Hurricane FAQ
This FAQ (Frequently Asked Questions) answers various questions regarding hurricanes, typhoons and tropical cyclones that have been posed
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NOAA's Atlantic Oceanographic and Meteorological Laboratory
Prior to the 20th century, hurricane names were inspired by everything from saints’ feast days, ship names, to unpopular politicians. In 1950, the National Hurricane Center officially began designating Atlantic hurricanes with code names and then women’s names. In 1979, naming responsibility was passed to a committee of the World Meteorological Organization who used alternating men and women’s names following the practice adopted by Australia’s Bureau of Meteorology three years earlier in 1975.

Currently, there are six yearly lists used in rotation found here. If a particularly damaging storm occurs, the name of that storm is retired. Storms retired in 2017 include Harvey, Irma, Maria, and Nate. If there are more storms than names on the list in a given season, an auxiliary name list is used. Lastly, if a storm happens to move across basins, it keeps the original name. The only time it is renamed is in the case that it dissipates to a tropical disturbance and then reforms.

In the Atlantic basin, tropical cyclone names are “retired” (not to be used again for a new storm) if it is deemed to be quite noteworthy because of the damage and/or deaths it caused. This is to prevent confusion with a historically well-known cyclone with a current one in the Atlantic basin. Sometimes names are removed for other reasons, such as cultural considerations or politics.

History of Hurricane Naming

For much of history, tropical cyclones were only given designations post facto. After they had come ashore and done much destruction, they would be commemorated by being named either for the Saint’s feast day they happened on (such as the San Felipe hurricanes in 1876 & 1928) or by some characteristic (the Salty hurricane 1810, the Yankee hurricane 1935).

The first use of a proper name for a tropical cyclone was by Clement Wragge, an Australian forecaster late in the 19th century. He first designated tropical cyclones by the letters of the Greek alphabet, then started using South Sea Island girls’ names. When the newly constituted Australian national government failed to create a federal weather bureau and appoint him director, Wragge began naming cyclones “after political figures whom he disliked. By properly naming a hurricane, the weatherman could publicly describe a politician (who perhaps was not too generous with weather-bureau appropriations) as ‘causing great distress’ or ‘wandering aimlessly about the Pacific.’ (Dunn and Miller 1960)

Although Wragge’s naming practice lapsed when his Queensland weather bureau closed in 1903, forty years later the idea inspired author George R. Stewart. In his 1941 novel “Storm”, a junior meteorologist named Pacific extratropical storms after former girlfriends. The novel was widely read, especially by US Army Air Forces and Navy meteorologists during World War II. When Reid Bryson, E.B. Buxton, and Bill Plumley were assigned to a USAAF base on Saipan in 1944 they had to forecast any tropical cyclones affecting operations. They decided (à la Stewart) to name them after their wives. In 1945, the armed services publicly adopted a list of women’s names for typhoons of the western Pacific using the names of officers’ wives assigned to forward forecast centers on Guam and the Philippines. However, the Air Forces were unable to persuade the U.S. Weather Bureau (USWB) to adopt a similar practice for Atlantic hurricanes.

Starting in 1947, the Air Force Hurricane Office in Miami began designating tropical cyclones of the North Atlantic Ocean using the Army/Navy phonetic alphabet (Able-Baker-Charlie-etc.) in internal communications. During the busy 1950 hurricane season there were three hurricanes occurring simultaneously in the Atlantic basin, causing considerable confusion. Grady Norton of the USWB’s Miami Hurricane Warning Center then decided to use the Air Force’s naming system in public bulletins and in his year-end summary. By the next year, these names began appearing in newspaper articles.

This practice proved popular. However, in 1952 a new International phonetic alphabet was adopted (Alpha-Beta-Charlie-etc.) which caused some confusion about which names were to be used. So in 1953, the US Weather Bureau finally acceded to the Armed Services’ practice of using women’s names. This was both controversial and popular. In 1978, under political pressure, the US National Hurricane Center (NHC) requested that the WMO’s Region IV Hurricane Committee (which had just taken control of the list) switch to a hurricane name list that alternated men’s and women’s names following the practice adopted by Australia’s Bureau of Meteorology in 1975. This was first implemented in the eastern Pacific then in 1979 in the Atlantic.

A rare hurricane near Hawaii in 1950 was called Hiki (Hawai’ian for Able). In 1957, three storms were detected in the Central Pacific, and the military forecast centers called them Kanoa, Della and Nina. In 1959, another hurricane threatened the islands and the Weather Bureau designated it “Dot”. The next year an official name list for tropical cyclones was drawn up for the Northeast Pacific basin. In 1978, both men’s and women’s names were utilized, and in 1979 a separate list was created for the Central Pacific (from 140°W to 180°W) using Hawaiian names.

