This blog was written just before departing for the GLOBE Learning Expedition meeting in South Africa. I’ll be posting some additional blogs about the meeting in the coming weeks. In the meantime, after you read this blog, check out the GLOBE home page for student blogs and photos!

The weather report always tells you the wind direction and speed reported by a weather station near you. Sometimes you hear about the strong winds in the “jet stream” that exists several kilometers above the ground.

Did you ever wonder how strong the winds are in a thunderstorm? The up and down winds, I mean. You can make a rough guess on how strong the updraft in a thunderstorm is, if you have hail.

On the night of 4 June 2008, we had hail, so I decided to see how big it was. There are two ways to do this. You can go out and collect the hail, and measure it before it melts (which I have done), or you can take a picture of the hail – with a ruler or something to compare the hail to, and measure the size of the hailstones from a photograph.

Figure 1. Picture of hail on our back porch, 1830 Local Daylight Time, 4 June 2008. Typical size is one centimeter in diameter. Since the slate surface was warm some of the hail that fell earlier may have melted some. Location: north part of Boulder, Colorado, USA.

Figure 2. As in Figure 1, but hail on the grass. Typical size is 1 centimeter in diameter. The grass was cool enough so that the hail wasn’t melting as much as in the first picture.

In both pictures, the larger hailstones are typically about a centimeter in diameter, with a few that even larger. I don’t think there was much melting after the hailstones hit the ground, because I was taking the pictures as the hail was falling.

How can hail size tell you how strong the updraft is? The updraft has to be strong enough to hold the hail while it is growing. In other words, the hail continues to grow until its downward speed (which goes up with size and weight) is greater than the upward speed of the air.

Hail fall speed is determined by a balance between two forces: the downward pull of gravity and the drag force (air resistance) on the hailstone created by the air. As the hailstone falls faster, the air resistance gets bigger. Gravity of course stays the same. When the drag force is equal to the force of gravity, the hailstone reaches a constant downward speed, called its terminal velocity or terminal fall speed. The updraft has to be this strong to keep the hail from falling.

So we use the terminal fall speed to estimate the updraft speed. The hail will fall to the ground when the updraft weakens slightly, or when the hailstorm travels out of the updraft horizontally.

People have estimated the terminal fall speed of hail using equations, and they have measured it. I actually saw scientists measuring the fall speed of artificial hailstones (same shape and density as hailstones, but not ice) by dropping them down a stairwell that extended vertically about seven stories. Assuming a story is about 3.7 meters, that’s about 26 meters. Sometimes scientists measure the fall speeds of hail in nature. They can photograph them falling with a high-speed camera using strobe lights that flash on at regular intervals. Or they can measure hail vertical speed with a Doppler radar pointing straight up. It is more likely that the “natural” hailstones reached their terminal fall speeds than those in the stairwell.

Knight and Knight (2001) argue that the terminal fall speed is related to:

1. Air density (hail falls faster through thinner air)
2. Hailstone density (less dense hailstones fall more slowly)
3. Drag coefficient (the effectiveness of the air in slowing down the hailstones)

The shape of the hailstone is also important, but Knight and Knight assume the hailstones are spherical to keep the problem simple.

The graph shows how hail terminal velocity (or fall speed) is related to hail diameter.

Figure 3. Hail fall speed (and hence updraft needed) as a function of hail diameter. Red curves are from Knight and Knight (2001); Black points read off figure in http://www.jdkoontz.com/articles/hail.pdf.

For our one-centimeter hailstone, the graph shows a range of values, based on assumptions on air density at the height the hail is forming (taken by Knight and Knight as somewhere around 5.5 kilometer above sea level, where the air pressure is about 500 millibars or hectoPascals, temperature 253.16 K), drag coefficient, and the ice density in the hailstones. I picked up the hailstones, and they appeared to be solid ice rather than soft, so the ice density was probably about 0.9 grams per cubic centimeter. This suggests the updraft speed was between 13 and 18 meters per second, or between 29 miles per hour and 40 miles per hour.

