1. How are US hurricane strengths ranked ?

The USA utilizes the Saffir-Simpson hurricane intensity scale (Simpson and Riehl 1981) to give an estimate of the potential flooding and damage to property given a hurricane's estimated intensity.

Note: Pressure and storm surge relationships are not valid in the Central Pacific Basin.

Note : Classification by central pressure was ended in the 1990s, and wind speed alone is now used. These estimates of the central pressure that accompany each category are for reference only.

Note : These surge values are for reference only. The actual storm surge experienced will depend on offshore bathymetry and onshore terrain and construction.

Expectation of Tropical Cyclone Wind-Related Damage in Hawaii
Based on Saffir-Simpson damage scale modified for Western Pacific typhoons (Lander and Guard: http://www.typhoon2000.ph/tropical_SS.htm). These guidelines are experimental and should be used only as an approximation. Damage in some cases may result from lesser winds than indicated.

2. How are Australian tropical cyclones ranked ?

Cyclone Severity Categories - From the Australian Bureau of Meteorology

An estimate of cyclone severity is now included in all tropical cyclone advices. The table below provides information concerning effects due to wind which are typical of cyclones in the various categories. Remember that the system is not designed to give an exact statement of conditions at individual locations but will give a general idea of the expected worst conditions. Categories of cyclone severity range from " 1 " for weak cyclones to "5" for the most severe cyclones. Accordingly, the risk of property and crop damage, shore erosion and danger to life increases from low for a category 1 to very high for a category 5 cyclone. Using this severity scale, communities will be able to assess the degree of cyclone threat and take appropriate action. It must be emphasized that the category refers to the severity in the zone of maximum winds and therefore the effects felt at individual locations may not be exactly as described in the accompanying table.

Damage will vary from location to location depending upon factors such as:

how far you are from the zone of maximum winds, how exposed the location is, building standards, vegetation type, resultant flooding.

It should also be noted that the category does not refer to storm tides; if a storm tide is expected it will be mentioned separately in the cyclone warning.

3. Why do tropical cyclone winds rotate counter-clockwise (clockwise) in the Northern (Southern) Hemisphere ?

The earth's rotation sets up an apparent force (called the Coriolis force) that pulls the winds to the right in the Northern Hemisphere (and to the left in the Southern Hemisphere). When a low pressure system starts to form north of the equator, the surface winds will flow inward trying to fill in the low and will be deflected to the right and a counter-clockwise rotation will be initiated. The opposite (a deflection to the left and a clockwise rotation) will occur south of the equator.

NOTE: This force is too tiny to effect rotation in, for example, water that is going down the drains of sinks and toilets. The rotation in those will be determined by the geometry of the container and the original motion of the water. Thus one can find both clockwise and counter- clockwise flowing drains no matter what hemisphere you are located.

4. What does maximum sustained wind mean? How does it relate to gusts in tropical cyclones?

Tropical cyclone forecasts in the United States use a 1 min averaging time for reporting the sustained (i.e. relatively long-lasting) winds. The maximum sustained wind in the advisories and forecasts for tropical storms and hurricanes are the highest 1 min surface winds occurring within the circulation of the system. These surface winds are those observed (or, more often, estimated) to occur at the standard meteorological height of 10 m (33 ft) in an unobstructed exposure (i.e., not blocked by buildings or trees).

Since the inauguration of the Automatic Surface Observation System (ASOS) the National Weather Service has adopted a two minute average standard for its sustained wind definition. This is because the ASOS stations average and report their wind data over a two minute period. There is a conversion factor of about 1.04 to change a two minute average wind into a one minute average.

Gusts are a few seconds (3-5 s) wind peak. Typically, in a hurricane environment the value for a peak gust is about 20-25% higher than a 1 min sustained wind. After a tropical cyclone makes landfall, the sustained winds tend to weaken rather quickly because of increase roughness over land. However, the gusts can be an even higher percentage of these weakened sustained winds due to increased turbulence.

One complication with the use of the 1 min averaging time for the standard for sustained wind in the Atlantic and Northeast Pacific tropical cyclone basins (where the United States has the official World Meteorological Organization tropical cyclone advisory responsibilities) is that in most of the rest of the world, a 10 min averaging time is utilized for sustained wind. While one can utilize a simple ratio to convert from peak 10 min wind to peak 1 min wind (roughly 12% higher for the latter), such systematic differences to make inter-basin comparison of tropical cyclones around the world problematic.

5. How does the damage that hurricanes cause increase as a function of wind speed ?

To rephrase the question: Would a minimal 74 mph hurricane cause one half of the damage that a major hurricane with 148 mph winds? No, the amount of damage (at least experienced along the U.S. mainland) does not increase linearly with the wind speed. Instead, the damage produced increases exponentially with the winds. The 148 mph hurricane (a category 4 on the Saffir-Simpson Scale) may produce - on average - up to 250 times the damage of a minimal category 1 hurricane!