The Northwest Pacific basin tropical cyclones were given women’s names officially starting in 1945 and men’s names were also included beginning in 1979. As of 1 January 2000, tropical cyclones in the Northwest Pacific basin are now being named from a new and very different list. The new names contributed by all the nations and territories that are members of the WMO’s Typhoon Committee. These newly selected names have two major differences from the rest of the world’s tropical cyclone name rosters.

The names by and large are not personal names. There are a few men’s and women’s names, but the majority are names of flowers, animals, birds, trees, or even foods, etc, while some are descriptive adjectives.

The names will not be allotted in alphabetical order, but are arranged by contributing nation with the countries being alphabetized.

The Philippine weather service PAGASA maintains their own separate list of names for any tropical system that threatens their archipelago.

For many years the Indian Ocean cyclones were given alphanumeric designators. The Southwest Indian Ocean tropical cyclones were first named during the 1960/1961 season. The North Indian Ocean region tropical cyclones were named as of 2006.

The Australian and South Pacific region (east of 90E, south of the equator) started giving women’s names to the storms for the 1964/1965 season and both men’s and women’s names for the 1974/1975 season. For the 2008/2009 season the three separate name lists of the different BoM forecast centers were consolidated into one list.

A rare South Atlantic storm in 2004 was post facto given the name Catarina. Another such system in 2010 was designated Anita after the fact. Starting in 2011, a name list was begun for the South Atlantic basin using mostly Brazilian designations.

Reference:

Dunn, G.E. and B.I. Miller (1960): Atlantic Hurricanes, Louisiana State Univ. Press, Baton Rouge, Louisiana, 377pp

Skilton, Liz, (2019): Tempest, Louisiana State Univ. Press, Baton Rouge, Louisiana, 306pp

Since 1978, the United Nations’ World Meteorological Organization, a group representing some 120 different countries, has used pre-determined lists of names for tropical storms for each ocean basin of the world. The Atlantic basin, which falls under Regional Association IV, has a six year supply of names with 21 names for each year. Why 21 names? Well, the letters Q, U, X, Y and Z are not used because names beginning with those letters are in short supply (you would need at least 3 male and 3 female names for each letter, plus a backup supply for those retired). Think about it; how many men and women do you know whose names begin with these letters?

When a damage or casualty producing storm like Mitch, Andrew, or Katrina strikes, the country most affected by the storm may recommend to the World Meteorological Organization’s Regional Association that the name be “retired.” Retiring a name is an act of respect for its victims, and reduces confusion in the insurance, legal or scientific literature. A retired name is replaced with a like-gender name beginning with the same letter. For example, Honduras recommended (1998) the name Mitch be retired and proposed the replacement name, Matthew, for consideration (and vote) by the 25-member countries of the Regional Association-IV. Eighty-three names have been retired in the Atlantic basin.

The names used on the list must meet some fundamental criteria. They should be short, and readily understood when broadcast. Further the names must be culturally sensitive and not convey some unintended and potentially inflammatory meaning. The potential for misunderstanding increases when you figure that in the Atlantic basin there are twenty-four countries, reflecting an international mix of English, Spanish and French cultures.

Typically, over the historical record, about one storm each year causes so much death and destruction that its name is considered for retirement. This means that in a “normal” year, the odds are about 1 in 8 of requiring a replacement name, given that over the last 57 years (of reliable record) we’ve averaged slightly over 8 tropical storms and hurricanes per season (actually 8.6). So, it’s more likely that letters/ names toward the front of the alphabet (letters A through H) might be retired.

The Region IV Naming Committee has a rather large file folder of nominated names that have already been submitted. The next time the need arises and it’s a storm affecting mainly the United States, the Committee will be casting about for a replacement tropical cyclone name. They will take out this file to make a selection. But as we say, it’s pure chance from there.

The Automated Tropical Cyclone Forecast (ATCF) system was developed for the Joint Typhoon Warning Center in 1988. It is used by computer software to identify tropical cyclones and assist in the generation of forecast messages. In order to distinguish different tropical cyclones that might be occurring simultaneously, a distinct alphanumeric code is assigned to each cyclone once it develops a closed circulation. This code system was adopted by other warning centers in order to facilitate the passing of storm information and reduce confusion.

The code designation consists of two letters designating the oceanic basin (“AL” for Atlantic, “EP” for Eastern Pacific, “CP” for Central Pacific and “WP” for Western Pacific), a two-digit number designating the sequential number of that particular cyclone for that basin in the year, and lastly a four-digit year number. So, the first depression to form in the Atlantic for 2001 would be AL012001, the third depression for the Central Pacific in 1999 would be CP031999.

A cyclone retains its ATCF code designation as long as it remains a distinct tropical vortex. Even if it becomes a named tropical storm or hurricane the software will still track it by its ATCF code.

