Storm
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FAQ

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FAQ:  HURRICANES, TYPHOONS, AND TROPICAL CYCLONES
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OUTLINE

Part I:
Section A : BASIC DEFINITIONS 
Section B : TROPICAL CYCLONE NAMES
Section C : TROPICAL CYCLONE MYTHS

Part II
Section D : TROPICAL CYCLONE WINDS
Section E : TROPICAL CYCLONE RECORDS

Part III
Section F : TROPICAL CYCLONE FORECASTING
Section G : TROPICAL CYCLONE CLIMATOLOGY
Section H : TROPICAL CYCLONE OBSERVATION

Part IV
Section I : Real Time Information
Section J : Historical Information 
Section K : Preparedness Information

Section D: Tropical Cyclone Winds

1. D1) How are Atlantic hurricanes ranked?
2. D2) How are Australian tropical cyclones ranked?
3. D3) Why do tropical cyclones' winds rotate counter-clockwise (clockwise) in the Northern (Southern) Hemisphere?
4. D4) How do I convert from mph to knots (to m/s) and from inches of mercury to mb (to hPa)?
5. D5) How does the damage that hurricanes cause increase as a function of wind speed?
6. D6) Why are the strongest winds in a hurricane typically on the right side of the storm?
7. D7) How much energy does a hurricane release?

Section E: Tropical Cyclone Records

1. E1) Which is the most intense tropical cyclone on record?
2. E2) Which tropical cyclone intensified the fastest?
3. E3) Which tropical cyclone has produced the highest storm surge?
4. E4) What are the largest rainfalls associated with tropical cyclones?
5. E5) Which are the largest and smallest tropical cyclones on record?
6. E6) Which tropical cyclone lasted the longest?
7. E7) Which tropical cyclones have caused the most deaths and most damage?
8. E8) What are the average, most, and least tropical cyclones occurring in each basin?
9. E9) What are the most and least tropical cyclones occurring in the Atlantic basin and striking the USA?
10. E10) For the U.S., what are the 30 most intense, 30 costliest, and 30 highest death toll hurricanes on record?
11. E11) What tropical storms and hurricanes have moved from the Atlantic to the Northeast Pacific or vice versa?

D1) How are Atlantic hurricanes ranked?

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

1:  MINIMAL:  Damage primarily to shrubbery, trees, foliage, and unanchored  homes. No real damage to other structures. Some damage to poorly constructed signs. Low-lying coastal roads inundated, minor pier damage, some small craft in exposed anchorage torn from moorings. Example:  Hurricane Earl (1998)

2:  MODERATE:  Considerable damage to shrubbery and tree foliage; some    trees blown down.  Major damage to exposed mobile homes.  Extensive damage to poorly constructed signs.  Some damage to roofing materials of buildings; some window and door damage.  No major damage to buildings.  Coast roads and low-lying escape routes inland cut by rising water 2 to 4 hours before arrival of hurricane center. Considerable damage to piers.  Marinas flooded.  Small craft in unprotected anchorages torn from moorings.  Evacuation of some shoreline residences and low-lying areas required.  Example:  Hurricane Georges (1998)

3:  EXTENSIVE:  Foliage torn from trees; large trees blown down. Practically all poorly constructed signs blown down. Some damage to roofing materials of buildings; some wind and door damage. Some structural damage to small buildings. Mobile homes destroyed. Serious flooding at coast and many smaller structures near coast destroyed; larger structures near coast damaged by battering waves and floating debris. Low-lying escape routes inland cut by rising water 3 to 5 hours before hurricane center arrives. Flat terrain 5 feet of less above sea level flooded inland 8 miles or more. Evacuation of low-lying residences within several blocks of shoreline possibly required. Example:  Hurricane Fran (1996)

4:  EXTREME:  Shrubs and trees blown down; all signs down.  Extensive damage to roofing materials, windows and doors. Complete failures of roofs on many small residences. Complete destruction of mobile homes. Flat terrain 10 feet of less above sea level flooded inland as far as 6 miles. Major damage to lower floors of structures near shore due to flooding and battering by waves and floating debris. Low-lying escape routes inland cut by rising water 3 to 5 hours before hurricane center arrives. Major erosion of beaches. Massive evacuation of all residences within 500 yards of shore possibly required, and of single-  story residences within 2 miles of shore. Example:  Hurricane Andrew (1992)

