high pressure?

A: High pressure over the Great Basin (also known as the Intermountain West -- essentially that area between the Sierra and the Rockies that includes Nevada, Utah, eastern Washington, eastern Oregon and much of Idaho) can form at any time of the year. The reason that the high pressure forms is the track of the jet stream, typically taking the shape of a large ridge over the entire West.

Why this high is particularly important in the fall is because the difference in temperature between the Great Basin and Southeastern California can enhance the pressure difference, thereby increasing the pressure gradient and increasing the offshore wind speed. Another factor, in addition to the seasonally stronger offshore winds, is the fact that, in the fall, Southern California is typically at the end of its dry season and awaiting the rains that typically kick in over the winter. Dry fuels, warm fall temperatures and strong offshore winds all combine to increase the fire threat.

Here is an excellent Santa Ana FAQ that you might find enlightening. -- Bob Swanson

This question was submitted by Kathleen Weight.

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Q: Are there websites that show past weather conditions?

A: If you want to see what was going on across the U.S. on a particular date and time, I recommend the Hydrometeorological Prediction Center’s (HPC) surface analysis archive. Simply select date and time and you’ll get a map showing the placement of high and low pressure areas as well as fronts and selected observations from around the country.

If you are looking for climate information (temperature, rainfall, etc.) for a particular location, the best place to start is your local National Weather Service office website. Pick out your local office on this map. Most NWS office websites are set up the same way. You’ll want to first find the “Climate” section in the left navigation and click “Local.” This will bring up an interface where you can access a lot of climate info for different observation sites in your general area. -- Bob Swanson

This question was submitted by Emily.

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Q: How many days a year does Chicago receive snow?

A: The best resource for snow is the National Climatic Data Center’s Snow Climatology site. You can get a variety of historical snowfall information in Chicago by clicking through that site.

According to that site, Chicago’s Midway International Airport has about 25 days with measurable snowfall each year. (“Measurable” snow is defined as greater than 0.1 inch; snowfall less than 0.1 inch is registered as a "trace"). January is the month with the most days of snow, on average, with about seven snowy days a year. -- Doyle Rice

This question was submitted by David Shinault.

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Q: Aircraft icing is a threat to aviation at 32 degrees -- is it more or less of a problem at lower temperatures?

A: Aircraft icing can occur both when the plane is on the ground as well as in-flight. On-the-ground icing results from freezing rain conditions where temperatures at the surface are at or below freezing. An airplane sitting on the tarmac getting ready for takeoff can be coated in a layer of ice as supercooled (less than 32°F) rain droplets freeze instantly upon contact with objects that are at or below freezing. Freezing rain can occur when the air temperature at the surface is 32°F, but it can also occur when the surface air is even colder.

In the case of in-flight icing, it is actually more of a threat at temperatures less then 32°F. Keep in mind that I’m not talking about temperatures at the ground, but rather the temperature of the air at flight level. For in-flight icing, it is not ice in the clouds that adheres to the aircraft, but rather supercooled liquid water droplets that freeze upon impact with the leading edge of an airfoil. Liquid water droplets can exist in this supercooled state at temperatures ranging from 32°F to -40°F, but are most commonly encountered in clouds in a narrower temperature range. In fact, more than 50% of in-flight icing occurs at temperatures between 10 and 18°F. -- Bob Swanson

This question was submitted by Bonnie Miller.

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Q: When it's raining, can you have relative humidity of less than 100%?

A: Yes, particularly at the outset of the rainfall. Keep in mind that, when it rains, the air is saturated, or even supersaturated, within the clouds where the rain forms (a relative humidity measurement within the clouds would read close to 100%). However, the air does not need to be, and rarely is, saturated at ground level where the relative humidity is typically measured (usually at the nearest airport).

As the rain falls from the cloud base toward the ground into drier air, some of it is evaporated. In fact, sometimes the air between the cloud and ground is so dry that the raindrops evaporate entirely before reaching the ground. Precipitation that evaporates before reaching the ground is called virga. Rainfall that reaches the ground will evaporate to some extent, increasing the relative humidity at the Earth’s surface. The more rain that falls and the more time allowed for evaporation of some of the rainfall, the more the relative humidity will climb. Sometimes a fog will form following rainfall, indicating relative humidity approaching 100% at the surface. -- Bob Swanson

This question was submitted by Preston Olson.

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Q: Since the beginning of summer, has Buffalo been cooler and wetter than normal?

A: For the 3 summer months, June and August were slightly cooler than average, while July was slightly warmer than average. As for precipitation, June and August had above-average rainfall, while July had below-average rainfall. Here are the temperature and precipitation totals by month, for Buffalo, along with the averages:

May 2008 Temp: 53.4 degrees (avg. is 57.0)
May 2008 Precip: 2.54 inches (avg. is 3.35)

June 2008 Temp: 67.9 (avg. is 68.5)
June 2008 Precip: 4.91 (avg. is 3.82)

July 2008 Temp: 71.4 (avg. is 70.8)
July 2008 Precip: 2.80 (avg. is 3.14)

Aug 2008 Temp:  68.5 (avg. is 69.1)
Aug 2008 Precip: 5.33 (avg. is 3.87)

Sept. 2008 Temp:  64.2 (avg is 61.5)
Sept. 2008 Precip: 3.96 (avg. is 3.84)

All this data is from the National Weather Service forecast office in Buffalo.

The official high temperature for Buffalo so far this year is 89 degrees, recorded on June 9. -- Doyle Rice

This question was submitted by Earl W. Robinson.

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Q: Why is Houston so humid?

A: Since humidity is essentially a measure of how much water vapor is in the air (actually, relative humidity is a percentage measure of the amount of water vapor in the air compared to the amount of water vapor which would saturate the air at a given temperature and pressure), it is the proximity of Houston to water (namely the Gulf of Mexico) which makes for such a humid climate.

Here is a link to a table of the average morning and afternoon humidities (month by month) for major U.S. cities. If you were to filter through all this data looking for the highest average afternoon humidities, you'd find that the most humid (at least as measured by relative humidity) cities are in Alaska.

It might seem strange for Alaska to be more humid than Houston. The key is that relative humidity is only one way (and not always the best way) of measuring the amount of water vapor in the air.

You might find this article interesting. It explains why dew point is often a much more useful measure. -- Bob Swanson

This question was submitted by Constanza Fishel.

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Q: How can there be tornadoes in November?

Q: Tornadoes can actually occur any time of the year, given the right conditions (the right conditions being an ample supply of warm, moist air to generate thunderstorms and a favorable weather pattern to have sufficient wind shear to produce rotation within the thunderstorm). These conditions are most often met in the late spring and early summer, which is why that period is typically thought of as “tornado season.”

The main supplier of warm, moist air is the Gulf of Mexico and the main supplier of wind shear is the polar jet stream, so tornado forecasters look for times when both are in supply over the same area. In the early spring and in the fall, the jet stream is often located across the contiguous 48 states and can at times dip all the way to the Gulf Coast. With the proximity to the Gulf, this is why early and late season tornadoes tend to occur in the Southeast. As the jet stream retreats northward in the late spring/early summer, tornadoes become more frequent in the “tornado alley” of the Plains and Midwest. The jet stream retreats farther northward and weakens in the summer (sometimes located as far north as southern Canada) which is why the few tornadoes produced during the summer tend to occur from the northern Plains through the upper Midwest and into the Northeast. The jet stream is very strong and active during the winter, but lack of widespread warmth decreases the chances for winter tornadoes (those that do occur tend to be in the South).

You can check out the chances for tornadoes during the year at your location at this National Severe Storms Laboratory website -- Bob Swanson

This question was submitted by Brooke Stamback.

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Q: When can North Georgia's first snow be expected?

Q: I was unable to find a climatology for the average date of first snowfall – perhaps because snow is a relatively uncommon event (even in the North Georgia mountains). However, I was able to skim through this climate narrative from the National Weather Service office in Peachtree City to come up with some general snow climatology:

Northeast mountains -- Snow falls an average of 5 days each year, producing average seasonal total snowfall of about 4 to 6 inches.

North Georgia (excluding the mountains) -- The average annual total is 1 or 2 inches in the northern counties. Usually this snowfall occurs on just one or two days.

Central Georgia -- The average annual total is less than one inch.

If you have a particular location in mind, you can drill down through the options (first choosing the “Station Snow Climatology” option) of this website to find the average date of first snow. Dalton, Ga., for example, has had a December snow of at least one inch on six occasions during the period of record, resulting in an average date of Dec. 20 for the first inch of snowfall.

You might also be interested in checking out this PowerPoint presentation about Georgia’s climate. -- Bob Swanson

This question was submitted by Don Clinton.

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Q: What will this winter be like (specifically for Annandale, Va.)?

A: NOAA does not issue its winter forecast until next week, so check back as I’m sure there will be wire stories and perhaps a blog entry about it.

For the time being, you can check out the forecast for the next three months from the Climate Prediction Center, for both precipitation and temperature. Currently, neither of these projections show any signal for extremes (above average or below average) in precipitation or temperature in Northern Virginia. Any projection to a smaller scale (specifically, Washington, D.C., or even more specifically to the Annandale area) is completely meaningless.

For what it's worth, Accuweather issued its winter forecast a couple of weeks ago. The forecasters at AccuWeather call for a "cold slap in the face" for the East, but offer little in the way of scienctific underpinning for such a claim. Perhaps they are simply using the law of averages. A review of NOAA winter roundup from the past several years shows that temperatures have been unusually warm across the East since at least 2004/2005. It stands to reason that a cold winter is statistically possible.

Here are links to the state-by-state average temperatures (compared to long-term averages) for the past 4 winters: 2007/2008, 2006/2007, 2005/2006, 2004/2005 -- Bob Swanson

This question was submitted by Joan Gammon.

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Q: Is there a website that shows the position and forecast motion of fronts/winds?

A: Your choices are many, but a site that I use on a daily basis is the Hydrometeorological Prediction Center. You can get still images or a loop of fronts through about 36 hours. Unfortunately, these images only feature the contiguous 48 states (I assume from your email address that you live in Alaska). For your purposes, you might find this National Weather Service website more useful as it has fronts for Alaska on Days 1 and 2. You can also get a North American view that includes Alaska for days 3-7.

You'll note that the surface maps only include fronts and isobars (lines of equal pressure) and don't explicitly show forecast wind speeds and directions. You might play around with this National Weather Service website to get more specific wind information. -- Bob Swanson

This question was submitted by Bill Crain.

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Q: Why does rain appear to form "out of nowhere" and move ashore when I watch radar loops?

A: The reason that the rain seems to generate itself out of nowhere is due to the limited range of the land-based radars you are watching. From your location in Jacksonville, N.C., your closest radar sites are located in Morehead City as well as Wilmington, and are only picking up on the rain as it gets within range of the radar. There is plenty more rain farther out over the water, you just can't "see" it until it gets within range, so it appears to be magically appearing out of nowhere. The range of most National Weather Service Doppler Radar (WSR-88) is about 80 miles (up to 140 miles when there is heavy rain or snow). There are no NWS radar facilities over the water.

You'll notice the same thing when a hurricane approaches the coast (the N.C. coast or any other coast for that matter). While the storm is over open water, meteorologists will feature satellite images (either visible or infrared) of the clouds that make up the hurricane. It is not until the hurricane approaches the coast that you will start to see radar loops (from land-based sites) of the rain within the storm.