AL90, AL92, 92L from the Tropical Discussions

Oftentimes, hurricane specialists become curious about disturbances in the tropics long before they form into tropical depressions and are given a tropical cyclone number. In order to alert forecasting centers that they are investigating such a disturbance and that they wish to have it tracked by the various forecast models, the specialist will attach a 9-series number to it. The first such disturbance of the year will be designated 90, the next 91, and so on until 99. After that, they restart the sequence with 90 again. The purpose of these numbers is to clarify which disturbance they are tracking as there are often more than one happening at the same time.

To further clarify matters, each number is accompanied by a two-letter code designating which tropical cyclone basin the disturbance is in. “AL” is used for the Atlantic basin (including the Caribbean Sea and Gulf of Mexico), “EP” for the Eastern Pacific, “CP” for Central Pacific, and “WP” for the Western Pacific.

In discussions, these designations will be shortened to 90L, 91L, and so forth. They may also be referred to as ‘Invest 90L’. However, once a disturbance is designated a tropical depression this 9-series number will be dropped and an ATCF code number will be assigned in its place.

You may also occasionally see an 8-series number, such as AL82. This means that this is a test investigation. There is no particular disturbance that the specialists are interested in, they’re just running a test of the system to make sure communications and software are running properly.

In order for a tropical cyclone to form, several atmospheric and marine conditions must be met.

Temperature & Humidity: Ocean waters should be 80° Fahrenheit at the surface and warm for a depth of 150 feet, because warm ocean waters fuel the heat engines of tropical cyclones. They also need an atmosphere which cools fast enough with increasing height so that the difference between the top and bottom of the atmosphere can create thunderstorm conditions. A moist mid-troposphere (3 miles high) is also needed because dry air ingested into thunderstorms at mid-level can kill the circulation.

Spin & Location: The Coriolis force is an apparent force that deflects movement to the right coming from the Northern hemisphere and to the left coming from the Southern hemisphere. The force is greatest at the poles and zero at the equator, so the storm must be at least 300 miles from the equator in order for the Coriolis force to create the spin. This force causes hurricanes in the Northern hemisphere to rotate counter-clockwise, and in the southern hemisphere to rotate clockwise. This spin may play some role in helping tropical cyclones to organize. (As a side note: the Coriolis force is not strong enough to affect small containers such as in sinks and toilets. The notion that the water flushes the other way in the opposite hemisphere is a myth.)

Wind: Low vertical wind shear (the change of wind speed and direction with height) between the surface and the upper troposphere favors the thunderstorm formation, which provides the energy for tropical cyclones. Too much wind shear will disrupt or weaken the convection.

Having these conditions met is necessary but not sufficient, as many disturbances that appear to have favorable conditions do not develop. Past work (Velasco and Fritsch 1987, Chen and Frank 1993, Emanuel 1993) has identified that large thunderstorm systems (called mesoscale convective complexes) often produce an inertially stable, warm core vortex in the trailing altostratus decks of the MCC. These mesovortices have a horizontal scale of approximately 100 to 200 km [75 to 150 mi], are strongest in the mid-troposphere (5 km [3 mi]) and have no appreciable signature at the surface. Zehr (1992) hypothesizes that genesis of the tropical cyclones occurs in two stages:

stage 1 occurs when the called mesoscale convective complex produces a mesoscale vortex. Stage 2 occurs when a second blow up of convection at the mesoscale vortex initiates the intensification process of lowering central pressure and increasing swirling winds.

References: Graham, N. E., and T. P. Barnett, 1987: Sea surface temperature, surface wind divergence, and convection over tropical oceans. Science, No.238, pp. 657-659.

Gray, W.M. (1968): “A global view of the origin of tropical disturbances and storms” Mon. Wea. Rev., 96, pp.669-700

Gray, W.M. (1979): “Hurricanes: Their formation, structure and likely role in the tropical circulation” Meteorology Over Tropical Oceans. D. B. Shaw (Ed.), Roy. Meteor. Soc., James Glaisher House, Grenville Place, Bracknell, Berkshire, RG12 1BX, pp.155-218

Chen, S.A., and W.M. Frank (1993): “A numerical study of the genesis of extratropical convective mesovortices. Part I: Evolution and dynamics” J. Atmos. Sci., 50, pp.2401-2426

Emanuel, K.A. (1993): “The physics of tropical cyclogenesis over the Eastern Pacific. Tropical Cyclone Disasters J. Lighthill, Z. Zhemin, G. J. Holland, K. Emanuel (Eds.), Peking University Press, Beijing, 136-142

Palmen, E. H., 1948: On the formation and structure of tropical cyclones. Geophysica , Univ. of Helsinki, Vol. 3, 1948, pp. 26-38.

Velasco, I., and J.M. Fritsch (1987): “Mesoscale convective complexes in the Americas” J. Geophys. Res., 92, pp.9561-9613

Zehr, R.M. (1992): “Tropical cyclogenesis in the western North Pacific. NOAA Technical Report NESDIS 61, U. S. Department of Commerce, Washington, DC 20233, 181 pp.