Pielke and Landsea (1998) analyzed the damage caused by various categories of U.S. landfalling tropical storms and hurricanes after normalizing by the inflation rate, increases in wealth and coastal population changes. Tropical cyclones from 1925 through 1995 were tabulated in terms of 1995 U.S. dollars.

The following table summarizes the findings:

* The Potential Damage values provide a reference value if one assigns the median damage caused by a category 1 hurricane to be "1". The rapid increase in damage as the categories go up is apparent. (The value for Category 5 hurricanes in brackets may not be representative of true amounts because of the very small sample [two] available.)

Other intersting findings:

• Mean annual damage in mainland US is $4,900,000,000.
• The worst U.S. hurricane damage - after normalizing to today's population, wealth and dollars - is no longer Hurricane Andrew, but is instead the 1926 Great Miami Hurricane. If this storm hit in the mid-1990s, it is estimated that it would cause over $70 BILLION in South Florida and then an additional $10 BILLION in the Florida panhandle and Alabama.
• The United States has at least a 1 in 6 chance of experiencing losses related to hurricanes of at least $10 BILLION on average.
• Even though the major hurricanes (the category 3, 4 and 5 storms) comprise only 21% of all US landfalling tropical cyclones, they account for 83% of all of the damage.
• Damages have NOT been on the increase once one normalizes for inflation, wealth, and coastal population changes. Instead one sees that hurricane damages that were fairly low during the first two decades of the 20th Century, quite high in the 1920s and 1940s to 1960s, and substantially lower in the 1970s and 1980s. Only during the early 1990s does damage approach the high level of impacts seen back in the 1940s through the 1960s. Thus recent hurricane damages are NOT unprecedented.

6. Why are the strongest winds in a hurricane typically on the right side of the storm ?

The right side of the storm is defined with respect to the storm's motion: if the hurricane is moving to the west, the right side would be to the north of the storm; if the hurricane is moving to the north, the right side would be to the east of the storm.

In general, the strongest winds in a hurricane are found on the right side of the storm because the motion of the hurricane also contributes to its swirling winds. A hurricane with a 90 mph [145 km/hr] winds while stationary would have winds up to 100 mph [160 km/hr] on the right side and only 80 mph [130 km/hr] on the left side if it began moving (any direction) at 10 mph [16 km/hr].

Note that forecasting center advisories already take this asymmetry into account and, in this case, would state that the highest winds were 100 mph [160 km/hr].

For tropical cyclones in the Southern Hemisphere, these differences are reversed: the strongest winds are on the left side of the storm. This is because the winds swirl clockwise south of the equator in tropical cyclones.

7. How much energy does a hurricane release ?

Hurricanes can be thought of as a heat engine; obtaining heat from the warm, humid air over the tropical ocean, and releasing this heat through the condensation of water vapor into water droplets in deep thunderstorms of the eyewall and rainbands, then giving off a cold exhaust in the upper levels of the troposphere (~12 km/8 mi up).

One can look at the energetics of a hurricane in two ways:

1. the total amount of energy released by the condensation of water droplets or
2. the amount of kinetic energy generated to maintain the strong swirling winds of the hurricane (Emanuel 1999).

It turns out that the vast majority of the heat released in the condensation process is used to cause rising motions in the thunderstorms and only a small portion drives the storm's horizontal winds.

Method 1) - Total energy released through cloud/rain formation:

An average hurricane produces 1.5 cm/day (0.6 inches/day) of rain inside a circle of radius 665 km (360 n.mi) (Gray 1981). (More rain falls in the inner portion of hurricane around the eyewall, less in the outer rainbands.) Converting this to a volume of rain gives 2.1 x 10¹⁶ cm³/day. A cubic cm of rain weighs 1 gm. Using the latent heat of condensation, this amount of rain produced gives

5.2 x 10¹⁹ Joules/day or
6.0 x 10¹⁴ Watts

This is equivalent to 200 times the world-wide electrical generating capacity - an incredible amount of energy produced!

Method 2) - Total kinetic energy (wind energy) generated:

For a mature hurricane, the amount of kinetic energy generated is equal to that being dissipated due to friction. The dissipation rate per unit area is air density times the drag coefficient times the wind speed cubed (See Emanuel 1999 for details). One could either integrate a typical wind profile over a range of radii from the hurricane's center to the outer radius encompassing the storm, or assume an average wind speed for the inner core of the hurricane. Doing the latter and using 40 m/s (90 mph) winds on a scale of radius 60 km (40 n.mi.), one gets a wind dissipation rate (wind generation rate) of

1.3 x 10¹⁷ Joules/day or
1.5 x 10¹²Watts

This is equivalent to about half the world-wide electrical generating capacity - also an amazing amount of energy being produced!