5:  CATASTROPHIC:  Shrubs and trees blown down; considerable damage to roofs of buildings; all signs down. Very severe and extensive damage to windows and doors. Complete failure of roofs on many residences and industrial buildings. Extensive shattering of glass in windows and doors. Some complete building failures. Small buildings overturned or blown away. Complete destruction of mobile homes. Major damage to lower floors of all structures less than 15 feet above sea level within 500 yards of shore. Low-lying escape routes inland cut by rising water 3 to 5 hours before hurricane center arrives. Massive evacuation of residential areas on low ground within 5 to 10 miles of shore possibly required.  Example: Hurricane Camille (1969)

    Note that tropical storms are not on this scale, but can produce extensive damage with rainfall-produced flooding. Note also that category 3, 4, and 5 hurricanes are collectively referred to as major hurricanes. These intense hurricanes cause over 83% of the damage in the USA even though they account for only 21% of tropical cyclone landfalls (Pielke and Landsea 1998).

    Note that in comparison with the Australian scale (subject D2), Australian 1 and most of Australian 2 are within the tropical storm categorization (i.e. would not be on the Saffir-Simpson scale). An Australian 3 would be approximately equal to either a Saffir-Simpson category 1 or 2 hurricane. An Australian 4 would be about the same as a Saffir-Simpson category 3 or 4 hurricane. An Australian 5 would be about the same as a Saffir-Simpson category 5 hurricane.

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D2) How are Australian tropical cyclones ranked?

     The Australian forecasters have developed a scale for tropical cyclone intensity for storms in their area of responsibility - 90 to 160E (Holland 1993).  Note that the sustained winds are based upon a 10 min averaging period instead of the USA 1 minute period.

There are further comments on this scale in subject D1).

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D3) Why do tropical cyclones' winds rotate counter-clockwise (clockwise) in the Northern (Southern) Hemisphere?

    The reason is that 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).  So when a low pressure 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.  If you don't believe this, test it out for yourself.

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D4) How do I convert from mph to knots (to m/s) and from inches of mercury to mb (to hPa)?

For winds:  (knots = nautical miles per hour)

            1 mile per hour (mph) = 0.864 knots (kt)
            1 mph = 1.609 kilometers per hour (kph)
            1 mph = 0.4470 meters per second (m/s)
            1 kt = 1.853 kph
            1 kt = 0.5148 m/s
            1 m/s = 3.600 kph

For pressures:  1 inch of mercury = 33.86 mb = 33.86 hPa

For distances:  1 ft = 0.3048 m

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D5) How does the damage that hurricanes cause increase as a function of wind speed?

    Or 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 just 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 interesting findings:

1. * Mean annual damage in mainland US is $4,900,000,000.
2. * 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 about $70 BILLION in South Florida and then an additional $10 BILLION in the Florida panhandle and Alabama.
3. * The United States has at least a 1 in 6 chance of experiencing losses related to hurricanes of at least $10 BILLION on average.
4. * 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.
5. * 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 then substantially lower again 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.

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D6) Why are the strongest winds in a hurricane typically on the right side of the storm?

    First, 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 in moving to the north, the right side would be to the east of the storm, etc.

    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 the U.S. National Hurricane Center and other 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 (see D3).

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D7) How much energy does a hurricane release?

    Hurricanes can be thought of, to a first approximation, as a heat engine; obtaining its heat input through the warm humid air over the tropical ocean, 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: * the total amount of energy released by the condensation of water droplets or ... * 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 10 to 16th cm3/day. A cubic cm of rain also weighs 1 gm. Using the latent heat of condensation, this amount of rain produced gives 5.2 x 10 to 19th Joules/day or 6.0 x 10 to 14th 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 1.5 x 10 to 12th Watts. 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. However, 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.

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E1) Which is the most intense tropical cyclone on record?

     Typhoon Tip in the Northwest Pacific Ocean on 12 October 1979 was measured to have a central pressure of 870 mb and estimated surface sustained winds of 85 m/s (165 kt) (Dunnavan & Diercks 1980).  Typhoon Nancy on 12 September, 1961 is listed in the best track data for the Northwest Pacific region as having an estimated maximum sustained winds of 185 kt with a central pressure of 888 mb.  However, it is now recognized (Black 1992) that the maximum sustained winds estimated for typhoons during the 1940s to 1960s were too strong and that the 185 kt (and numerous 160 kt to 180 kt reports) is somewhat too high.