According to the U.S. National Severe Storms Laboratory website, a one-centimeter hailstone falls at about nine meters per second – meaning that the updraft has to be that strong. This means the air had to be moving upward at 32 kilometers per hour or 20 miles per hour. This is more consistent with the less-dense hail.

So – to be safe, I would say the updraft overhead was between 9 meters per second and 18 meters per second. There are too many factors that we don’t really know to get much more accurate than that. This is between 32 and 65 kilometers an hour, or between 20 and 40 miles per hour.

The Encyclopedia of Climate and Weather (New York, Oxford University Press, Stephen Schneider) quotes a 47 meter per second fall speed (or necessary updraft) for a 14.4 centimeter hailstone, translates to a little over 100 miles per hour!

So – next time you have a hailstorm, measure the diameter of some hailstones to find out roughly how strong the updraft was! But if the hail is large, either photograph it from a safe place or wait until the large hail has stopped. If you don’t have a camera, collect some hail stones, put them in a plastic bag, and put them in a freezer until you have time to measure them.

Related blog: “More about Hail,” (No 19, 1 November 2006).

Reference:

Knight, Charles, and Nancy Knight, 2001: Hailstorms. In Severe Convective Storms, C. A. Doswell III, Ed., Meteorological Monographs, volume 28, No. 50. Published by the American Meteorological Society

According to the most recent report by the Intergovernmental Panel on Climate Change, land use change has a relatively minor impact on the recent rise in global average temperature.

Yet, as stressed in an earlier set of blogs on Iowa Dew Points and an apparent increase in stormy activity regionally, land use seems to be quite important at local and even regional scales.

Why the difference?

According to an article by Raddatz in a recent issue of Agricultural and Forest Meteorology, about 3.6% of Earth’s surface is covered in crops, and about 6.6% is in pasture. Figures 1 and 2 show what these percentages look like. Even if all the temperature trends associated with changes in land cover were all in the same direction, it would require large changes indeed to show up significantly in the average global temperature for a whole year.

Figure 1. Fraction of Earth’s surface covered with crops, rounded up to 4%.

Figure 2. Fraction of Earth’s surface covered with pasture, rounded up to 7%.

Further, crops grow actively only part of the year. So, for example, winter wheat or corn will lead to cooler maximum temperatures during their growing season. (Recall this is because more of the sun’s energy hitting the surface is going into evapotranspiration and less into heating). During the rest of the year, the stubble or plowed ground would have a different effect from surrounding green vegetation. In fact, the dormant fields could be warmer than grasslands if the grasses are green. Thus it is not surprising that converting natural land cover to crops or pasture does not always have the same effect on temperature change. In some areas, there is a cooling effect (e.g., if more moisture is being evaporated or transpired, or if more sunlight is reflected), while in other areas, there is a warming effect (e.g., more sunlight absorbed, less evaporation or transpiration). And finally, as pointed out in the Iowa Dew Points blogs, regional changes in land cover could have an indirect effect, like shifting the wind patterns. This can in some cases decrease the local influence on temperature. Figure 3 illustrates the “mixed” effect of crops.

Figure 3. Possible scenario showing effects of crops on the global average temperature, with the “red” representing a net warming effect over the whole year, and the “blue” representing a net cooling effect. This is only to illustrate that the net effect of crops will be mixed, rather than saying what the effect will be.

As noted in earlier blogs, cities also affect the temperatures, but they occupy a tiny fraction of Earth’s surface.

This does not mean that land use isn’t important. We live on land, which occupies only 30% of the globe. If we change our percentages of Earth’s surface to percentages of Earth’s land surface, they become bigger – 3.6% of Earth’s surface becomes 12% of Earth’s land surface, and 6.6% of Earth’s surface becomes 22% of Earth’s land surface! Furthermore, we humans aren’t evenly distributed: there are vast parts of Earth that are uninhabited. Human influence on land cover is where humans are. And, of course, those seasonal effects on temperature are important to us if they are happening where we live.

Figure 4. Fraction (12/100) of land surface on Earth covered with crops.

Figure 5. Fraction (22/100) of land surface on Earth covered with pasture.