Why aren't radar images available from satellite? Unlike visible and infrared images (which are passive in that the satellite is only receiving visible or infrared wavelengths of light), radar is an active process. This means that pulses of radio waves are sent from the radar site toward the precipitation, some of these radar waves bounce off the precipitation (sometimes shifting the frequency of the waves) back toward the radar site. Measuring the time between transmission and reception of a returned pulse, as well as any shift in frequency, allows the radar to calculate the distance of the precipitation as well as its direction of movement (allowing meteorologists to infer the winds, as well as the rainfall, within  the storm).

The active nature of radar as opposed to visible and infrared images adds a whole other level of complexity and is not practical for most weather satellites. This isn't to say that radar imaging is not used aboard satellites -- it is just a different type, synthetic aperture radar and is used primarily for mapping rather than for meteorological use. -- Bob Swanson

This question was submitted by Jim Varner.

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How long does it take rain/snow to hit the ground?

Today's focus graphic actually grew out of an "Ask the Weather Guys" question that I answered last week. I had to make some assumptions in the calculations, namely that the objects (rain, snow, skydiver) have already reached terminal velocity at 10,000 feet.

As can be seen by comparing terminal velocities, a skydiver, falling faster than the precipitation, would be pelted from below while falling through rain or snow.

When I spoke to Ssgt. Marc Owens, he said that the Golden Knights jump from a variety of altitudes -- most frequently from 12,500 feet. It typically takes 67 seconds for them to fall to 2,000 feet, at which point they deploy their parachutes. The final 2,000 feet is typically covered in 60 to 90 seconds. (Graphic reprinted from USA TODAY newspaper)

Q: If there is low pressure in one part of the world, does that mean there must be high pressure somewhere else in the world?

A: Yes, essentially due to the fact that the atmosphere is a closed system, with little or no matter entering or leaving it. The gravitational pull on the constituent gases (primarily molecular nitrogen, N2, and molecular oxygen, O2) keeps most of the mass of the atmosphere in the lowest 6 miles of the atmosphere (what we call the troposphere). The thickness of this life-giving layer of atmosphere is less than 1% of the planet’s radius – sometimes the troposphere is likened to an onion skin or apple skin. You might be interested in doing a rough calculation of the mass of the atmosphere –  Mississippi State's Jeff Haby has a nice writeup.

Somewhat similar to squeezing a balloon (or perhaps the “sit on one side of the waterbed” example, though both examples are not truly accurate as they both constrain the motion of the air or water, while the atmosphere has no such constraints apart from gravity), a change in pressure in one location often induces changes in pressure at other locations. For example, when it comes to low pressure, it’s tough to beat a strong hurricane. It is not uncommon to have sinking air (subsidence) around the periphery of a hurricane to compensate for all the rising air within the storm. This is why there is often very calm, sunny weather sometimes a day before and shortly after a hurricane’s passage.

I would suspect that the many ways that the atmosphere compensates for vertical and horizontal motions would keep a world-wide averaged pressure pretty much constant. -- Bob Swanson

This question was submitted by Bill Wallace.

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Q: How long does it take a raindrop to hit the earth from 10,000 feet?

A: This answer may come with a few assumptions and the math may use some rounding, so please excuse my imprecision and bear with my back-of-the-envelope calculations.

Let’s first of all assume that the raindrop leaves the base of the cloud (at 10,000 feet) as an average-size raindrop (1 to 2 millimeters in diameter) and having already reached its terminal velocity. That is to say that the force of gravity pulling the raindrop toward the center of the earth is equally balanced by the air resistance encountered by the falling drop. Therefore, with the forces acting on the drop being equal and opposite, there is no net force on the drop and it will not accelerate, but rather will remain at a constant velocity until reaching the ground.

I’ll use the value from this table for a 1.6 mm raindrop – it would have fallspeed of  5.65 meters/second. Now, simply converting 10,000 feet into meters yields about 3,048 meters. Dividing this distance by the fallspeed yields almost 540 seconds, or almost 9 minutes.

This is a good time to also make the point that falling raindrops are not shaped like tear drops (as is so often depicted). Average-sized raindrops tend to look like hamburger buns, as air resistance and surface tension serve to squish the bottom of the drop. I did a “weather focus graphic” on the topic last year. -- Bob Swanson

This question was submitted by Tony and Cheryle Brix.

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Q: Despite triple-digit heat, why are thunderstorms rare events in Kuwait?

A: While temperatures top triple-digits easily and often during the summer, relative humidity does not. In fact, at Kuwait International Airport, relative humidity in the afternoon averages 9% in June and July, and 11% in August. So the air is extremely hot, but also extremely dry, unlike the warm, moist air (and summertime thunderstorms) that parks over Florida in the summer.

As is described in this lengthy climate narrative, summer thunderstorms in Kuwait, when they do occur, are often dry. This means that they may produce lightning, but the rainfall they produce evaporates as it falls through dry air on its way to the ground.

You might find this NOAA article about climate in the Middle East to be interesting reading. -- Bob Swanson

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Q: Can barometric pressure be calculated if air temperature and relative humidity are known?

A: If only there were a simple relationship between temperature, humidity and sea-level pressure. Unfortunately, the atmosphere is far more complicated, so there is no simple answer to your question.

First, let me convince you that a given temperature and humidity do not necessarily give you a certain barometric pressure (and vice versa). Take a look at a recent surface analysis (for example, 10/29/2008 at 18Z -- you’ll have to scroll down and enter those values and choose the United States B/W version of the map). Once you see the map, you’ll notice a bunch of squiggly lines around centers of high and low pressure (denoted by little “H”s and “L”s). These lines are called isobars and connect locations that have the same barometric pressure. You’ll notice that the line (1020 millibars) that runs through Myrtle Beach, S.C., also runs through Tucson, Ariz., and any other number of locations in between (through the Great Lakes, the northern Plains and the Northwest along the way). All of these spots have widely different temperatures and humidities, yet they all have the same pressure.

How can this be? Temperature measures the average kinetic energy of the air molecules that hit your thermometer. Relative humidity is a measure of the number of water vapor molecules in the air near your sensor. Both of these readings are essentially sampling the air at the surface. However, barometric pressure is a measure of the pressure exerted by the column of air directly above your barometer all the way to the edge of the atmosphere. Essentially, the barometer is measuring the weight of that large column of air. That’s why there is no simple answer to your question. You might think that if you were to magically go from a temperature at the surface of 79° and a relative humidity of 78% (fairly warm, moist and low density air) to a temperature of 35 and a humidity of 30% (relatively cool, dry and denser air), that the pressure would go up since the air at the surface is now denser (heavier, therefore exerting more pressure). However, since the barometric pressure at the surface takes into account the weight of the entire column, we cannot rule out that, as the surface is getting much denser, something equally crazy could be happening high aloft in the atmosphere. Perhaps much warmer air is moving in aloft, making that portion of the column less dense and essentially counteracting the pressure increase at the bottom of the column.

Of course, this scenario is quite magical in that this change in temperature and humidity is instantaneous, while things in the real world work more gradually. The point is that there is no simple calculation that will yield a pressure given a temperature and a relative humidity. -- Bob Swanson

This question was submitted by Kirk Robinson.

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Q: What should my in-house humidity be during the wintertime?

A: Your best bet is to get a quality hygrometer to keep an eye on your in-house humidity level. Indoor humidity can vary greatly from the outdoor humidity that you might see reported on your local news.

Conventional wisdom is that, for best indoor comfort and health, a relative humidity of about 45% is ideal. Winter’s dry air can lead to health problems like dry skin, scratchy throat, and nosebleeds. Humidifiers can provide relief by adding moisture to the air and slowing evaporation from the body. However, too much moisture can lead to condensation, mold and rot. A house with relative humidity less that 30% is too dry. One with relative humidity over 50% is too wet. -- Bob Swanson

This question was submitted by Pearl Williams.

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Q: On a recent flight, I saw flashes of light. Was this lightning?

A: I went back into the NEXRAD archive to see if there were any storms in the vicinity of your flight path (from Washington, D.C., to Atlanta). There was a small line of showers and perhaps a few thunderstorms stretching from western New York into the Ohio Valley. The Storm Prediction Center mentioned the possibility of thunderstorms along this front, which (according to my back-of-the-envelope calculations) was about 225 miles from you at takeoff. Unfortunately, archived lightning data (from the National Lightning Detection Network) is held by a private company, Vaisala, so I can’t tell if there were actual lightning strikes on the end of the line of showers and storms during that time period.

Anyway, assuming there were some thunderstorms, could they be seen from such a distance? According to this “distance to the horizon” calculator, when cruising at 30,000 feet, the horizon is about 200 miles away. Now factor in the height of the hypothetical thunderstorms in the Ohio Valley – perhaps 20,000 to 30,000 feet – and it’s not out of the realm of possibility that you could have been seeing lightning flashes from these storms. -- Bob Swanson

This question was submitted by Kristin Bartholomew.

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Q: Is it a decrease in pressure that creates hail within thunderstorms?

A: In order to tackle this question, let's learn a little bit about the motion of air within the troposphere.

Typically, air is mainly composed of molecular nitrogen, N2, (78% of the atmosphere) and molecular oxygen, O2 (21% of the atmosphere). When air is more humid than normal (has more water vapor per unit volume), it is actually less dense than when there is less water vapor. The reason being is that the molecular weight of water vapor, H2O, is about 18, while the molecular weight of N2 is 28 and O2 is 32. More water vapor molecules in a given volume of air take the place of some of the N2 and O2 molecules and make a given volume of air less heavy (less dense). That’s why the moist air heads for the ceiling of your bathroom and is easily sucked out with your ceiling fan when you take a shower.

Now, a little bit about the atmosphere. Since air molecules have mass, they are acted upon by gravity and have weight. The majority of air molecules are clustered near the surface of the earth and become fewer and fewer the higher you go. We say that the air gets thinner (try taking a deep breath atop Mt. Everest), but really what is happening is that air pressure is lower at higher elevations. The pressure at sea-level is around 1000 mb (1013.2 mb is standard sea-level pressure) while the pressure at 18,000 feet is about half that (500 mb).

Now, let’s get back in the shower and follow that warm, moist air that got sucked out of your bathroom and exhausted out of the vent on your roof. If that warm, moist air is less dense than the surrounding air, it will continue to rise. As it does and moves into areas with fewer air molecules (lower pressure) the air parcel expands. Thermodynamically, since it takes work to expand, the air molecules use energy to do this work and this lowers the temperature – that is why rising air cools (and conversely, why sinking air warms). This is the same principle behind your cooling propane tank example. When air cools to the dew point temperature, net condensation of water vapor can occur. Generally when this occurs above the freezing level in clouds, water vapor condenses onto existing liquid cloud droplets as well as ice crystals, causing them to grow. This condensation also releases energy (latent heat) which can warm other parcels of air, making them less dense than their surroundings and allowing the thunderstorm to grow even higher into the atmosphere.

Despite the cooling of a rising parcel, it will continue to rise as long as it remains less dense than the surrounding environment. That is why such strong updrafts can occur on warm, humid days when there is cold air aloft. It’s like stacking large heavy boxes on top of little light boxes. It’s an unstable atmosphere that wants to overturn. Updrafts form that carry warm, moist air high into the atmosphere. Generally, an updraft speed of 24 to 34 mph is required to support the formation of hailstones.

The long and the short is that it is not the pressure drop directly that causes the formation of hailstones, but rather the cooling induced by the expansion of air parcels as they move into regions of lower pressure. -- Bob Swanson

This question was submitted by Richard Kraemer.

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Q: One inch of snow typically equals how much rainfall?