In addition to hurricane-favorable conditions such as temperature and humidity, many repeating atmospheric phenomenon contribute to causing and intensifying tropical cyclones. For example, African Easterly Waves (AEW) are winds in the lower troposphere (ocean surface to 3 miles above) that originate and travel from Africa at speeds of about 3-mph westward as a result of the African Easterly Jet. These winds are seen from April until November. About 85% of intense hurricanes and about 60% of smaller storms have their origin in African Easterly Waves.

The Saharan Air Layer (SAL) is another significant seeding phenomenon affecting tropical storms. It is a mass of dry, mineral-rich, dusty air that forms over the Sahara from late spring to early fall and moves over the tropical North Atlantic every 3-5 days at speeds of 22-55mph (10-25 meters per second). These air masses are 1-2 miles deep and exist in the lower troposphere. They can be as wide as the continental US and have significant moderating impacts on tropical cyclone intensity and formation because the dry, intense air can deprive the storm of moisture and wind shear can interfere with its convection. However, disturbances on the periphery of the Saharan Air Layer can receive a boost in their convection and spin.

An upper atmospheric perturbation known as the Madden-Julian Oscillation (MJO) can travel around the globe on a time-scale of weeks. As its positive phase passes over an area it can bring favorable conditions for convection, while its negative phase can suppress it. This can affect forming tropical cyclones either giving them a boost or hindering them.

The climatic fluctuation in the Pacific Ocean known as the El Niño-Southern Oscillation (ENSO) can affect Atlantic tropical cyclone development by increasing or decreasing (depending on ENSO phase) the vertical wind shear over the western side of the basin.

The Pacific Decadal Oscillation (PDO) and Atlantic Multi-decadal Oscillation (AMO) are oceanic temperature fluctuations occurring over tens of years. They can have a profound influence on the overall tropical cyclone activity over the world’s tropical oceans. For example, when the tropical North Atlantic Ocean is warmer than usual, hurricanes tend to form more often and become stronger. See more in the Tropical Cyclone Climatology Section on Atlantic Multi-decadal Variability.

Cape Verde-type hurricanes are Atlantic basin tropical cyclones that develop into tropical storms fairly close (<1000 km [600 mi] or so) to the Cape Verde Islands and then become hurricanes before reaching the Caribbean. Typically, this may occur in August and September, but in rare years (like 1995) this may occur in late July and/or early October. The numbers range from none to around five per year – with an average of 2 per year.

References: Dunn, G. E., 1940: “Cyclogenesis in the tropical Atlantic” Bull. Amer. Meteor. Soc., 21, pp.215-229

Riehl, H., 1945: “Waves in the easterlies and the polar front in the tropics” Misc. Rep. No. 17, Department of Meteorology, University of Chicago, 79 pp.

Burpee, R. W., (1972): “The origin and structure of easterly waves in the lower troposphere of North Africa” J. Atmos. Sci., 29, pp.77-90

Burpee, R. W., (1974): “Characteristics of the North African easterly waves during the summers of 1968 and 1969” J. Atmos. Sci., 31, pp.1556-1570

Landsea, C.W. (1993): “A climatology of intense (or major) Atlantic hurricanes” Mon. Wea. Rev., 121, pp.1703-1713

Avila, L. A., and R. J. Pasch, 1995: “Atlantic tropical systems of 1993” Mon. Wea. Rev., 123, pp.887-896

When a tropical disturbance organizes into a tropical depression, the thunderstorms will begin to line up in spiral bands along the inflowing wind. The winds will begin to increase, and eventually the inner bands will close off into an eyewall, surrounding a central calm area known as the eye. This usually happens around the time wind speeds reach hurricane force. When the hurricane reaches its mature stage, eyewall replacement cycles may begin. Each cycle will be accompanied by fluctuations in the strength of the storm. Peak winds may diminish when a new eyewall replaces the old, but then re-strengthen as the new eyewall becomes established.

If the storm passes through an area of high vertical wind shear or dry air the storm could be weakened. However, if it continues to pick up moisture from a warm environment, then it could become a major hurricane.

Hurricanes are driven by larger scale circulation patterns. The predominant pattern in the tropics is the Subtropical ridge, a semi-permanent high pressure cell roughly located near the Tropic of Cancer or Capricorn (23°26′ N or S). In the Atlantic this ridge is often called the Bermuda High due to its location. South of the ridge the circulation drives tropical cyclones westward with a slight poleward component. But when the cyclone reaches the westward edge of the ridge it will tend to move around the high first poleward then easterly. This is known as recurvature.

This motion means that many Atlantic hurricanes may recurve back out to sea without ever making landfall. If a hurricane reaches the mid-latitudes, it can interact with fronts. Often the energy and moisture of tropical cyclones will be absorbed into such fronts, transitioning into extratropical low pressure storms. Studies have shown that this process can increase the unpredictability of mid-latitude weather downstream for days following.