Climate researchers know this. You have mainly heard about the predicted average annual temperature because it is the easiest “measure” that sums up all the details in one convenient number. This is particularly helpful in sending the message to governments, businesses, and individuals that Earth is getting warmer. Once you think about how this will affect how you live, one temperature is clearly not enough. You want to know how the temperature varies seasonally and where you live.

Also, climate models cover both a large area (the whole Earth) and a long time (decades), so they are expensive to run and require the biggest computers. And, they have to account for the various things that affect climate from the outside – the variability in the sun, the variation in greenhouse gases and particles, etc. By comparison, weather models are run for at most around 10 days or so. To make the runs doable, climate models work on points too far apart to really represent smaller-scale atmospheric motions (”weather”), smaller-scale regional effects (such as vegetation changes), and even terrain.

Some climate scientists have looked at regional climate changes by running weather-type (regional) models to describe what’s happening at the boundaries of a smaller area – such as the United States. (If the model only covers the United States, it still has to account for what the wind brings from the outside!) These models have taught us something. However, there is a worry that the climate models supplying the boundary information are themselves faulty because the effects of “weather” could affect the details of the local wind, temperature, etc.

So, in spite of the enormous costs, climate scientists are just now starting to run climate models on computers or clusters of computers working together that are powerful enough that global climate models can represent these regional changes better. The first such computer system was the Japanese Earth Simulator. It enabled model runs with grid points spaced at one-tenth the distance of most climate models. As more and more people start focusing on regional climate, and more computers with the capability of the Earth Simulator become available, the issues of our effect on Earth’s surface will be studied more intensely.

And our discussion didn’t even consider more long-term effects, such as the effect of land use on changes in greenhouse gas concentrations! Nor have we considered the effects of forests.

Reference: Raddatz, R.L., 2006. Evidence for the influence of agriculture on weather and climate through the transformation and management of vegetation: Illustrated by examples from the Canadian Prairies. Agricultural and Forest Meteorology, 142, 186-202.

I’m going to interrupt blogging about surprising liquid puddles and soil temperature to talk about the Second Pole-to-Pole Videoconference, which took place yesterday (8 April 2008). Several scientists participated, as did five schools: in Ushuaia, Argentina, the Escuela Provincial No. 38 Julio Argentina Roca; and in Alaska, the Randy Smith Middle School (Fairbanks), Moosewood Farm Home School (Fairbanks), Wasilla High School (Wasilla), and Innoko River School (Shageluk). The Web Conference was hosted by the GLOBE Seasons and Biomes Earth System Science team, at the University of Alaska at Fairbanks.

Figure 1. Locations of the schools in Alaska. Courtesy Dr. Elena Sparrow

Flgure 2. Location of Ushuaia, which is near the southern tip of South America. Part of Antarctica appears on the southern part of the map.

The focus was on climate change, in particular:

1. The most important seasonal indicators (things that change with season)
2. Whether they are being impacted by climate change (if so, how?)
3. How students could study these indicators to see if they are impacted by climate change.

As was the case last year, the students had an opportunity to ask questions of the students at the other schools as well as the scientists, but the conversation was more structured. We organized the conversations into three rounds. In Round 1, the Alaskan and Argentinean students were to ask each other about signs of seasonal change or share their own observations. In Round 2, the focus was on how to narrow questions down enough so that students could investigate them. And in Round 3, we were supposed to discuss the ways the investigations could be done.

The questions in Round 1 were wide-ranging. Why do leaves change color? Why is the soil frozen when the air is warm? Does the melting of permafrost cause damage to buildings and trees? Are glaciers disappearing? Do scientists use Native knowledge in their research? How does climate change affect plants and animals?

We learned that soil below ground warms and cools with the seasons more slowly than the air, and – the farther you go down, the less the temperature changes (this is also discussed in the previous few blogs). We also learned that the changes in the lower layers of the soil took place after the changes higher up (in scientific terms, the changes in the lower layers lags the changes in the upper layers). So a student was able to guess that late summer is the best time to test for permafrost, rather than the height of summer, when the sun angle is the highest.