A: “Typically” is the operative word in your question. Off the cuff, most meteorologists would say that there is a 10:1 ratio of snow to water (meaning that an inch of liquid water would result in 10 inches of snow). However, wet snows can result in ratios as low as 3:1 and very dry snows can result in ratios of 30:1 (in extreme cases, the ratio can be upwards of 100:1). So much depends on the wind and temperature profile of the atmosphere. According to the National Snow and Ice Data Center:

While many snows that fall at temperatures close to 32°F and snows accompanied by strong winds do contain approximately one inch of water per ten inches of snowfall, the ratio is not generally accurate. Ten inches of fresh snow can contain as little as 0.10 inches of water up to 4 inches depending on crystal structure, wind speed, temperature, and other factors. The majority of U.S. snows fall with a water-to-snow ratio of between 0.04 and 0.10.

I haven’t found any long-term studies of snow-liquid equivalents specifically for western North Carolina, but this NCSU student poster of recent snow events shows a tremendous range in such ratios. A longer-term study from St. Louis University does indeed show close to a 10:1 snow:liquid average for western North Carolina. -- Bob Swanson

This question was submitted by Johnny Woody.

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Q: What are Florida's all-time hottest and coldest temperatures?

A: Florida's all-time high temperature was 109 degrees, set on June 29, 1931, in Monticello, Fla.; the lowest temperature was the -2 degree reading, set on Feb. 13, 1899, in Tallahassee. This is according to the National Climatic Data Center, where all of the USA’s official weather and climate records are kept.

All 50 states' record high and low temperature readings can be found on these USA TODAY resource pages:
Each state's high temperature record
Each state's low temperature record
--Doyle Rice

This question was submitted by Karen Kane.

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Q: I live in a valley. Do I get fewer daylight hours?

A: Yes, being in a valley where your horizon is obstructed by terrain does have an impact on the amount of daylight hours. Sunrise is defined as the moment that the top of the solar disc rises above the horizon and sunset is when the top of the solar disc dips below the horizon. The U.S. Naval Observatory has a calculator that will tell you the sunset/sunrise for your location.

I’m unsure of this, but I seriously doubt that local terrain is part of the calculation process – I think it goes strictly by latitude and longitude. It might be interesting for you to compare the time (using a synchronized clock such as the one on your cell phone) that the sun sets below the mountains to your west with the value given by the USNO table. My suspicion is that your time of sunset will be at least a few minutes earlier than the value given in the table. -- Bob Swanson

This question was submitted by Mike Vendetti.

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Q: Is the west-to-east motion of the jet stream related to the rotation of the earth?

A: Yes, the general west-to-east flow of the many jet streams (polar front and subtropical jets), both in the Northern and Southern Hemispheres, is related to the Earth's rotation. The very basic idea is that air at the equator has more momentum than air at higher latitudes. Air at the equator is moving eastward at more than 1,000 miles per hour. Air at 30°N traces out a smaller circle during the 24 hours of the day so its speed is less than 1,000 mph. Similarly for air at 60°N. Because air moving toward the poles is carrying more eastward momentum, it results in west-to-east jet streams.

Another way to think of this is to picture a figure skater in place of the earth’s axis of rotation. With her arms extended, her hands are at the equator as she spins. Now think of air moving poleward, curving toward the axis of rotation as it moves toward the pole. This is equivalent to the figure skater drawing her hands inward closer to her own axis of rotation. By conservation of momentum, she spins faster as she does this.

Let's see how this works in both hemispheres. In the Northern Hemisphere the wind starts blowing from south to north (poleward). The Coriolis effect turns it to the right, making it flow from west to east. South of the equator, the wind begins blowing from north toward the south (again poleward). Here the Coriolis effect turns it to its left. The result is also a west-to-east jet stream in the Southern Hemisphere.

You might find this excellent National Weather Service webpage useful in understanding jet streams. -- Bob Swanson

This question was submitted by Bill Hills.

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Q: Why didn't Hurricane Omar move from east to west through the Caribbean?

A: While most tropical cyclones (tropical storms and hurricanes) do tend to move from east to west through the Caribbean, late-season storms have a climatological tendency to move northward or even northeastward (as Omar did) through the Caribbean. This was the topic of a recent weather focus graphic.

Here are maps that show the locations of origin and prevailing tracks of tropical cyclones month-by-month in the Atlantic basin.

As it gets later in the fall, mid-latitude westerlies become stronger and tend to have more influence the motion of storms in the Caribbean. In the case of Omar, an upper-level trough off the U.S. Eastern Seaboard produced southwesterly steering winds over the Caribbean. -- Bob Swanson

This question was submitted by Hector Correa.

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Q: I live in southwestern Pennsylvania. Do winter winds blow from the north?

A: No, as is common in many parts of the country, prevailing winds in southwestern Pennsylvania are from the west and southwest -- even during the winter months. Check out this National Climatic Data Center website to see prevailing winds by month for many locations around the nation.

While the annually averaged winds in southwestern Pennsylvania are from the west, that is not to say that the weather pattern cannot produce winds from the north. Indeed this can and does happen when high pressure is centered over south central Canada or the upper Great Lakes. This weather pattern would indeed bring cold, polar air from Canada into southwestern Pennsylvania, but this isn’t the predominant weather pattern either in the winter or any other season for that matter. -- Bob Swanson

This question was submitted by Ed Smith.

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Q: Why don't tornadoes ever form in the Caribbean?

A: Rather than saying why the Caribbean so rarely sees tornadoes, I’d rather explain why the U.S. sees so many tornadoes (far more than any other country). The existence of a large north-south mountain range (the Rockies) that affects the flow of the jet stream, combined with available moist air from the Gulf are two key ingredients in making the U.S. the tornado capital of the world.

The Caribbean simply does not have similar topography conducive to tornado formation. While rare, tornadoes have occasionally occurred in the Caribbean. -- Bob Swanson

This question was submitted by Marsha Nelson.

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Q: What can cause a dramatic drop in barometric pressure?

A: Typically a dramatic change in pressure is the result of the passage of a cold front. In advance of a cold front, you’ll typically find falling pressures, with the pressure minimum associated with the passage of the front itself then gradual rises in the wake of the front.

There are even greater pressure drops in the case of tropical cyclones. In fact, Hurricane Wilma in 2005 dropped from 28.94 inches to 26.05 in the span of 24 hours.

Keith Heidorn, The Weather Doctor, has a very nice article about how to forecast weather by watching your barometer. -- Bob Swanson

This question was submitted by Joyce Douroux.

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Q: Why does the weather become clear and cool after a thunderstorm?

A: The clearing that occurs after a thunderstorm can partly be explained by the particulates that get washed out of the air due to the rainfall. When these tiny particles are abundant, they act as condensation nuclei (that is, water vapor molecules condense on them) and as they grow, they scatter sunlight. This is why it can look hazy on a hot, humid day. Raindrops can scrub these haze particles out of the sky as they fall to earth.

As for the cooling during a thunderstorm, a summer thunderstorm can produce some nice cooling due to its outflow. These are the winds that can precede the storm itself. Typically they result as dry mid-level air is drawn into the rain shaft of the storm. The downward momentum of the rain can drive this air downward and evaporational cooling of the raindrops in the dry air (a process that takes energy) can make the air even cooler and denser. This cool, dense air hits the ground and spreads out in all directions and can produce a sudden drop in temperature. This cool air can act like a mini cold front (sometimes referred to as a gust front), lifting other warm moist air and triggering additional thunderstorms. This is why forecasters keep an eye on “outflow boundaries” from thunderstorms that have fallen apart as possible locations for additional thunderstorms to initiate. -- Bob Swanson

This question was submitted by Valerie Harden.

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Q: Does warm air hold more moisture than cold air?

A: Because moisture (water vapor) is part of air (mainly nitrogen and oxygen, with other trace gases including water vapor), I hesitate to say that “air” has any temperature-dependent “capacity” or ability to “hold” water vapor. What I can say is that water molecules are constantly changing phases (solid, liquid and gas) and that the rate of change is temperature dependent (there are other factors involved as well). When there is more energy available (higher temperature), there is a net evaporation (more molecules evaporate than condense). When temperature decreases, there is a net condensation.

So in the case of cloud formation, it isn’t that rising air cools and has less “holding capacity” for moisture, but rather that in a colder environment, there is net condensation, allowing for the growth of cloud droplets or even ice crystals.

Alistair Fraser, retired professor of meteorology at Penn State University, has an excellent explanation on his "Bad Meteorology" website.  Fraser correctly points out that, in the absence of nitrogen, oxygen or any other gases, this same behavior of water molecules would go on. Air (nitrogen and oxygen) has no more capacity to hold water vapor than water vapor has a capacity for nitrogen and oxygen. -- Bob Swanson

This question was submitted by Gordon Malcolm.

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Q: Why don't the Great Lakes have tides?

A: There is apparently some disagreement among experts as to whether the Great Lakes have tides, but there is no doubt that, even if they do, they are miniscule (1 to 4 cm) compared to tides experienced at coastal locations on oceans (particularly the Atlantic Coast).

I would suspect that it is the relatively small volume of water (even in Lake Superior) as well as the fact that the Great Lakes are essentially closed off from other bodies of water. Such restrictions to the free flow of water can have an impact on tidal cycles. For example, while the East Coast of the U.S. has semi-diurnal tides (two high/two low each day), the Gulf Coast only has diurnal tides (one high/one low each day). This has to do with the fact that ocean water can only enter or exit the Gulf of Mexico through the narrow Yucatan Channel or the Florida Straits. -- Bob Swanson

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Q: At what time of day does the temperature reach its highest point in Texas?

A: There is no single answer, since weather conditions and time of year can have an impact on the diurnal cycle of temperatures. However, as an example, here are the times of high temperatures around the Lone State State from Sept. 17, 2008:

Dallas = 83°F at 4:27 p.m.
Austin = 84°F at 2:38 p.m.
Houston = 79°F at 2:52 p.m.
El Paso = 79°F at 3:10 p.m.
Lubbock = 79°F at 3:17 p.m.
Brownsville = 84°F at 2:00 p.m.

The general trend is that, on a typical late summer/early fall day, the hottest time is in the mid-afternoon, not at local noon. I think the general range of 2:00 to 6:00 p.m. is pretty good. Most National Weather Service offices issue daily climate reports around 5:30 p.m. or 6:00 p.m. This allows them enough time to capture the high temperature at an observation station, but still allows television broadcasters to get this information onto the local evening news.

This NOAA website offers some support (if it comes in a graph, it has to be meaningful) for the late afternoon time period as being the hottest. -- Bob Swanson

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Q: Why is New York City cooler than San Francisco? Aren't they both on the ocean?

A: Part of the reason is latitude. New York is at nearly 41°N, while San Francisco is at about 37.5°N. Even more important is that the prevailing winds for both cities are from the west. This means that while air flows into San Francisco off the Pacific Ocean, air flowing into New York City comes from off the land. This makes the moderating effect on San Francisco more pronounced.

Here is a table of average monthly high temperatures (for the 30 year period from 1971-2000) for San Francisco (Airport) and New York City (Central Park). Note that the first column is the number of years in the record (30) and the remaining columns are the average high temperature each month, January through December, with the final column being the annual average high temperature:

SAN FRANCISCO AP, CA             30   55.9   59.3   61.2   64.3   66.8   69.9   71.1   71.7   72.7   69.7   62.0   56.1   65.1
NEW YORK C.PARK, NY              30   38.0   41.0   49.8   60.7   70.9   79.0   84.2   82.4   74.7   63.5   53.1   42.9   61.7

You’ll note that while the annual average is cooler in New York (61.7) than San Francisco (65.1), what is most striking is that the range in New York (38 degrees in January to 84 degrees in July) compared to the San Francisco (56 degrees in January, September being the warmest month at nearly 73 degrees). This graphic explains why summer arrives late in San Francisco. -- Bob Swanson

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Q: When a hurricane is a specific distance from shore, why is the expected time of landfall so changeable?