     Note that Hurricane Gilbert's estimated 888 mb lowest pressure in mid-September 1988 is the most intense [as measured by lowest sea level pressure] for the Atlantic basin (Willoughby et al 1989), it is almost 20 mb weaker (higher) than the above Typhoon Tip of the Northwest Pacific Ocean.

     While the central pressures for the Northwest Pacific typhoons are the lowest globally, the North Atlantic hurricanes have provided sustained wind speeds possibly comparable to the Northwest Pacific.  From the best track database, both Hurricane Camille (1969) and Hurricane Allen (1980) have winds that are estimated to be 165 kt.  Measurements of such winds are inherently going to be suspect as instruments often are completely destroyed or damaged at these speeds.

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E2) Which tropical cyclone intensified the fastest?

     Typhoon Forrest in September 1983 in the Northwest Pacific Ocean deepened by 100 mb (976 to 876 mb) in just under 24 hr (Roger Edson, personal communication).  Estimated surface sustained winds increased a maximum of 30 kt in 6 hr and 85 kt in one day (from 65 to 150 kt).

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E3) Which tropical cyclone has produced the highest storm surge?

     The Bathurst Bay Hurricane produced a 13 m (about 42 ft) surge in Bathurst Bay, Australia in 1899 (Whittingham 1958).

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E4) What are the largest rainfalls associated with tropical cyclones?

12 hr:  1144 mm (45.0") at Foc-Foc, La Reunion Island in Tropical Cyclone
        Denise, 7-8 January, 1966.
24 hr:  1825 mm (71.8") at Foc-Foc, La Reunion Island in Tropical Cyclone
        Denise, 7-8 January, 1966.
48 hr:  2467 mm (97.1") at Aurere, La Reunion Island 8-10 April, 1958.
72 hr:  3240 mm (127.6") at Grand-Ilet, La Reunion Island in Tropical
        Cyclone Hyacinthe, 24-27 January, 1980.
10 d:   5678 mm (223.5") at Commerson, La Reunion Island in Tropical
        Cyclone Hyacinthe, 18-27 January, 1980.
(Holland 1993)

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E5) Which are the largest and smallest tropical cyclones on record?

     Typhoon Tip had gale force winds (39 mph or 34 kt or 15 m/s) which extended out for 1100 kmi (675 mi) in radius in the Northwest Pacific on 12 October, 1979 (Dunnavan and Diercks 1980).  Tropical Cyclone Tracy had gale force winds that only extended 50 km (30 mi) radius when it struck Darwin, Australia, on 24 December, 1974 (Bureau of Meteorology 1977).

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E6) Which tropical cyclone lasted the longest?

     Hurricane/Typhoon John lasted 31 days as it traveled both the Northeast and Northwest Pacific basins during August and September 1994. (It formed in the Northeast Pacific, reached hurricane force there, moved across the dateline and was renamed Typhoon John, and then finally re-curved back across the dateline and renamed Hurricane John again.) Hurricane Ginger was a tropical cyclone for 28 days in the North Atlantic Ocean back in 1971.

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E7) Which tropical cyclones have caused the most deaths and most damage?

     "The death toll in the infamous Bangladesh Cyclone of 1970 has had several estimates, some wildly speculative, but it seems certain that at least 300,000 people died from the associated storm tide [surge] in the low-lying deltas." (Holland 1993)

     The largest damage caused by a tropical cyclone as estimated by monetary amounts has been Hurricane Andrew (1992) as it struck the Bahamas, Florida and Louisiana, USA:  US $26.5 *Billion*.  Most of this figure was due to destruction in southeast Florida.  However, if one normalizes hurricane damage by inflation, wealth changes and coastal county population increases, then the worst 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 (updated from Pielke and Landsea 1998).

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E8) What are the average, most, and least tropical cyclones
          occurring in each basin?