We discovered that scientists are using Native knowledge in their research in many parts of the world, including not only Alaska and Canada, but also in Australia. We learned that magpies are coming farther north to Shageluk, and there are more pine grosbeaks than there used to be, although a student in Fairbanks didn’t notice any changes. We also learned that tree line is moving up in the mountains near Ushuaia.

In Round 2, questions focused on some fascinating things to investigate, including changes in the snowboarding season (of interest to students in both hemispheres), changes in temperature and precipitation, and succession of species after wildland fires. In fact, the students at Shageluk are already investigating the succession of species of some land recovering from a forest fire (see pictures at the Shageluk web site). The discussion of temperatures taught us the difference between maritime (Ushuaia and Wasilla) and continental (Fairbanks and Shageluk) climates: Ushuaia rarely gets below freezing, but Fairbanks has temperatures as low as -40 (same in Fahrenheit and Celsius), although such cold temperatures aren’t as common and persistent as they used to be). The discussion of snowboarding led to suggestions of investigating how long ski areas remain open, interviewing someone at a ski area about what conditions are good for snowboarding, thinking about what makes snow last (amount of precipitation, timing of precipitation, temperature). Two intriguing observations were that there were both more cumulus clouds in Ushuaia than there used to be, and more heavy rains.

With so many ideas generated in Round 2, some investigations were already outlined in some detail by the time we got to Round 3 – especially related to snowboarding. But snowboarding ideas continued to come up. A ski area had closed in Ushuaia, because its elevation was too low in the warming climate; and students in both hemispheres thought snowboarding might be an interesting thing to investigate together. Since the seasons are opposite, the study could be continuous.

Some new ideas also emerged about items to investigate. How about looking at when people take off or put on snow tires? Is that a good indicator of climate change? What about using frost tubes to monitor freezing and thawing in the soil in Ushuaia as well as Alaska? And how would frost-tube measurements relate to air temperature or the times that lakes and rivers freeze? And one could investigate the long-term seasonal geographic changes in diseases (mosquito-borne diseases, corn diseases).

It was pointed out to us that using a simple variable like temperature could yield some fascinating results beyond averages and simple trends. Is there a trend in how many days that the temperature stays above freezing? How about for the number of days when temperatures stay below freezing? How does this relate to precipitation? Clouds?

Also, we were reminded that not all changes we see are due to climate change – we humans are changing our environments in many other ways, such as destroying wilderness areas. And that trends we see in a few years can be quite different from the long-term trend. (That is, one cold winter doesn’t mean that it is getting colder on the long term.)

Through this rich mix of ideas for research topics and data to look at, the students continuously asked about each others’ lives. One of the most fascinating exchanges took place toward the end of the videoconference, when a student from Alaska asked the students in Ushuaia what kind animals they had and what kind of wildlife they ate. The Ushuaia students listed foxes, llamas, beaver, rabbit, birds, and penguins as the animals they had; and said that they ate rabbits, fish, and some beavers (but mostly tourists ate beavers). The beavers were apparently introduced to the region in 1946, and there are no natural enemies, so people are being encouraged to eat them.

A student from Shugaluk closed the discussion section of the conference by putting things in perspective. Yes, skate boarding and dog mushing are interesting, but for the Native peoples of the far North, their very way of life is being threatened. Earlier, a student in Ushuaia said that a glacier that was supplying water to the city was melting and would be gone in a few decades, leading to a shortage of drinking water. As one of the scientists said earlier, like the canary in the coal mine that warned of dangerous gases in a mine– the people in the Polar regions are the first to see the real danger in climate change. We need to remember this as we begin to take steps to try to slow down climate change and its impacts.

NOTES IN CLOSING:

There will be a web chat and web forum April 10-11. The purpose is to help students develop research ideas and projects, and interact with scientists. Links to the chat and forum can be found on the Pole-to-Pole Videoconference page of GLOBE Web site.

Three PowerPoint presentations describe the science and people of Ushuaia. They are also available on the Videoconference page at the above link.