A: A distance specified by the National Hurricane Center is not necessarily the distance of the hurricane from the eventual point of landfall. The "cone of uncertainty" in a forecast track can encompass a large area and changes to the track can drastically affect the distance to shore. If the storm slows down or speeds up, this can also change the time the storm takes to make landfall.

In the case of Hurricane Ike, the storm was 700 miles east of Brownsville, Texas, moving to the northwest at 8 mph at 8 p.m. ET, Wed., Sept.10, 2008. The forecast track at the time had the storm making landfall on the Matagorda Peninsula (it wound up making landfall farther north up the coast) in the early morning hours of Saturday, Sept. 13 (the timing proved to be quite accurate). So the storm made landfall about 56 hours after the advisory was issued.

Certainly the math doesn’t add up -- if you divide 700 miles/8 mph, you would expect landfall in something more like 88 hours. Then again, the storm didn’t and wasn’t expected to make landfall at Brownsville, Texas. The National Hurricane Center uses some of the larger population centers and convenient geographic reference points (the same advisory placed the storm 345 miles south-southeast of the mouth of the Mississippi River) to give some context for the location of the storm. Distance is not given to the expected location of landfall – probably because the NHC constantly stresses that folks should not become so focused on the “skinny black line” that makes up the center of the “cone of uncertainty.” Thus, even though the storm was 700 miles from Brownsville, its distance from the eventual landfall location was likely shorter. Also, the forward speed of the storm can and does change speed. In fact, Ike sped up as it approached the coast, eventually moving at 12 to 13 mph during the final 30 hours before landfall.--Bob Swanson

This question was submitted by Rick Hill.

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Q: How can snow fall when the temperature is above freezing?

A: In order for snow to reach the ground, ideally the entire air column from surface to cloud base would be below freezing. However, such is not always the case. As I'm sure you’ve experienced, snow can and does fall when surface temperatures are above freezing. The general forecast rule of thumb is to look at temperatures at around 5,000 feet. If the temperature at that level and at levels higher in the atmosphere is at freezing or below, snow will be possible. However, if there is above-freezing air at lower levels, the snowflakes may melt as they fall to the ground.

While I've seen anecdotal reports of snow falling with surface temperatures in the mid-40s, that would require an extremely shallow warm layer, with much colder air immediately above the surface. According to the National Snow and Ice Data Center, as a general rule, snow will not form if the ground temperature is 41 degrees or higher.

Check out this NSIDC webpage for an excellent FAQ about snow. Learn more about winter precipitation with this USA TODAY interactive graphic. -- Bob Swanson

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Q: How can you determine if there will be dew overnight?

A: The same conditions that can create fog can also result in dew formation. Clear skies and light winds allow surface temperatures to drop rapidly during the overnight hours. If the air temperature cools to the dew point temperature you can get fog. You can also get dew or, if the dew point temperature is below freezing, frost.

Usually for areas away from water, a limiting factor is available moisture. The amount of water vapor in the air is typically low in the winter and temperatures in the summer are so high (with relatively short nights) that not enough cooling occurs overnight to drop the air temperature to the dew point. Thus, dews and fogs tend to be more common during the spring and fall. That said, available moisture is rarely a problem over bodies of water. As temperatures cool in the fall, steam fog is a common occurrence near bodies of water.--Bob Swanson

This question was submitted by Doug Coyner.

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Q: What is the frost line?

A: The frost line marks how far down frost will penetrate soil. The beams of a structure that establish the foundation, known as footings, are placed below this point during construction to ensure minimal, or no, movement. Areas have different frost lines, depending on air temperature above the soil, sun exposure, ability to reflect light, snow cover, as well as the heat conductivity, thickness, and water content of the soil.

This map shows the frost lines for all states across the nation. Northern Maine and northern Minnesota have the deepest frost lines in the lower 48 states, with an average frost depth of more than five feet.For more about how ground freezes, check out this USA TODAY resource page.-- Paige Dearing

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Q: Why is the weather along an ocean coastline usually milder than the weather farther inland?

A: Proximity to water does indeed have a moderating effect on coastal areas. This is because of the larger heat capacity of water compared to land. This means that it takes water longer to heat up as well as to cool down. On a day-to-day basis this means that, as inland areas heat rapidly after sunrise, surface air is heated and rises, creating low pressure inland. Air above coastal waters is cooler and moves in to take the place of the rising air inland. This is the cooling sea breeze enjoyed by coastal residents. High temperatures along the coast will typically be cooler during the day. The vast reservoir of energy contained in the water also keeps coastal areas warmer at night. This is not unlike the urban heat island effect in which concrete and pavement release energy, keeping urban areas warmer at night than rural areas.-- Bob Swanson

This question was submitted by Kelly Walls.

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Q: How do meteorologists decide which computer model to use in creating a forecast?

A: Different computer models can and do show different forecast outcomes, based on the equations that are built into the respective models. Human forecasters, over time, learn which models tend to perform better in certain situations (some do better with snow, others do better in forecasting heavy rain events). These are called model biases and can vary depending on season and region. An individual forecaster tends to have his/her own preferred models and can abandon model guidance altogether if he/she feels that the models are “barking up the wrong tree.” This is sometimes the case when models are not capturing some of the microscale climate tendencies that a human forecaster may be familiar with after living/working in a particular location over time.-- Bob Swanson

This question was submitted by Kathy Gremaud.

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Q: What’s the sunniest part of North America?

A: After checking the weather records for the USA and Canada (I wasn’t able to find reliable data for Mexico), I found that Yuma, Ariz., takes the prize as the USA’s sunniest city, by either of two ways this statistic can be measured. First, the city averages 242 clear days per year, the most of any major U.S. location.

Another way of measuring “sunniness” is by the percentage of possible sunshine a city receives each year. By this measurement, Yuma again is the winner, as the city receives 90% of the possible sunshine annually. Other very sunny U.S. cities are also in the Desert Southwest, including Phoenix, Tucson, and Las Vegas.

These charts from the National Climatic Data Center show how many cloudy vs. clear days there are for many U.S. cities, as well as the percentage of possible sunshine.

No Canadian regions were even close to the U.S. Desert Southwest in terms of annual sunshine. Southern parts of the central prairie provinces of Saskatchewan and Alberta have the most sunshine in Canada -- about 50 percent of the possible amount -- due to their distance from the oceans.

As for Mexico, I would suspect that desert locations in northern Mexico near the Arizona border would also be very sunny, potentially as high as the Desert Southwest of the USA.-- Doyle Rice

This question was submitted by Pamela R. Roberson.

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Q: Why did the remnants of Hurricane Ike cause so much trouble in the Midwest?

A: Tropical systems are supposed to always lose strength over land. Hurricane Ike was no different in that sense. Its central pressure increased and its maximum winds decreased steadily as it moved inland.

After making landfall as a strong Category 2 early Saturday morning, Sept. 13, 2008, the storm had weakened to a tropical storm over Texas by the afternoon. The storm weakened further and was a tropical depression just 12 hours later (1 a.m. CT, Sunday, Sept. 14) over western Arkansas. By 4 a.m., the National Hurricane Center was no longer issuing advisories on the storm and had turned over responsibility to the Hydrometeorological Prediction Center.

The HPC continued to issue advisories on the remnants of Ike on Sunday and at each successive advisory the central pressure was higher and the center of circulation weaker.

So, to get to your central question, why did the storm seem to do so much damage in the Midwest? The basic answer is that, while the storm itself weakened, it got squeezed between a trough in the jet stream over the Rockies and a ridge over the Southeast. At the same time, it was absorbed by surface cold front moving through the Midwest and eventually the Ohio Valley. The remnants of Ike fed plenty of moisture into the cold front to produce heavy rainfall (some heavy rainfall had already fallen in the Midwest on Saturday with some of the moisture attributed to the remnants of Tropical Storm Lowell from the eastern Pacific) as well as thunderstorms.

The convective nature of thunderstorms allows them to tap into some of the stronger mid-level winds in the atmosphere, dragging them down to the surface in the form of downbursting winds. Such was the case with the remnants of Ike, as more than a dozen reports of damaging winds came in on Sunday as the whole system raced off to the northeast.-- Bob Swanson

This question was submitted by Ray Hamilton.

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Q: Do pilots have to constantly update their altimeters due to barometric pressure changes?

A: An altimeter tells a pilot the altitude of the plane above sea level by measuring the air pressure around the aircraft. This reading can be affected by ascent and descent of the aircraft, as well as changes in air pressure due to changing weather, so pilots have to adjust altimeters in-flight by getting altimeter settings via radio from airports along the way.

Jack Williams, former weather editor here at USA TODAY, has an excellent article explaining altimeters.

This question was submitted by Steve Johnson and answered by Bob Swanson.

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Q: What’s the highest dew point ever recorded?

A: Supposedly, the highest dew point on record is the phenomenal 95-degree reading measured in Dhahran, Saudi Arabia, on July 8, 2003, according to the book Extreme Weather by Christopher Burt. I say supposedly because weather record expert Randy Cerveny from Arizona State University reports that there is some controversy with that measurement, due to inaccurate instrumentation. Cerveny is the keeper of the World Meteorological Organization's World Weather / Climate Extremes Archive.

Regardless of the record, locations near the Persian Gulf, the Red Sea and the Gulf of Aden are some of the most hot and humid spots on the planet, due to the extremely high temperatures of the sea water there. Dew points in the sultry 80s are commonplace there.

In the USA, dew points as high as the mid-80s have been recorded. However, Cerveny reports that dew points are not part of the “official” extremes monitored by the National Weather Service, so no official record dew point exists for the USA. Official records only exist for temperature, precipitation, wind, hail, and snow. -- Doyle Rice

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Q: Can wind speeds be estimated by looking at wind socks, flags, etc.?

The FAA has established criteria for wind socks at airports. They should show the true direction of the wind at speeds as low as 3.5 mph and should be fully extended at speeds as low as 17.25 mph. Flags can vary depending on location, elevation, and composition, but many can become outstretched at wind speeds from 10 to 20 mph. Because the strength of the wind can affect ball flight, flags for pins on golf courses are sold that help golfers estimate winds.

You can take your cues for wind speeds by observing the world around you. That’s how, in 1805, Sir Francis Beaufort came up with the Beaufort wind scale that is still in use today. It has 13 wind categories that correspond to wind effects observable at sea and on land.-- Bob Swanson

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Q: Why are winds so strong and consistent in the fall?

A: The strength of fall winds derives from the migration of the mid-latitude jet stream (typically referred to simply as the "jet stream"). This river of air, six to nine miles high in the atmosphere, forms along the boundary between cold, polar air and warm, subtropical air. The jet stream is typically located across the southern USA in the winter and shifts toward the Canadian border during the summer. During the transitional seasons of spring and fall, this ribbon of strong winds aloft often roars across the continental USA.

This jet stream increases the speed of all winds above the Earth's surface. In early fall, there is still enough daytime heating to create warm air pockets that rise vigorously from the ground. Strong winds aloft sink to replace this rising air, creating gusty winds. --Bob Swanson

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Why do hurricanes move from east to west?

Today's "weather focus graphic" was inspired by a Montessori schoolteacher, Debbie Mazzei, who sent in the question on behalf of her students. In addition to the general circulation of winds around the globe, another important factor in steering tropical systems in the Atlantic is the strength and position of the Bermuda High.