However, some hurricanes will make landfall. Striking an island, especially a mountainous one, could cause its circulation to break down. If it hits a continent, a hurricane will be cut off from its supply of warm, moist maritime air. It will also begin to draw in dry continental air, which combined with increased friction over land leads to the weakening and eventual death of the hurricane. Over mountainous terrain this will be a quick end. But over flat areas, it may take two to three days to break down the circulation. Even then you are still left with a large pocket of tropical moisture which can cause substantial inland flooding. There have been studies on the rate of storm decay once they make landfall (Demaria Kaplan Decay Model).

References: Willoughby, H.E. (1990a): “Temporal changes of the primary circulation in tropical cyclones” J. Atmos. Sci., 47, pp.242-264

Willoughby, H.E., J.A. Clos, and M.G. Shoreibah (1982): “Concentric eye walls, secondary wind maxima, and the evolution of the hurricane vortex” J. Atmos. Sci., 39, pp.395-411

Powell, M.D., and S.H. Houston, 1996: “Hurricane Andrew’s wind field at landfall in South Florida. Part II: Applications to real -time analysis and preliminary damage assessment” Wea. Forecasting, 11, pp.329-349

Tuleya, R.E. (1994): “Tropical storm development and decay: Sensitivity to surface boundary conditions” Mon. Wea. Rev., 122, pp.291-304

Tuleya, R.E. and Y. Kurihara (1978): “A numerical simulation of the landfall of tropical cyclones” J. Atmos. Sci., 35, pp.242-257

Storm surge is an abnormal rise of water generated by a storm’s winds blowing onshore.

Storm tide is the combination of the storm surge and astronomical tide as a result of a storm. Storm surge is caused by the force of high wind speeds acting on the ocean surface combined with the forward speed of the storm. The height of a storms surge is determined by the approaching angle of the storm as well as the coastline characteristics, such as the shape of the continental shelf and local geographic features, such as inlets.

The degree of vulnerability of any stretch of coast is dependent on a number of factors which includes the central pressure, intensity, forward speed, storm size, angle of approach, width and slope of the off-shore continental shelf, and local bays and inlets. The figure above illustrates the degree of storm surge threat for a “worst case scenario” Category 4 hurricane normalized along the coastline of the eastern and Gulf coasts of the United States.

The SLOSH Model

The Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model is the computer model utilized by the National Oceanic and Atmospheric Administration (NOAA) for coastal inundation risk assessment and the operational prediction of storm surge.

The eastern seaboard and Gulf Coast of the United States, Puerto Rico, the Bahamas, the Virgin Islands, and Hawaii, are subdivided into 39 regions or “basins.” These areas represent sections of the coastline that are centered upon particularly susceptible features: inlets, large coastal centers of population, low-lying topography, and ports. The SLOSH model computes the maximum potential impact of the storm in these “computational domains” based on storm intensity, track, and estimates of storm size provided by hurricane specialists at the National Hurricane Center (NHC).

Currently, SLOSH basins are being updated at an average rate of 6 basins per year. SLOSH basin updates are ultimately governed by the Interagency Coordinating Committee on Hurricanes (ICCOH). ICCOH manages hazard and post-storm analysis for the Hurricane Evacuation Studies under FEMA’s Hurricane Program. Updates are driven by a number of different factors such as: changes to a basin’s topography/bathymetry due to a hurricane event, degree of vulnerability to storm surge, availability of new data, changes to the coast, and the addition of engineered flood protection devices (e.g. levees).

Sometimes these updates include higher grid size resolution to improve surge representation, increasing areas covered by hypothetical tracks for improved accuracy, conversion to updated vertical reference datums, and including the latest topography or bathymetric data for better representation of barrier, gaps, passes, and other local features.

The SLOSH model can generate several different products:

Deterministic runs

This is an operational product based on the official NHC track and intensity forecast of a tropical cyclone. Operational SLOSH runs are generated whenever a hurricane warning is issued, approximately 36 hours prior to arrival of tropical storm winds. It is run every 6 hours coinciding with the full advisory package. This is a single run product which can result in uncertainty because it is STRONGLY dependent on the accuracy of the storm track and timing. This product is intended to provide valuable surge information in support of rescue and recovery efforts.

Probabilistic (P-surge) runs

This is a graphical product using an ensemble of many SLOSH runs to create a Probabilistic Storm Surge (P-Surge) product. This is intended to be used operationally so it is based on NHC’s official advisory. P-Surge uses SLOSH-based simulations which are based on statistics of past performance of the advisories. These different SLOSH simulations are based on the distribution of:

Cross-track error (impacts landfall location)

Along-track error (impacts forward speed and timing)

Intensity error (impacts pressure)

Size error (impacts size)

P-Surge is available whenever a hurricane watch or warning is in effect. It is posted on the NHC webpage within approximately 30 minutes after the advisory release time.