Either method is an enormous amount energy being generated by hurricanes. One can see that the amount of energy released in a hurricane (by creating clouds/rain) that actually goes to maintaining the hurricane's spiraling winds is a huge ratio of 400 to 1.

8. What are concentric eyewall cycles (or eyewall replacement cycles) and why do they cause a hurricane's maximum winds to weaken?

Concentric eyewall cycles (or eyewall replacement cycles ) naturally occur in intense tropical cyclones , i.e. major hurricanes (winds > 50 m/s, 100 kt, 115 mph) or Catories 3, 4, and 5 on the Saffir-Simpson scale. As tropical cyclones reach this threshold of intensity, they usually - but not always - have an eyewall and radius of maximum winds that contract to a very small size, around 10 to 25 km [5 to 15 mi]. At this point, some of the outer rainbands may organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and momentum. During this phase, the tropical cyclone is weakening (i.e. the maximum winds die off a bit and the central pressure goes up). Eventually the outer eyewall replaces the inner one completely and the storm can be the same intensity as it was previously or, in some cases, even stronger. A concentric eyewall cycle occurred in Hurricane Andrew (1992) before landfall near Miami: a strong intensity was reached, an outer eyewall formed, this contracted in concert with a pronounced weakening of the storm, and as the outer eyewall completely replaced the original one the hurricane reintensified. Another example is Hurricane Allen (1980) which went through repeated eyewall replacement cycles -- going from Categrory 5 to Category 3 status several times. To learn more about concentric eyewall cycles, read Willoughby et al. (1982) and Willoughby (1990a).

It was the discovery of concentric eyewall cycles that was partially responsible for the end of the U.S. Government's hurricane modification experiment Project STORMFURY, since what the scientists had hoped to produce through seeding was happening frequently as a natural part of hurricane dynamics.

9. What causes each hurricane to have a different maximum wind speed for a given minimum sea-level pressure ?

The basic horizontal balance in a tropical cyclone above the boundary layer is between the sum of the Coriolis acceleration and the centripetal acceleration, and the horizontal pressure gradient force. This balance is referred to as gradient balance, where the Coriolis acceleration is defined as the horizontal velocity of an air parcel, v, times the Coriolis parameter, f. Centripetal acceleration is defined as the acceleration on a parcel of air moving in a curved path, directed toward the center of curvature of the path, with magnitude v2/r, where v is the horizontal velocity of the parcel and r the radius of curvature of the path. The centripetal acceleration alters the original two-force geostrophic balance and creates a non-geostrophic gradient wind. The reason that different peak winds can result in different central pressures is caused by the fact that the radius, r, of the peak wind varies. A storm with 40 m/s peak winds with a 100 km RMW will have a much lower pressure drop than one with a 25 km RMW.

10. Why do hurricane force winds start at 64 knots ?

In 1805-06 Commander Francis Beaufort RN (later Admiral Sir Francis Beaufort) devised a descriptive wind scale in an effort to standardize wind reports in ship's logs. His scale divided wind speeds into 14 Forces (soon after pared down to thirteen) with each Force assigned a number, a common name, and a description of the effects such a wind would have on a sailing ship. And since the worst storm an Atlantic sailor was likely to run into was a hurricane that name was applied to the top Force on the scale.

During the 19th Century, with the manufacture of accurate anemometers, actual numerical values were assigned to each Force level, but it wasn't until 1926 (with revisions in 1939 and 1946) that the International Meteorological Committee (predecessor of the WMO) adopted a universal scale of wind speed values. It was a progressive scale with the range of speed for Forces increasing as you go higher. Thus Force 1 is only 3 knots in range, while the Force 11 is eight knots in range. So Force 12 starts out at 64 knots (74 mph, 33 m/s).

There is nothing magical in this number, and since hurricane force winds are a rare experience chances are the committee which decided on this number didn't do so because of any real observations during a hurricane. Indeed the Smeaton-Rouse wind scale in 1759 pegged hurricane force at 70 knots (80 mph, 36 m/s). Just the same, when a tropical cyclone has maximum winds of approximately these speeds we do see the mature structure (eye, eyewall, spiral rainbands) begin to form, so there is some utility with setting hurricane force in this neighborhood.

References

Hamblyn, Richard "The Invention of Clouds : How an Amateur Meteorologist Forged the Language of the Skies", (2001) Farrar, Straus, and Giroux New York, NY

DeBlieu, Jan "Wind : How the Flow of Air Has Shaped Life, Myth, and the Land" (1998) Houghton Mifflin Co. New York, NY

FAQ REFERENCES