This sub-tropical area of high pressure is typically located over the eastern Atlantic in winter, but moves westward toward Bermuda during the summer. Tropical systems move westward along the southern boundary of the high, then move northward on the western edge. The location and strength of the Bermuda high can determine whether a tropical cyclone recurves out over ocean water or if it is steered into the U.S., either into the Gulf or Atlantic Coast.

Send your suggestions for future weather focus graphics to weatherguys@usatoday.com. 

(Graphic reprinted from USA TODAY print edition. Click to enlarge.)

Q: What’s the fall weather forecast for the USA?

A: The Climate Prediction Center forecasts that most of the USA should enjoy a warmer-than-average fall, with the greatest likelihood of unusual warmth across New England and southern parts of Texas, New Mexico and Arizona. Only the Southeast, Northwest, and California are expected to have equal chances of a warmer- or cooler-than-average fall season. This means the climate signal wasn't strong enough to determine what the temperatures should be in those locations for the season.

As for precipitation, the areas with the highest chance of a wet autumn are across northern New England and South Florida. The rest of the nation should see average amounts of precipitation.-- Doyle Rice

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Q: What was the strongest hurricane ever?

A: In recorded history, the strongest tropical system was 1979’s Supertyphoon Tip, which maxed out with sustained winds of 190 mph. The strongest hurricane in the Atlantic basin was 2005’s Hurricane Wilma, with estimated sustained winds of 185 mph in the Caribbean. By the time Wilma made U.S. landfall near Cape Romano, Fla., it was a Category 3 with sustained winds around 115 mph.

As for strongest hurricanes at U.S. landfall, the 1935 Labor Day storm that hit the Florida Keys tops the list – its central pressure at landfall was 892 mb (26.35 inches of mercury). Estimated wind speeds at landfall range from 160 mph to nearly 200 mph.-- Bob Swanson

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Q: Why does the Atlantic Coast have two high tides and two low tides per day, while the Gulf Coast has only one of each?

The Gulf of Mexico does indeed have diurnal tides (one high, one low per day) while the Atlantic Coast has semi-diurnal tides (two high, two low per day).

The explanation of the semi-diurnal tide is rather straightforward, as high tide occurs when a point on Earth is aligned with the gravitational pull of the moon (and to a lesser extent, the sun). This happens when moon is highest in the sky, as well as approximately 12 hours later when the Earth has rotated and the moon is on the other side of the Earth. Low tides occur when a point on Earth is oriented at 90 degrees to the main axis of gravitational pull – which also happens twice a day as the earth rotates.

Indeed, if the Earth were covered with a uniform depth of water, all points on Earth would experience two high tides and two low tides per day. It is the blocking effect of landmasses that complicates the process, prohibiting the free flow of water and contributing to unusual tide cycles such as the diurnal (one high, one low) tide cycle in the Gulf of Mexico and mixed tide cycles, such as those that occur along the West Coast. Remember that the only way water can get into or out of the Gulf is through the narrow Yucatan Channel as well as the Florida Straits.-- Bob Swanson

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Q: Why are "millibars" sometimes used instead of "inches" to measure barometric pressure?

A: Millibars are used by most countries' weather services to measure air pressure, other than the USA, which still uses inches of mercury. Also, most scientists in the USA and around the world use millibars in their research. Additionally, measurements of air pressure above the surface throughout the atmosphere are also usually given in millibars.

Millibars are the same as hectoPascals, the official metric unit of air pressure that is another term you may hear in other countries.

The standard atmospheric pressure at sea level is 1,013 millibars, which equals 29.92 inches of mercury. -- Doyle Rice

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Q: Does a wind direction of 225 degrees mean the wind is blowing to or from the southwest?

A: The convention in weather is to name winds according to the direction they are coming from. Thus, an easterly wind is blowing from the east toward the west. In the case of degree readings, 0 degrees is true north, 90 degrees is east, 180 degrees is south and 270 degrees is west. When the wind direction is given as 225 degrees, that means the wind is blowing from the southwest (225 is halfway between 180 -- south -- and 270 -- west) and toward the northeast.

Confusion arises when directions are used in other contexts. For example, a “southerly” wind means that is blowing from the south, but a ship that takes a “southerly” course is moving toward the south.-- Bob Swanson

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Q: At what radio frequency does lightning transmit?

A: Lightning emits a wide range of radio frequencies, but is mainly concentrated in the very low frequencies, ranging from a few hundred hertz to 10 kHz. Lightning can produce radio waves farther up the spectrum, which is why it can sometimes interfere with AM radio (535 kHz – 1700 kHz) reception, but is never a problem with FM radio reception.

You can listen to lightning live on NASA’s online VLF online receiver. You can also build your own lightning radio detector that is tuned to 300 kHz, as it is a fairly uncluttered frequency that will pretty much only yield lightning static. -- Bob Swanson

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Q: Which state has been hit by the most hurricanes?

A: Florida leads the nation in hurricane landfalls, with a total of 114 strikes since records began in 1851, according to the Atlantic Oceanographic and Meteorological Laboratory. That represents about 40 percent of the 286 hurricanes that have hit the USA. Texas is second with 62 landfalls and Louisiana third with 52. Every state along the Gulf and East Coasts has been struck by at least one hurricane since 1851.

Additionally, 83 percent of all Category 4 or 5 hurricanes have hit either Florida or Texas. Category 4 hurricanes have wind speeds of greater than 130 mph.

This map from the National Climatic Data Center (PDF) shows the location of all U.S. hurricane strikes since 1950 — Doyle Rice

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Q: When the temperature reaches 100° or more, does the humidity go down?

A: Relative humidity essentially measures the ratio of the amount of water vapor in the air compared to the amount that it could possibly hold. Thus, low relative humidity means that the air is really dry (it only contains a small fraction of the water vapor that it could possibly hold). High relative humidity means that the air is close to saturation, meaning that it cannot possibly hold any more water vapor.

Typically, the air temperature changes (due to the heating of the sun) more than the amount of water vapor in the air does over the course of the day. That is why, if you watch your local weather report or check an internet weather observation for a location of your choice several times a day, you’ll notice that the relative humidity changes throughout the day. In the morning around dawn, the temperature is relatively cool. During the day, air temperature increases, often peaking in the afternoon between 2 p.m. and 5 p.m. Since the capacity for water vapor increases with temperature, and if the amount of water vapor remains essentially the same throughout the day, the air will be closer to saturation in the morning when the air is cool than it will be in the afternoon. Therefore, you’ll notice a cycle (a diurnal pattern) where the relative humidity is high in the morning and decreases through the hottest part of the day then starts to climb as evening sets in and air temperatures cool again.

Therefore, to answer your question, the relative humidity would likely be going down when the temperature reached 100° on its way up. But there is nothing special about the 100 degree mark. Assuming that the morning temperature started at say, 75°, the relative humidity was decreasing all day on the way to 100°. Keep in mind that this all assumes that there is no drastic change in the air mass that would affect the dew point temperature dramatically. -- Bob Swanson

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Q: Are there any states that have never hit 100°?

A: Actually, every state in the USA has seen the thermometer reach triple-digits, but only just barely in both Alaska and Hawaii.

According to NOAA's record of the highest recorded temperatures, Alaska peaked at 100° on Jun. 27, 1915 in Fort Yukon. Hawaii saw that same reading 16 years later on Apr. 27, 1931 in Pahala. Technically, these triple-digit temperatures occurred before Alaska and Hawaii became U.S. states, which didn't happen until 1959.

Check out this National Climatic Data Center website for a listing of the record high temperature for each state in the nation.

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Q: How soon after sunset does it get dark?

Civil twilight is the period after the top of the sun's disk crosses the horizon and the center of the sun is 6 degrees below the horizon. After civil twilight, artificial illumination is typically required to carry on outdoor activities. The amount of time between sunset and complete darkness will vary depending on latitude. For example, Barrow, Alaska will have 1 hour, 37 minutes of civil twilight this evening, while Miami will have only 23 minutes. -- Bob Swanson

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Q: On a recent night, flashes of multi-colored light, seemingly continuous, filled the sky above overcast skies, yet there was no rain and I heard no thunder. Was this lightning or something else?

A: Lightning is always accompanied by thunder. However, lightning is an electromagnetic wave that requires no medium to travel. That is why lightning can often be seen as far as the horizon will allow. Thunder is a sound wave that requires a medium (in this case air) in which to travel. Such a wave will be dampened out with distance. Typically, thunder is not heard if the lightning strike is more than 10 to 15 miles away. Obviously, during your light show, the storms were sufficiently far away that you were seeing the lightning, but not hearing the thunder – sometimes this is referred to as "heat lightning."

Lightning also varies in the way it travels, sometimes moving only between clouds without touching down on the ground. Cloud flashes account for bolts that extend out into the air and sheet lightning, or intra-cloud lightning, stays within a cloud. Spider lightning spreads out horizontally in the sky, flashing on the underside of stratiform clouds.

As for the different colors you saw, this has to do with scattering and absorption of certain frequencies of the white light produced by the lightning.

You can learn more about lightning with this USA TODAY interactive graphic. -- Bob Swanson

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Q: In a cold front, cold air cuts under warm air. In a warm front, warm air overrides cold air. What happens in an occluded front?

A: In the case of an occluded front, the cold air from the west or northwest interacts directly with the cold air that is to the north of the warm front. If the cold air from the west is colder and more dense, it will lift the not-as-cold air, producing precipitation akin to what happens with a normal cold front (heavy, showery precipitation). This type of occlusion is called a cold-occlusion. In some instances, particularly in the Northwest during the winter, cold air off the Pacific might actually be warmer than some of the cold, polar air inland. In this case the advancing cold air from the Pacific would be less dense than the air it encounters and would be forced to override the colder, denser air. The weather associated with such a warm-occlusion is more similar to that of a warm front. -- Bob Swanson

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Q: Is an overpass a safe place to shelter during a tornado?

A: An overpass is actually one of the worst places one could choose to ride out a tornado. Unfortunately, despite efforts to educate the public regarding the danger, many people still think that this is a good idea – largely due to widely-viewed video from a TV news crew that chose to take refuge among the girders under an overpass (and survived). The truth is, they were extremely lucky and did not take a direct hit from the relatively weak tornado.

A viaduct or overpass is more like a vacuum than a safe haven during a tornado. Winds speed up as they travel through these overpasses, increasing the speed of flying debris which is the number one cause of death and injury during tornadoes.

Roger Edwards of the Storm Prediction Center in Norman, Okla., makes the following important points about the danger of sheltering under overpasses in the online tornado FAQ:

Deadly flying debris can still be blasted into the spaces between bridge and grade -- and impaled in any people hiding there.

Even when strongly gripping the girders (if they exist), people may be blown loose, out from under the bridge and into the open -- possibly well up into the tornado itself. Chances for survival are not good if that happens.

The bridge itself may fail, peeling apart and creating large flying objects, or even collapsing down onto people underneath. The structural integrity of many bridges in tornado winds is unknown -- even for those which may look sturdy.

Whether or not the tornado hits, parking on traffic lanes is illegal and dangerous to yourself and others. It creates a potentially deadly hazard for others, who may plow into your vehicle at full highway speeds in the rain, hail, and/or dust. Also, it can trap people in the storm's path against their will, or block emergency vehicles from saving lives.

In the May 3, 1999 tornado that hit Moore, Okla., there were two fatalities and numerous injuries when people attempted to shelter under overpasses on I-44 and I-35. A third fatality was counted as a traffic death because the victim remained in his truck (parked under a rural overpass) when the vehicle was blown out and thrown 30 yards.