Maximum Envelope of Water (MEOW) runs

This is an ensemble product representing the maximum height of storm surge water in a given basin grid cell using hypothetical storms run with the same:

Category (intensity)

Forward speed

Storm trajectory

Initial tide level

Internally a number of parallel SLOSH runs with same intensity, forward speed, storm trajectory, and initial tide level are performed for the basin. The only difference in runs is that each is conducted at some distance to the left or to the right of the main track (typically at the center of the grid). Each component run computes a storm surge value for each grid cell. For example, five parallel runs may yield storm surge values of 4.1, 7.1, 5.3, 6.3, and 3.8 feet. In this case, the MEOW for the cell is 7.1 ft. It is unknown (to the user) which track generated the MEOW for a particular cell, so it is entirely possible that the MEOW values for adjacent cells may have come from different runs. MEOWs are used to incorporate the uncertainties associated with a given forecast and help eliminate the possibility that a critical storm track will be missed in which extreme storm surge values are generated. MEOWs provide a worst case scenario for a particular category, forward speed, storm trajectory, and initial tide level incorporating uncertainty in forecast landfall location. The results are typically generated from several thousand SLOSH runs for each basin. Over 80 MEOWs have been generated for some basins. This product provides useful information aiding in hurricane evacuation planning.

Maximum of MEOW (MOM) runs

This is an ensemble product of maximum storm surge heights for all hurricanes of a given category regardless of forward speed, storm trajectory, landfall location, etc. MOMs are created internally by pooling all the MEOWs for a given basin separated by category and tide level (zero/high), and selecting the MEOW with the greatest storm surge value for each basin grid cell regardless of the forward speed, storm trajectory, landfall location, etc. This procedure is done for each category of storm. Essentially, there is 1 MOM per storm category and tide level (zero/high). MOMs represent the worst case scenario for a given category of storm under “perfect” storm conditions. The MOMs provide useful information aiding in hurricane evacuation planning and are also used to develop the nation’s evacuation zones.

Strengths and limitations of SLOSH

The SLOSH model is computationally efficient resulting in fast computer runs. It is able to resolve flow through barriers, gaps, and passes and model deep passes between bodies of water. It also resolves inland inundation and the overtopping of barrier systems, levees, and roads. It can even resolve coastal reflections of surges such as coastally trapped Kelvin waves. However it does not model the impacts of waves on top of the surge, account for normal river flow or rain flooding, nor does it explicitly model the astronomical tide (although operational runs can be run with different water level anomalies to model conditions at the onset of operational runs).

Surprisingly, not much lightning occurs in the inner core (within about 100 km or 60 mi) of the tropical cyclone center. Only around a dozen or less cloud-to-ground strikes per hour occur around the eyewall of the storm, in strong contrast to an overland mid-latitude mesoscale convective complex which may be observed to have lightning flash rates of greater than 1000 per hour maintained for several hours.

Hurricane Andrew’s eyewall had less than 10 strikes per hour from the time it was over the Bahamas until after it made landfall along Louisiana, with several hours with no cloud-to-ground lightning at all (Molinari et al. 1994). However, lightning can be more common in the outer cores of the storms (beyond around 100 km or 60 mi) with flash rates on the order of 100s per hour.

This lack of inner core lightning is due to the relative weak nature of the eyewall thunderstorms. Because of the lack of surface heating over the ocean ocean and the “warm core” nature of the tropical cyclones, there is less buoyancy available to support the updrafts. Weaker updrafts lack the super-cooled water (e.g. water with a temperature less than 0° C or 32° F) that is crucial in charging up a thunderstorm by the interaction of ice crystals in the presence of liquid water (Black and Hallett 1986). The more common outer core lightning occurs in conjunction with the presence of convectively-active rainbands (Samsury and Orville 1994).

One of the exciting possibilities that recent lightning studies have suggested is that changes in the inner core strikes – though the number of strikes is usually quite low – may provide a useful forecast tool for intensification of tropical cyclones. Black (1975) suggested that bursts of inner core convection which are accompanied by increases in electrical activity may indicate that the tropical cyclone will soon commence a deepening in intensity. Analyses of Hurricanes Diana (1984), Florence (1988) and Andrew (1992), as well as an unnamed tropical storm in 1987 indicate that this is often true (Lyons and Keen 1994 and Molinari et al. 1994).

References: Molinari, J., P.K. Moore, V.P. Idone, R.W. Henderson, and A.B. Saljoughy (1994): “Cloud-to-ground lightning in Hurricane Andrew” J. Geophys. Res., pp.16665-16676

Black, R.A., and J. Hallett (1986): “Observations of the distribution of ice in hurricanes” J. Atmos. Sci., 43, pp.802-822

Samsury, C.E., and R.E. Orville, 1994: “Cloud-to-ground lightning in tropical cyclones: A study of Hurricanes Hugo (1989) and Jerry (1989)” Mon. Wea. Rev., 122, pp.1887-1896

Black, P.G., (1975): “Some aspects of tropical storm structure revealed by handheld-camera photographs from space” Skylab Explores the Earth, NASA, pp.417-461

Lyons, W.A., and C. S. Keen (1994): “Observations of lightning in convective supercells within tropical storms and hurricanes” Mon. Wea. Rev., 122, pp.1897-1916

The ocean’s primary direct response to a hurricane is a cooling of the sea surface temperature (SST). How does this occur? When the strong winds of a hurricane move over the ocean they churn-up much cooler water from below. The net result is that the SST of the ocean after storm passage can be lowered by several degrees Celsius (up to 10° Fahrenheit).