    Based on data from 1968-1989 (1968/69 to 1989/90 for the Southern Hemisphere):

    Note that the data includes subtropical storms in the Atlantic basin numbers. (Neumann 1993)

    Starting in 1944, systematic aircraft reconnaissance was commenced for monitoring both tropical cyclones and disturbances that had the potential to develop into tropical cyclones.  This is why both Neumann et al. (1993) and Landsea (1993) recommend utilizing data since 1944 for computing climatological statistics.  However, for tropical cyclones striking the USA East and Gulf coasts - because of highly populated coast lines, data with good reliability extends back to around 1899.  Thus, the following records hold for the entire Atlantic basin (from 1944-1998) and for the USA coastline (1899-1998):

    For the Northeast Pacific, the records stand at maximums of 27 tropical storms/hurricanes in 1992 and 16 hurricanes in 1990.  Reliable records go back in this basin to around 1966 when geostationary satellite coverage began.

For the Northwest Pacific, the peak year stands at 1964 with 39 tropical storms, 26 of which became typhoons.  Reliable records for this basin begin around 1960.

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E9) What are the most and least tropical cyclones occurring in
 the Atlantic basin and striking the USA?

    Starting in 1944, systematic aircraft reconnaissance was commenced for monitoring both tropical cyclones and disturbances that had the potential to develop into tropical cyclones.  This is why both Neumann et al. (1993) and Landsea (1993) recommend utilizing data since 1944 for computing climatological statistics.  However, for tropical cyclones striking the USA East and Gulf coasts - because of highly populated coast lines, data with good reliability extends back to around 1899.  Thus, the following records hold for the entire Atlantic basin (from 1944-1998) and for the USA coastline (1899-1998):

Annual Storms Totals - with individual years for the numbers of named storms

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E10) For the U.S., what are the 30 most intense, 30 costliest, and 30 highest death toll hurricanes on record?

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Top 30 Most Intense USA (continental) hurricanes from 1900-1998:
(at time of landfall with landfall area). Updated from Hebert et al. (1997):
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Top 30 Damaging Hurricanes - From 1900-1998 (Normalized to 1998
dollars by inflation, wealth increases, and coastal county population
changes. Updated from Pielke and Landsea (1998).
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Top 30 Deadliest USA (continental) hurricanes from 1900-1998.
Updated from Hebert et al. (1997):
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E11) What tropical storms and hurricanes have moved from the Atlantic to the Northeast Pacific or vice versa?

(Stephen Caparotta, D. Walston, Steven Young and Gary Padgett compiled this list.)

    Here is a list of tropical cyclones that have crossed from the Atlantic basin to the Northeast Pacific and vice versa.  The tropical cyclone must have been of at least tropical storm strength in both basins (i.e. sustained winds of at least 34 kt, or 18 m/s).  This record only goes back to 1949.  Before the advent of geostationary satellite pictures in the mid-1960s, the number of Northeast Pacific tropical cyclones was undercounted by a factor of 2 or 3.  Thus the lack of many of these events during the 1960s and earlier is mainly due to simply missing the Northeast Pacific TCs.

    There has not been a recorded case where the same tropical cyclone crossed into the Northeast Pacific then crossed back into the Atlantic.

1. Atlantic Hurricane Cesar (July 1996) became Northeast Pacific Hurricane Douglas.
2. Atlantic Tropical Storm Bret (August 1993) became Hurricane Greg in the Northeast Pacific.
3. Northeast Pacific Hurricane Cosme became Atlantic Tropical Storm Allison (June 1989).
4. Atlantic Hurricane Joan (October 1988) became Northeast Pacific Hurricane Miriam.
5. Atlantic Hurricane Greta (September 1978) became Northeast Pacific Hurricane Olivia.
6. Atlantic Hurricane Fifi (September 1974) became Northeast Pacific Tropical Storm Orlene.
7. Atlantic Hurricane Irene (September 1971) became Northeast Pacific Tropical Storm Olivia.
8. Atlantic Hurricane Hattie (October-November 1961) became Northeast Pacific Tropical Storm Simone.
9. A Northeast Pacific Tropical Storm (September-October 1949) became an Atlantic Hurricane (Storm #10) and made landfall in TX.

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References

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Hurricane FAQ is Provided Courtesy of:
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Chris Landsea                                                              .
NOAA AOML/Hurricane Research Division           Voice:  (305) 361-4357
4301 Rickenbacker Causeway                      Fax:    (305) 361-4402
Miami, Florida 33149                 Internet:   landsea@aoml.noaa.gov 
http://www.aoml.noaa.gov/hrd/Landsea/landsea_bio.html 
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