If caught in a vehicle out in open country (and if traffic allows and the tornado is relatively distant), Roger Edwards recommends that one try to outrun the tornado by car. If traffic is jammed or the tornado is too close, the best bet is to look for nearby sturdy buildings. If there are none, get out of the vehicle and lie in a low spot and protect your head with your hands and arms. Be aware that tornadoes often are accompanied by heavy rainfall, so avoid areas that might be flood traps. -- Bob Swanson

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(A woman and her two children huddle under a bridge as a tornado  nears outside Newcastle, Okla., on Monday, May 3, 1999. Photo by J. Pat Carter, AP)

Q: During a thunderstorm, is it safer to be in a wood-frame house or a metal-frame house trailer?

A: Just like the relative safety of being inside a car, the metal frame of a house trailer acts like a shell, keeping the charge of the lightning away from the living space. A wooden house with a lightning protection system of air terminals and surge suppressors that are properly connected and grounded offers equally good protection against a lightning strike.

In either type of building, it is still wise to stay off corded phones, away from plumbing and plugged-in electronic devices during thunderstorms, as lightning can get into the living space through telephone lines, plumbing and electrical wiring.

A wooden home may be preferable during a thunderstorm in that it is typically a more substantial building. Lightning is not the only threat from thunderstorms; at times, damaging winds and isolated tornadoes can accompany severe thunderstorms. In that case, I’d rather be in the structure that can withstand the strongest winds.

Learn more about lightning safety on this USA TODAY chat with John Jensenius, lightning expert and meteorologist with the National Weather Service. Interested readers should also check out The Institute of Electrical and Electronics Engineers' guide, How to Protect Your House and Its Contents from Lightning. -- Bob Swanson

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Q: What is a "bow echo?"

A: It’s an intense line of strong thunderstorms that looks like an archer’s bow when seen on weather radar. Bow echoes are often associated with small tornadoes and damaging winds, which can roar up to 100 mph. These powerful storms can range in size from about 10 to over 100 miles in length. Here's a link to a USA TODAY resource page that includes a graphic that explains bow echoes.

The phrase was first coined by famed severe storm expert Ted Fujita in the late 1970s, who noticed how bands of rain showers or thunderstorms "bow out" when strong, damaging winds reach the surface and spread out like pancake batter.-- Doyle Rice

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Q: What are the sunniest locations in the USA for an August vacation?

A: As is to be expected, if you're looking for sun, California and the Desert Southwest is the way to go. Here are the locations that received the greatest percentage of possible sunshine during the month of August:

Redding, Calif. (97%)
Fresno, Calif. (96%)
Sacramento, Calif. (96%)
Reno, Nev. (92%)
Yuma, Ariz. (91%)
Las Vegas, Nev. (88%)
Phoenix, Ariz. (85%)
Boise, Idaho (85%)
Winnemucca, Nev. ( 85%)
Los Angeles, Calif. (83%)

Keep in mind that the Southwest can get pretty hot during this time of the year as well -- check forecasts and climate normals before you make your travel plans.

These charts from the National Climatic Data Center show how many cloudy vs. clear days there are for many U.S. cities, as well as the percentage of possible sunshine. -- Bob Swanson

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Q: When I run at my local state park, I often encounter pockets of cooler air. What causes these pockets of cooler temperatures to occur?

A: These "pockets" of cooler air are part of your park's microclimate, which means small areas can have different climates based on their individual elevations and exposure. Water, hills and concrete are the biggest influences on microclimates. Thicker tree cover or proximity to water may account for chilly patches within your local state park. You are more likely to encounter cooler microclimates within a park, opposed to running in a neighborhood, because of creeks, wind-blocking hills and the absence of sidewalks, which release absorbed heat. -- Paige Dearing

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Q: For health reasons, I want to vacation in an arid location. Which states have the lowest humidity in the summer?

A: Head for the hills, literally. Parts of the Intermountain West, located between the Sierra Nevada and Rocky Mountains, can at times feature very low, single-digit relative humidity during the summer. The region is extremely dry both because of its isolation from large bodies of water as well as its elevation.

Dew point helps to determine a locale's relative humidity; the closer a dew point is to the actual temperature, the higher the humidity is. Intellicast provides a HUMIDITYcast map that forecasts daily humidity levels nationally, at six-hour intervals.

Humidity actually helps bronchitis by adding moisture as air is breathed into dry, inflamed lungs. Allergens, dust and airborne pollutants aggravate lungs' bronchioles, which are their small airways. Chronic bronchitis is the result of longtime irritation, from smoking or allergies, whereas acute bronchitis accounts for the infection caused by bacteria or a virus. -- Paige Dearing

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Q: Has it ever snowed in the summer in Chicago?

A: Yes. Snow has indeed fallen during meteorological summer, which runs from June through August. A trace of snow was recorded in downtown Chicago on June 2, 1910. Outside of the summer months, trace amounts have fallen on Sept. 25, in both 1942 and 1948. The earliest measurable snow, 0.1 inch or more, fell on October 18, both in 1972 and 1989. The latest measurable snow recorded at Midway Airport fell on May 11, 1966. Chicago averages 38 inches of snow annually.-- Bob Swanson

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Q: What causes the wind to increase during the warm part of the day, and decrease at night?

A: Because of the heating of the Earth’s surface on sunny afternoons, air near the surface is heated rapidly and rises. As these columns of warm air (called thermals) rise into the atmosphere, winds from 3,000 to 5,000 feet can mix down to take the place of the rising air in the thermals. Since winds aloft do not feel the friction of the Earth’s surface (trees, buildings, etc.), they are typically stronger and it is the momentum of these winds aloft when they mix down to the surface that you feel as wind gusts on a sunny afternoon.

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Q: When is the best time and where is the best place to see the aurora borealis in the USA?

A: The aurora borealis, or Northern Lights, results from an interaction between solar particles and the Earth's geomagnetic field. This field is strongest at the poles, so best viewing is available in Alaska, followed by northern tier states such as Minnesota and North Dakota. March and September offer the best combination of solar activity, dark skies and comfortable temperatures.

For more on this phenomenon, check out this USA TODAY resource page.

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Q: Recently, Fairbanks, Alaska, was hotter than Flagstaff, Arizona. How is this possible?

A: Despite the difference in latitude, the two cities are both relatively warm in the summer, thanks to the high sun angle in the Northern Hemisphere. The average highs for Flagstaff during the summer are: 78.7° for June, 82.2° for July and 79.7° for August. The average highs for Fairbanks aren't much cooler, measuring at 70.9° for June, 73° for July and 66.3° for August. On the day in question (July 15), Flagstaff endured thunderstorms and 0.66 inches of rain, making it a bit cooler than Fairbanks, where skies were partly sunny.

Because of its high latitude location, seasonal sun angles largely influence Fairbanks' weather. As for Flagstaff, it too sees highly variable weather, largely influenced by its elevation. At around 7,000 feet, Flagstaff's high elevation gives residents and visitors mild summers and cool winters, as anyone would find high up on mountains or at similar high elevations.

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Q: What's the 24-hour rainfall record in the USA?

A: A record of 43 inches fell on Alvin, Texas, between July 25 and 26 in 1979, the heaviest 24-hour rainfall in U.S. history. Tropical Storm Claudette generated the rain and the accompanying 52 mph winds.

As for other wet and wild records, Hilo, Hawaii, takes the first place for the wettest and rainiest city in the USA, averaging 128 inches of rainfall annually and 277 rainy days out of the year.

And talk about a downpour: Unionville, Md., was pounded with 1.23 inch of rain in one minute alone on July 4, 1965. For more information on precipitation, check out this Weather Guys resource page.

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Q: Is there any connection between dust devils in the Southwest and tornadoes in the Plains?

A: Apart from the fact that the period of peak (May through June) dust devil activity in the Southwest tends to coincide with the period of peak tornado activity (April through June), the two are unrelated. In fact, dust devils have nothing to do with the formation of tornadoes – they each form under different processes.

Dust devils form due to surface temperature contrasts that arise from differential heating of varying surfaces. Tornadoes form as a result of changes in wind speed and direction (wind shear) within a parent thunderstorm. Even a large dust devil that can reach heights of several thousand feet and last more than an hour does indeed dissipate when it runs out of warm, unstable air or the circulation is disturbed in some other way.

Q: What is a "thunderstorm day?"

A: It's a term meteorologists use to describe any day in which thunder is heard at a weather station (rain need not occur.) Central Florida leads the USA with about 90 "thunderstorm days" per year. Orlando, Tampa and Jacksonville are all in this "thunderstorm belt." Thunderstorm development is favorable throughout Florida due to the available warmth and moisture from the nearby Gulf of Mexico and the Atlantic Ocean.

Globally, parts of Africa and South America near the equator have the most thunderstorm days each year, with as many as 200 annually. -- Doyle Rice

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Q: Are official temperature readings taken in the shade?

A: Yes. Many National Weather Service observation stations use aspirated shields to shade temperature sensors. Older stations may use a Cotton Region Shelter, also known as a "Stevenson screen" or "Stevenson box." These white instrument shelters with louvered vents have been used since the 19th century and have even inspired poetry.

Certainly, a temperature sensor exposed to direct sunlight will result in erroneous readings. Likewise, it can feel a lot hotter when the human body is exposed to direct sunlight versus relaxing in the shade. How much hotter is a subject of debate. Howard Bernstein of WUSA-TV did a nice job of explaining why rules of thumb are hard to come by when it comes to temperature in the sun vs. temperature in the shade.

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Q: How many tornadoes have there been this year?

A: Preliminary data from the Storm Prediction Center indicates there have been more than 1,700 reports of tornadoes across the USA so far in 2008. However, that number will surely be reduced as the data is analyzed and duplicate reports of tornado sightings are eliminated. So far this year, tornadoes have killed 119 people in the USA.

About 1,000 tornadoes usually hit the USA each year. However, as the SPC notes, "the actual average is unknown, because tornado spotting and reporting methods have changed so much in the last several decades that the officially recorded tornado climatologies are believed to be incomplete. Also, in the course of recording thousands of tornadoes, errors are bound to occur. Events can be missed or mis-classified; and some non-damaging tornadoes in remote areas could still be unreported."

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Q: Can the hottest temperature of the day ever occur just before midnight?

A: As you likely have experienced, the day’s hottest temperature is typically in the afternoon, often between 3:00 and 5:00 p.m. local time. Likewise, the coldest temperature of the day is often observed at or shortly after sunrise. However, to get to your question, this is not true on all occasions.

The sun is not the only factor that can affect temperatures. A process called advection, in which the wind transports a colder or warmer air mass into an area, can sometimes overwhelm the heating of the sun. That is to say, a cold wind from the north can lower the temperatures during a day that is sunny (usually in the winter when the sun’s low angle makes it less of a factor in diurnal heating). Likewise, a warm southerly breeze can make the mercury climb, even on an overcast day.

At the last TV station I worked at (WJHL in Johnson City, Tenn.), our weather staff played a forecasting game where we would compare our individual forecast accuracy (we also competed against our television market competition, our local National Weather Service office, and the computer model guidance). Since we targeted our forecast high temperatures for what our viewers would experience in the afternoon, a warm advection event in the evening could really mess with the numbers and turn a good forecast ugly in a hurry. Similarly, since we were forecasting for a morning low around 6 or 7 a.m., a cold advection event through the day could make the temperature cooler shortly before midnight than it was early that morning (the same day). -- Bob Swanson

This question was submitted by reader Kevin L. Albert

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Q: When it is drizzling/raining, what is the relative humidity?