A warmer ocean can have intensifying effects because the warmer an ocean is, the easier it is for the liquid water to become vapor and fuel the storm’s clouds.

Figure 1 shows SSTs ranging between 25-27°C (77-81°F) several days after the passage of Hurricane Georges in 1998. As Figure 1 illustrates, Georges’ post-storm ‘cold wake’ along and to the right of the superimposed track is 3-5°C (6-9°F) cooler than the undisturbed SST to the west and south (i.e. red/orange regions are ~30°’C [86°’F]). The magnitude and distribution of the cooling pattern shown in this illustration is fairly typical for a post-storm SST analysis.

One important caveat to realize however is that most of the 3-5°C (6-9°F) ocean cooling shown in Figure 1 occurs well after the storm has moved away from the region (in this case several days after Georges made landfall). The amount of ocean cooling that occurs directly beneath the hurricane within the high wind region of the storm is a much more important question scientists would like to have answered. Why? Hurricanes get their energy from the warm ocean water beneath them. However, in order to get a more accurate estimate of just how much energy is being transferred from the sea to the storm, scientists need to know ocean temperature conditions directly beneath the hurricane. Unfortunately, with 150kph+ (100mph+) winds, 20m+ (60ft+) seas and heavy cloud cover being the norm in this region of the storm, direct (or even indirect) measurement of SST conditions within the storm’s “inner core” environment are very rare.

Thankfully in this case “very rare” does not mean “once in a lifetime”. Recently, scientists in AOML’s Hurricane Research Division (HRD) were able to get a better idea of how much SST cooling occurs directly under a hurricane by looking at many storms over a 28 year period. By combining these rare events, HRD scientists put together a “composite average” of ocean cooling directly under the storm.

Figure 2 illustrates that, on average, cooling patterns are a lot less than the post storm 3-5°C (6-9°F) cold wake estimates shown in Figure 1. In most cases, the ocean temperature under a hurricane will range somewhere between 0.2 and 1.2°C (0.4 and 2.2°F) cooler that the surrounding ocean environment. Exactly how much depends on many factors including ocean structure beneath the storm (i.e. location), storm speed, time of year and to a lesser extent, storm intensity (Cione and Uhlhorn 2003).

While the estimates in Figure 2 represent a dramatic improvement when it comes to more accurately representing actual SST cooling patterns experienced under a hurricane, even small errors in inner core SST can result in significant miscalculations when it comes to accurately assessing how much energy is transferred from the warm ocean environment directly to the hurricane. With all other factors being equal, being “off” by a mere 0.5°C (1°F) can be the difference between a storm that rapidly intensifies and one that falls apart! With that much at stake, scientists at HRD and other government and academic institutions are working to improve our ability to accurately estimate, observe and predict “under-the-storm” upper ocean conditions. These efforts include statistical studies, modeling efforts and enhanced observational capabilities designed to help scientists better assess upper ocean thermal conditions under the storm. It is believed that future forecasts of tropical cyclone intensity change will be significantly improved.

Reference:

Cione, J. J., and E. W. Uhlhorn, 2003: Sea Surface Temperature Variability in Hurricanes: Implications with Respect to Intensity Change. Monthly Weather Review, 131, 1783-1796.

The Eye is a roughly circular area of fair weather found at the center of a severe tropical storm. The eye is the region of the lowest pressure at the surface and the warmest temperatures at the top. Eye size ranges from 5-120 miles across, but most are 20-40 miles in diameter. Understanding exactly how the eye forms has been controversial. Some scientists believe the radial spreading of the wind creates a warm dry down flow from the upper atmosphere, and this forms the cloud-free eye. Others have think the latent heat release in the eyewall forces the subsidence in the storm center creating the eye.

The Eyewall is a ring of deep convection bordering the eye of the storm. This area has the highest surface winds in the tropical cyclone. Because air in the eye is slowly sinking, it creates an updraft in the eyewall. In particularly strong storms, concentric eyewall circles (or an “eyewall replacement cycle”) can occur. Eyewall replacement happens when a storm reaches its intensity threshold and the eye contracts to a smaller size (5-15 miles). Strong rain bands in the outer storm move inward towards the eye, robbing the inner eyewall of its moisture and momentum and weakening the storm.