Finally, I recall promising a student from Fairbanks that we would return to the topic of leaves changing color. Since we didn’t follow up on this question, I thought I would include a discussion here. The leaves change color because the chlorophyll, which gives the leaves their green color, disappears in the fall, so that other chemicals in the leaves give them their color. The chlorophyll, of course, is involved in photosynthesis, which provides plants the energy to grow. Different types of trees change different colors. For example, some maple trees turn bright red, while aspen trees turn yellow in the autumn. The weather actually affects how bright the colors are in the fall. In long term, the climate also affects the trees that can stay healthy in a given place. Thus the mix of trees, and hence the colors could change over many decades.

More information is available about leaf color under the Seasons and Phenology Learning Activities, Activity P5 “Investigating Leaf Pigments” in the Earth as a System Chapter of the GLOBE Teachers’ Guide.

The seasons and Biomes project is an effort to engage students in Earth system science studies as a way of learning science. It is a timely project for this fourth International Polar Year with many and intense collaborative research efforts on the physical, biological, and social components and their interactions. Changes in the Polar Regions affect the rest of the world and vice versa, since we are all connected in the earth system. I encourage students to conduct their own inquiry whether collectively as a class or in small groups, or individually. Students can use the many already-established GLOBE measurements in the areas of atmosphere/weather. soils/land cover/biology, hydrology, and plant phenology in their local areas (You can access the protocols by clicking on “For Teachers” on the menu bar at the top of the GLOBE homepage.) Soon there will be new measurement protocols such as fresh-water ice freeze-up and break-up protocols and a frost-tube protocol that will be posted on the GLOBE web site. Students can conduct a study on things that interest them as part of the upcoming GLOBE Student Research campaign.

Why are the icicles so long on our house?

On a recent walk just a day or two after our first snow, my husband and I noticed that we had the longest icicles in the neighborhood. Some houses built the same time as our house had icicles, but they were shorter. One new house had almost no icicles.

But what was the most fun, was our own house. The picture below shows our “champion” icicles.

Figure 1. Sketch of icicles on the east side of our house. The windows to the right of the icicles are about 1 meter high. The part to the right is the front part of the house; the part to the left is the back part of the house.

Notice that the icicles only cover the middle third of the side of the house. To the right and to the left, there are no icicles. Were we to walk on the roof, we would probably find the snow melted in the middle third of the roof, but not on the sides.

Why? Our house was built in stages. The front two-thirds were built were built in 1950. There was little insulation in the roof. A few months before I made this sketch, we tore out the old ceiling in the room in the front of the house and found that the insulation from 1950 was in poor condition, just like the insulation in the middle of house. The new insulation was much better. The picture confirms that the new insulation was working. No icicles implies no water from melting snow. This means that little heat was escaping through the roof, so there was little or no snowmelt on the roof.

Similarly, the back part of the house was built in 1979. When that part of the house was built, we made sure we had good thick insulation in the roof. There are no icicles on the new part of the house. Again – the insulation must be working.

Using the data from our house, can we explain why our house had the longest icicles? I’m guessing that the new house in our neighborhood that had almost no icicles had good insulation – just like the newer parts of our house and the room we just insulated. We could that the snow on the roof of the new house was fairly deep – there was little melting.

What about the older houses with shorter icicles? Let’s imagine an older house with about the same insulation as the old parts of our house (Figure 2). If this is true, the snow would melt at about the same rate (I am assuming that the roof was exposed to the same amount of sunlight per unit area). Why then would the icicles be shorter on the other (imaginary) house?

If you believe my assumptions, the answer is that the area of the roof “draining” toward the eaves (where the icicles grow) was smaller. Say the distance from the top to the icicles on our imaginary house is 5 meters, and the distance on our house is 10 meters. As the melted snow moves down from the top of the roof to the eaves, twice as much water reaches a given length along the eaves for the 10-meter roof (ours) compared to the five-meter roof. It follows that the icicles on our house would contain twice as much water and be longer than on the other house. The icicles may be not twice as long, because the icicles we had might be wider as well as longer.

Figure 2. View of a one-meter slice of our house (top) and an imaginary neighborhood house (bottom). More water is available to flow over the eaves for our house. We are looking at the two houses from the north.