A: The relative humidity will usually be at or near 100% when it is drizzling – meaning that the air is near saturation. This is because drizzle typically falls from low stratus clouds, meaning that the lower part of the atmosphere is pretty close to being saturated.

Relative humidity is not always 100% when it is raining. If rain is falling from a cumulonimbus cloud, the cloud base (where the air is saturated) may be thousands of feet above the surface. As raindrops fall through drier, unsaturated air, they will get smaller due to evaporation, but may still reach the surface – meaning that it is raining at your location even though the surface relative humidity is well below 100%. Of course, as this rainwater evaporates, water vapor in the surface air will increase and the relative humidity will climb. It may even reach 100%, in which case fog is likely to form.

For more information, check out this USA TODAY resource page about drizzle and rain.

This question was submitted by reader Mike Fromme.

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Q: What is graupel?

A: The formation of graupel is very similar to that of hail, which is why graupel is also known as “soft hail.” Both hailstones and graupel grow in size through a process called riming – supercooled water droplets and ice crystals freeze or stick to the initial hailstone as it rises and falls within a cloud. Hail tends to be associated with warm-season (spring, summer, even fall) thunderstorms. The more vigorous updrafts associated with warm-season storms helps to keep a hailstone in the cloud longer, allowing it to grow to a larger size (technically, hail is considered to be a dense ice pellet at least 5 millimeters in diameter). A hailstone can have concentric layers (like when you slice an onion down the middle) of hard and soft ice, depending on the temperature of the hailstone at the time the layer formed.

Graupel tends to be smaller (typically less than 5 millimeters) and is usually produced in cold-season storms. Less vigorous updrafts limit the size to which the ice can grow and the cold temperatures within the cloud result in ice and water droplets freezing directly to the embryo graupel – rather than melting and refreezing as ice. This direct-freezing process traps air and makes the graupel opaque and less dense than a hailstone would be, thus the moniker “soft hail.” On occasion, graupel can grow in thunderstorms and wind up as large as hailstones.

While sleet is also ice, it forms in an entirely different way than either hail or graupel. Sleet typically starts out as a snowflake as it leaves the base of a cloud. It then encounters a layer of above-freezing air that partially melts the falling snowflake. The partially-melted flake then refreezes as it falls through a layer of sub-freezing air and eventually hits the ground as an ice pellet. Check out this interactive graphic that describes the differences between snow, sleet and freezing rain.

This question was submitted by reader Lisa Shilhan.

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Q: Why does thunder rumble?

A: Sometimes thunder claps, sometimes it rumbles. Claps tend to be 0.2- to 2-second-long cracks that accompany close lightning. Lightning that occurs farther away tends to produce rumbles, rather than claps. This is because higher frequencies tend to be more rapidly absorbed by the surrounding environment, while the lower frequency waves sometimes travel distances of 10 to 15 miles (even 25 miles under the right conditions). These long-distance travelers can come from different parts of the lightning bolt, bounce against terrain and buildings, or be refracted by temperature variations in the atmosphere, resulting in a myriad of sound waves reaching an observer's ears at different times. When this happens from repeated lightning bolts from one storm, and other storms chime in, you can wind up with rumbling, or rolling, thunder.

This National Weather Service JetStream webpage has an excellent explanation of rumbling thunder (as well as instructive diagrams).

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Q: What is ball lightning?

A: Ball lightning is a short-lived floating sphere of energy that forms in the aftermath of a lightning strike. While human observation of the phenomenon is relatively rare, an estimated 5% of the world's population has witnessed ball lightning. Typically the size of a grapefruit, ball lightning can range from pea-size to the size of a beach ball. As for duration, ball lightning can last as short as a few seconds to as long as several minutes, with the average occurrence around 25 seconds.

USA TODAY WonderQuest columnist April Holladay has written about ball lighting, explaining where and why it happens.

In 2007, Pace VanDevender released the results of his own investigation of ball lightning, citing the discovery of what he defines as "Extreme Ball Lightning." He deemed ball lightning as extreme if it exhibited any of these characteristics: floats at about 1 meter/second, is lethal or potentially lethal, causes significant damage, contains an estimated 100,000 to 1 billion joules of energy and can excavate tons of earth, to name a few.

This question was submitted by reader Steve Johnson and was answered by Paige Dearing and Bob Swanson of the USA TODAY weather staff.

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Q: What’s the difference between hurricanes, typhoons and cyclones?

A: They are three different names for the same type of storm, collectively known as “tropical cyclones.” What they’re called depends on where they form. Hurricanes form in the Atlantic Ocean (which includes the Caribbean and Gulf of Mexico) and the eastern Pacific Ocean; typhoons form in the western Pacific Ocean, and cyclones in the Indian Ocean. This USA TODAY resource page shows the various hurricane, typhoon and cyclone basins around the world

A “cyclone” is also a generic meteorological term for any spinning area of low pressure, which includes both extratropical and tropical cyclones. It's also a term that at one time described tornadoes in the Midwest, hence the nickname of sports teams at Iowa State University.-- Doyle Rice

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Q: Does humidity make it warmer or cooler?

A: Some people claim that a “wet” cold feels colder than a “dry” cold. One possible reason for this is that we wear insulating clothing in the winter. The dead air spaces within the fill materials of a jacket or within the fibers of a sweater limit the loss of body heat, due to the low thermal conductivity of air. Thermal conductivity increases as relative humidity increases, so body heat is more quickly lost in more humid conditions, making you feel colder.

All other conditions being equal, a cold day with rain and/or fog feels colder than a dry day. If moisture is on the skin, heat will be lost through conduction of heat from the body to the water as well as the evaporative cooling effects of the water.

On the flip side, humidity can make a hot day feel even hotter – that’s why the “heat index” is given on hot summer days. Calculating the heat index involves the dew point, which is often a better measure of discomfort than relative humidity. Just like temperature tells you how hot or cold the air is, dew point can tell you how much moisture is in the air.

For example, a 45% relative humidity doesn’t sound too bad to most people. However, that is what a 100 degree temperature and a 75 degree dew point yields. These conditions yield a heat index of 114 degrees, which no one would consider comfortable. As a general rule, no matter what the temperature, a dew point of 65 degrees will feel somewhat muggy, and dew points above 75 degrees are downright oppressive.

To deal with excess heat and exertion, the body perspires, but it’s not the sweat that cools you. It is the evaporation of the sweat that takes energy from the body and makes you feel more comfortable. When the relative humidity in the air is high, sweat does not evaporate as easily, so excess energy is not carried away from the body as rapidly.

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Q: Why do cumulus clouds appear fluffy on top but flat on the bottom?

A: Cumulus clouds have flat bases and fluffy tops because they grow vertically. As warm air rises, it cools and eventually water vapor in the air condenses. The level at which this starts to happen is called the “lifting condensation level” (LCL) and is typically where you will see cumulus clouds begin to form. However, updrafts of warm air don’t just stop there. They continue to rise, with water vapor within the air continuing to condense, resulting in the fluffy, puffy, “cotton ball” nature of cumulus clouds.

The higher cumulus clouds go, the more likely weather will turn bad. Strong winds upward can stretch them into cumulonimbus clouds, which produce heavy rain and thunderstorms.

This question was submitted by reader James Hively and was answered by Paige Dearing of the USA TODAY weather staff.

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Q: What do high ozone levels mean for asthma sufferers?

A: Ozone harms all people's lungs, weakening their ability to fight disease as well as increasing the frequency and severity of asthma. According to Dr. Daniel Weber, ozone increases the lesions of lungs' alveoli, which are the small sacs where oxygen and carbon dioxide are exchanged and directly relates to asthma.

WebMD has a good video on ways asthmatics deal with ozone and why it affects them. The EPA also has an informative brochure about "Ozone and Your Health."

You can find out the daily ozone level and forecast for your local area, or anywhere nationally, using www.airnow.gov.

This question was submitted by reader Gayla Northam and was answered by Paige Dearing of the USA TODAY weather staff.

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Q: Can your ears pop due to pressure differences during a tornado?

A: The sensation of an ear popping is known as an ear barotrauma, when air pressure inside the ear differs from outside pressure. It usually occurs during altitude changes, which explains why your ears might pop during an airplane's takeoff or descent. Certainly, similar rapid changes in pressure can and do occur during a tornado.

The pressure within a tornado will vary depending on the storm's intensity as well as the victim's proximity to the center of the storm. The lowest pressure ever recorded in a tornado was 912 millibars, though it is theoretically estimated that pressures could drop to nearly 800 millibars within an EF5 tornado (the strongest classification on the Enhanced Fujita scale of tornado intensity). Considering that mean sea-level pressure is usually 1013.2 millibars, the pressure drops within a tornado can be rapid and significant and would be capable of popping one's ears.

Q: What does it mean for a river to crest?

A: The National Weather Service definition of a flood crest is, “the maximum height of a flood wave as it passes a location.” Which of course begs the question, what is a flood wave? It’s not an ocean wave that laps or pounds against the seashore. Rather, it is the rise and subsequent recession in stream or river flow due to any or all of the following – precipitation, snowmelt, dam failure or reservoir releases.

In a flood situation, hydrologists at National Weather Service offices issue forecast hydrographs. These hydrographs are computer simulations that take into account upstream riverflow, recent precipitation across a watershed and precipitation that is forecast to occur within 24 hours. Knowing how much water is flowing in the channel and how much additional water could be added allows hydrologists to forecast how high the river might rise at a particular location. We want the river to crest simply because that means that the river, failing any additional rainfall, will have reached its highest point and will subsequently recede.

If you want to check out examples of hydrographs, spend some time clicking on flood-stricken locations at the National Weather Service’s Advanced Hydrologic Prediction Service (AHPS) webpage.

This question was submitted by reader Carlos Harvey.

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Q: What countries have the most dramatic weather changes?

A: Locations far from the moderating influences of the oceans have the greatest annual variations in temperatures. These "continental climates" have warm summers and very cold winters.

In North America, central parts of the USA or Canada endure the greatest variations in average temperature. Winnipeg, Canada, has an annual temperature range of about 68 degrees: a frigid 0 degrees in the winter and a warm 70 degrees in the summer.

In Asia, parts of China, Russia, Mongolia, and Kazakhstan have the largest temperature spread. The location with the world’s largest difference between the lowest and highest recorded temperature is Verkhoyansk, Russia. That lovely spot in eastern Siberia has recorded both a low of -90 degrees and a high of 98 degrees, for a whopping 188 degree temperature range.

This question was submitted by reader Shelly R. Wood.

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Q: How do other countries measure up to the USA in tornado frequency?

A: Canada, particularly southern sections of its prairie provinces of Alberta, Saskatchewan and Manitoba, is no stranger to tornadoes. Strong tornadoes can also occur in Bangladesh, where tornado fatalities are common due to population density and poor building construction. Tornadoes also occur from western Europe into Russia, though many of these are rather weak. Some other countries that see tornadoes include South Africa, Argentina, Japan and Australia.

As for frequency, the U.S. is definitely king when it comes to tornadoes. According to the Storm Prediction Center, the U.S. has averaged 1,270 tornadoes per year over the past 10 years. Other countries don’t even come close. Christopher Burt, in his book, Extreme Weather, says that Russia likely takes second-place in the number of tornadoes each year, largely due to “its vast size and ‘potential for small tornadoes.’” Great Britain averages 30 to 35 weak tornadoes per year, both Australia and New Zealand average about 25 tornadoes per year, and Japan sees about 20 each year.

This question was submitted by reader Dan Keene.

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Q: How does vorticity advection create low pressure at the Earth's surface?

A: One of the better descriptions (and mental depictions) of vorticity and its advection comes from A World of Weather: Fundamentals of Meteorology by Jon Nese and Lee Grenci (both of Penn State University). Assuming you understand the trough and ridge pattern common at 500 mb (18,000 feet), you can think of a figure skater spinning fast (increase in absolute vorticity) around the base of a trough both due to cyclonic vorticity (the curved shape of the trough) and speed vorticity. As the skater moves east out of the base of the trough into an area of lower vorticity, he/she would have to have to spread out arms to decrease angular velocity. When this divergence occurs with winds aloft, it leads to surface convergence and the development of surface lows. As long as surface divergence (positive vorticity advection) exceeds surface convergence, the surface low will continue to deepen. When surface convergence exceeds divergence aloft, the low begins to fill.

This webpage might also help you in understanding vorticity advection.

This question was submitted by reader Jonathan Marker.

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Q: Is it possible that a hail stone can knock out someone?

A: Possible? Yes. Likely? Not very.

The storms that bring large hail big enough to cause serious bodily harm typically only batter areas in sparsely populated parts of the USA. The largest hailstone to hit U.S. soil was a seven-inch-wide chunk of ice, almost as big as a soccer ball, in Aurora, Neb., in June 2003. Previous records for huge hailstones were in Coffeyville, Kan., and Potter, Neb.

Death by hail is a seldom occurrence, but nearly $1 billion in property and crops are damaged every year by these ice rocks, according to National Geographic.

Next time a storm rolls your way, look for thick streamers in the clouds. Those visible streaks form when precipitation beings to fall and can mark the sign of an intense hailstorm.

This question was answered by Paige Dearing of the USA TODAY weather staff.

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Q: I live in Chicago and have never experienced a tornado -- is this because of Lake Michigan - or am I just lucky?

A: Chicago has experienced many tornadoes in its history, although the city has been spared recently. The state of Illinois has seen 647 tornadoes within the past decade, but only three have made an appearance in Chicago's Cook County. The most severe of the three caused about $50,000 in property damage, mostly to building rooftops in an industrial park, and the other two were recorded as being weak and blowing down trees, according to the National Climatic Data Center's storm events database.

The worst tornado in the Windy city's history hit April 21, 1967, killing 58 people and injuring over 1,000. It was part of a group of 19 tornadoes that ripped through northern Illinois, northern Missouri, southeast Iowa and southern lower Michigan.

Oak Lawn, which is located just southwest of Chicago, was one of the areas hit by the tornado. Because it is located in an urban area and by Lake Michigan, it is used as evidence to help refute the myth that claims cities and lakes protect areas from tornadoes.

Make sure to take the necessary safety precautions when tornado warnings are issued in your county, even though you've been lucky so far in avoiding twisters.

This question was submitted by reader Cecile, and was answered by Paige Dearing of the USA TODAY weather staff.

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Q: Why does it seem like violent weather hits at night?

A: People might be led to believe that severe weather occurs more at night because more deaths result from nighttime storm systems. Research by David Sutter of the University of Texas indicates that nocturnal tornadoes (midnight to 6 a.m.) result in a disproportionate number of fatalities -- there are 64% fewer fatalities from tornadoes occurring between 6 a.m. and 6 p.m. Those in the path of nocturnal tornadoes may sleep through alerts which they would otherwise recognize during the daytime, thus preventing them from taking the proper safety precautions.

Even though fatalities seem to be skewed toward the nighttime hours, tornadoes have been known to occur at any time of day. While peak times can vary according to location and time of year, many tornadoes happen between 3 p.m. and 9 p.m. For example, the National Weather Service reports that out of the 117 tornadoes that hit Oklahoma City, one of the most tornado-prone cities in the USA, from 1890 to 2007, about 60 percent of them (71) occurred between those hours. 

Pay attention to nighttime weather forecasts and plan accordingly if watches or warnings for severe thunderstorms or tornadoes are issued for your area. Check out the USA TODAY safety guides to prep yourself for any natural disasters that occur once the sun goes down.

This question was submitted by Michael E. Kiefer, Sr., and was answered by Paige Dearing of the USA TODAY weather staff.

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Q: What is the average snowfall in Atlanta?

A: Well, that's not a typical question we get in May, but let's take a stab at it. One of the best resources for snow statistics is the National Climatic Data Center's Snow Climatology site. According to this resource, Atlanta averages 1.8 inches of snow a year, based on statistics that go back 58 years. The most the city has ever received? 10.5 inches, in 1936.

This question was submitted by Debra Poane.

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Q: What is the warmest part of the day?

A: While temperature-time graphs will change depending on the time of year, location, latitude and weather conditions, the hottest time of the day is usually between 3:00 and 6:00 p.m. Most National Weather Service offices issue daily climate reports around 5:30 p.m. or 6:00 p.m. This allows them enough time to capture the high temperature at an observation station, but still allows television broadcasters to get this information onto the local news at 6:00 p.m.

This NOAA website offers some support for the late afternoon time period as being the hottest.

This question was submitted by Dave Dale.

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Q: If two days in a row are both sunny, why is one day ten degrees warmer than the other?

A: While the heating from the sun does play a role in warming the air locally, there isn’t a significant difference in the amount of solar heating on two consecutive sunny days. Quite often warmer air can blow in from the south (a process called warm advection) that can make a significant difference. Sometimes, depending on topography, wind from a direction other than south can also lead to warmer weather at a valley location (compressional heating due to downsloping).  In the case of Portland, downsloping offshore winds from the east tend to warm more than cooler onshore winds from the west.

What if there’s not much at the surface so that advection or downsloping aren’t big factors? On sunny days, as air at the surface is heated, it rises. Air from above can mix down to take the place of the air that is rising, sometimes from 5,000 feet or more. Since the air that is mixing down also undergoes compressional heating, if the air starts out warm aloft, it can lead to some toasty temperatures by the time it reaches the surface. Advection can occur at the ground, but it can also occur aloft. That is, temperatures aloft may be much greater from one day to the next, leading to a significant difference in high temperature on sunny days. On cloudy days, there is less mixing, so advection aloft has less of an effect on surface temperatures.

The recent heat wave in Portland likely resulted from a combination of these factors, but mainly it was the result of the warmer air building in aloft. A large ridge of high pressure built into the Northwest over the weekend. That ridge has moved eastward and has been replaced by a large trough, leading to the drastic swing in temperature that you have experienced.

This question was submitted by Matt Kindall of Portland, Ore.

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Q: How fast does the jet stream flow and is it a constant speed?

A: There's a reason why they are called "jet" streams.

These narrow currents of air whip by at a minimum of about 57 mph, but can reach much higher speeds. They form along the boundary between giant air masses of significantly different temperatures, usually at altitudes as high as 20,000 feet above the Earth's surface.

While always fast, jet streams don't travel at constant speeds. They accelerate or decelerate relative their proximity to centers of high and low pressure. Because of the large contrast between temperatures of polar air and warmer air, jet streams are the strongest in the winter, when they can reach speeds of up to 300 mph.

(Answered by Paige Dearing of the USA TODAY weather staff.)

This question was submitted by Terry Ulrey.

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Q: Is 'tornado alley' shifting eastward?

A: While the traditional “tornado alley” states you mention (Texas, Oklahoma, Kansas, et al) still tend to get the lion’s share of tornadoes, recent research has shown that killer tornadoes tend to frequent the Mid-South -- http://blogs.usatoday.com/weather/2008/02/usa-today-we-11.html . Reasons include greater population density, tendency for nocturnal tornadoes, number of people living in mobile homes, and the tendency for tornadoes to occur outside the peak time period for tornadoes (April through June), thus catching the public unprepared.

I think that the media coverage of killer tornadoes might give one the impression that there have been more storms in the past 5 years, but there is nothing to my knowledge to suggest that there has been a shift in any large scale weather patterns to bring more tornadic thunderstorms to your region. (Note: reader lives in northwestern Mississippi, close to Memphis.)

This question was submitted by Duane.

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Q: Isn't it unusual to have a Northern Hemisphere tropical cyclone occur in spring?

A: Each ocean basin has unique characteristics that make certain times of the year more conducive to tropical cyclone formation. For the Atlantic basin that most of us keep the closest eye on, hurricane season runs from June 1 through Nov. 30, with the climatological peak occurring from mid-August through mid-October. In the northern Indian Ocean, including the Bay of Bengal where Cyclone Nargis formed, there are two periods of peak activity – one in May and a more pronounced one in November. In between these two peak periods, tropical cyclone formation is relatively rare. Those that do occur during the peak periods tend to be the deadliest in the world – due to the low-lying flood prone delta regions (Burma, Bangladesh) as well as the high population densities. The Nov. 1970 cyclone in Bangladesh killed over 300,000 people.

The majority of cyclones occur over the Bay of Bengal compared to the Arabian Sea. May and November are associated with the transition seasons of the monsoon, as the monsoon trough is over water during these times. During the summer months, with the monsoon trough well inland, there is too much vertical shear over the northern Indian Ocean due to the Tropical Easterly Jet. This shear tends to tear apart cyclones before they have the chance to develop.

This question was submitted by Dan.

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Q: What are the differences between cyclones, typhoons and hurricanes?

A: Just as in real estate, cyclones, typhoons and hurricanes are differentiated strictly by location, location, location. The term cyclone is typically used in the Indian Ocean and around the Coral Sea off northeastern Australia. They’re called hurricanes in the Atlantic Ocean and the eastern Pacific Ocean, and typhoons in the western Pacific.

The term tropical cyclone, a low-pressure area in which the central core is warmer than the surrounding atmosphere, can be used as a catch-all term to describe all such storm systems. Tropical cyclones also include weaker systems such as tropical depressions (winds less than 39 mph) and tropical storms (winds 39 to 74 mph).

This question was submitted by Donna Moore.

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Q: What does it mean when the air pressure is high or low? ... What kind of weather can I expect?

A: The short answer is that high pressure usually brings clear, calm weather, while low pressure tends to bring cloudy, stormy weather.

A "high-pressure area" is an area where the air pressure is higher than the air around it. Air usually sinks in high-pressure areas, which prevents clouds and precipitation from forming. They're indicated by blue Hs on weather maps. 

A "low-pressure area," indicated by red Ls on weather maps, is where the air pressure is lower. Air rises in low-pressure areas, and as it rises, it cools and condenses into clouds and precipitation. The center of storms are areas of low pressure.

This question was submitted by Karen Warriner.

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Q: When is a day an official precipitation day? Does a trace count? What constitutes a trace?

A: According to the AMS (American Meteorological Society) glossary, a precipitation day is one in which at least .01 inches of precipitation is observed. When a trace is reported as the daily observed precipitation, generally less than .01 inches fell. Therefore, a day with just a trace of precipitation would not be considered a precipitation day by AMS standards.

Today's question was submitted by Gary B. Liska.

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Q: How long does it take a water molecule to return back into the atmosphere after a rain event?

A: It depends on what happens to that water molecule.If it remains at the surface or is taken up by a plant, evaporation will return it relatively quickly to the atmosphere as water vapor. However, if it runs off into a lake or river, returns to the ocean, becomes stored as groundwater or in ice, it can take considerably longer to return to the atmosphere. It’s estimated that in a 100-year period, a single water molecule spends 98 years in the ocean, 20 months as ice, about two weeks in lakes and rivers, and less than a week in the atmosphere

There's more about the phases of water on this USA TODAY resource page.