So the amount of water in the icicles is determined by the amount of snow upstream of (or straight up the roof from) the eaves.

21 December 2007

It is the last official day of the 2007 surface temperature field campaign. Although we will not know for several weeks the total number of schools that have participated and the total number of observations taken, it was a great success. It has been a lot of fun watching the observations come in and to post blogs a couple times a week. Of course, if you want to keep taking measurements into the new year, please do. I know of a couple schools that are going to do that.

Here is an update on the snow situation. In North America, the snow has been melting back a bit in the center and eastern parts of the United States (Figure 1). You will also notice that there is a little more snow coverage in Eastern Europe. That is a common occurrence in the winter for cold air to move back a forth from the Eastern to the Western Hemisphere. When the cold weather was affecting the United States, Eastern Europe was warm. Now, it has flipped a little.

Figure 1. Snow extent in the Northern Hemisphere from 20 December 2007.

In the Western Hemisphere, there has been a fundamental change in the atmospheric flow. When it was cold and there were several major snow storms in the United States, the flow was out of Canada which is a source of cold air in the winter (Figure 2). This type of flow is called meridional. The upper level flow is at around 5500 meters above sea level. It steers the weather systems. This week, the pattern has changed and the upper level flow is sending weather systems from the Pacific Ocean into the west coast of the United States. Then the systems track across the United States. This type of flow is called zonal. This is bringing in warmer and moister air and is one of the reasons that the temperatures across much of the central part of the United States have gone above freezing.

This shift in weather patterns is evident in the surface temperature observations that were recorded. On 17 December 2007, Rockhill Elementary School in Alliance, Ohio recorded a surface temperature 3.0° C where there was no snow in the parking lot to –5.4° C on their grassy field where there was 78 mm of snow. On 20 December 2007, the students at Rockhill Elementary School measured 4.5° C on the parking lot and 0.4° C on the snow covered field with 80 mm of snow. As you can see, the temperature of the snow pack warmed up to right around freezing.

It is the last official day of the 2007 surface temperature field campaign. Although we will not know for several weeks the total number of schools that have participated and the total number of observations taken, it was a great success. It has been a lot of fun watching the observations come in and to post blogs a couple times a week. Of course, if you want to keep taking measurements into the new year, please do. I know of a couple schools that are going to do that.

Here is an update on the snow situation. In North America, the snow has been melting back a bit in the center and eastern parts of the United States (Figure 1). You will also notice that there is a little more snow coverage in Eastern Europe. That is a common occurrence in the winter for cold air to move back a forth from the Eastern to the Western Hemisphere. When the cold weather was affecting the United States, Eastern Europe was warm. Now, it has flipped a little.

Figure 2. Upper level flow on 15 December 2007 (top) and 21 December 2007 (bottom).

In general, all of the measurements in the United States showed a warming from 17 December to 20 December. The warmest temperature for this past week was 12.4° C and was measured 20 December 2007 at Waynesboro High School in Waynesboro, Pennsylvania on their dry parking lot while the coldest temperature was measured at Eastwood Middle School, Pemberville, Ohio of –11.0° C where there was 83 mm of snow. If the students at Moosewood Farm Home School in Fairbanks, Alaska had reported this week, they would have reported the coldest temperature by far. The air temperature hovered between –35° C and –40° C. But, they did not report any temperatures. It was probably too cold for the students to go outside.

Below is a pair of figures that was recently published about the Arctic ice cover on the NASA Earth Observatory website along with an article (Figure 3). The 1978-2002 median value is shown as the yellow line (Median is the middle value of a bunch of numbers. For example, if you are one of five children, of ages 1, 3, 7, 11, and 13, the median age of the children in your family is 7 years). You can see that the ice melted in the Arctic Ocean to record low levels in September this year and recovered quite a bit by November. But, the ice extent is still far below the 1978-2002 median level.

Figure 3. Ice extent compared to the 1979-2002 median extent for November (top) and September (bottom) 2007. Maps of ice extent from NASA Earth Observatory.

Schools involved in the surface temperature field campaign to date:

Dr. C