Spiral Bands are long, narrow bands of rain and thunderstorms that are oriented in the same direction as the wind movement. They are caused by convection (the vertical movement of air masses) and they spiral into the center of the tropical cyclone. In contrast, the Moat of a storm usually refers to the region between the eyewall and an outer spiral band where rainfall is relatively lighter. Not all hurricanes have moats.

References: Hawkins, H.F., and D.T. Rubsam (1968): “Hurricane Hilda, 1964 : II Structure and budgets of the hurricane on October 1, 1964” Mon. Wea. Rev., 104, pp.418-442

Weatherford, C. and W.M. Gray (1988): “Typhoon structure as revealed by aircraft reconnaissance. Part II: Structural variability” Mon. Wea. Rev., 116, pp.1044-1056

Smith, R.K. (1980): “Tropical Cyclone Eye Dynamics.” J. Atmos. Sci., 37 (6), pp.1227-1232.

Willoughby, H.E. (1979): “Forced secondary circulations in hurricanes” J. Geophys. Res., 84, pp.3173-3183

Shapiro, L.J. and H.E. Willoughby (1982): “The Response of Balanced Hurricanes to Local Sources of Heat and Momentum” J. Atmos. Sci., 39 (2), pp.378-394

Willoughby, H.E. (1990a): “Temporal changes of the primary circulation in tropical cyclones” J. Atmos. Sci., 47, pp.242-264

Willoughby, H.E. (1995): “Mature structure and evolution. Global Perspectives on Tropical Cyclones, R.L. Elsberry (ed.). World Meteorological Organization, Report No. TCP-38; Geneva, Switzerland, 62 pp.

The energy released from a hurricane can be explained in two ways: the total amount of energy released by the condensation of water droplets (latent heat), or the amount of kinetic energy generated to maintain the strong, swirling winds of a hurricane. The vast majority of the latent heat released is used to drive the convection of a storm, but the total energy released from condensation is 200 times the world-wide electrical generating capacity, or 6.0 x 1014 watts per day.

If you measure the total kinetic energy instead, it comes out to about 1.5 x 1012 watts per day, or ½ of the world-wide electrical generating capacity. It would seem that although wind energy seems to be the most obvious energetic process, it is actually the latent release of heat that feeds a hurricane’s momentum.

To Calculate:

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.

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.), gets a wind dissipation rate (wind generation rate) of 1.3 x 1017 Joules/day (1.5 x 1012Watts).

Reference: Emanuel, K. A., (1999): “The power of a hurricane: An example of reckless driving on the information superhighway” Weather, 54, 107-108

The Atlantic hurricane season is June 1st to November 30th. In the East Pacific, it runs from May 15th to November 30th. Hurricane Awareness week runs from May 25th through May 31st and is a great time to get your hurricane kit and plans up to date. NOAA’s seasonal outlook is published here: NOAA Seasonal Outlook

Hurricanes have occurred outside of the official six month season , but these dates were selected to encompass the majority of Atlantic tropical cyclone activity (over 97%). When the Weather Bureau organized its new hurricane warning network in 1935 it scheduled a special telegraph line to connect the various centers to run from June 15th through November 15th. Those remained the start and end dates of the ‘official’ season until 1964, when it was decided to end the season on November 30th, and in 1965, when the start was moved to the beginning of June. These changes made the Atlantic hurricane season six months long and easier for people to remember.

Atlantic Ocean tropical cyclone activity over a year

The Atlantic basin shows a very peaked season from August through October, with 78% of the tropical storm days, 87% of the minor hurricane days, and 96% of the major hurricane days occurring then (Landsea (NHC) 1993). Maximum activity occurs in early to mid September. “Out of season” tropical cyclones primarily occur in May or December.

East Pacific Ocean tropical cyclone activity over a year

The Northeast Pacific basin has a broader peak with activity beginning in late May or early June and going until late October or early November with a peak in storminess in late August/early September. The National Hurricane Center’s official dates for this basin are from May 15th to November 30th.

West Pacific Ocean tropical cyclone activity over a year

The Northwest Pacific basin has tropical cyclones occurring all year round regularly. There is no official definition of typhoon season for this reason. There is a distinct minimum in February and the first half of March, and the main season goes from July to November with a peak in late August/early September.

The North Indian basin has a double peak of activity in May and November though tropical cyclones are seen from April to December. The severe cyclonic storms (>33 m/s winds [76 mph]) occur almost exclusively from April to June and late September to early December.

The Southwest Indian and Australian/Southeast Indian basins have very similar annual cycles with tropical cyclones beginning in late October/early November, reaching a peak in activity from mid-January to early May. The Australian/Southeast Indian basin February lull in activity is a bit more pronounced than the Southwest Indian basin’s lull.

The Australian/Southwest Pacific basin begin with tropical cyclone activity in late October/early November, reaches a single peak in March, and then fades out in early May.

Globally, September is the most active month and May is the least active month. (Neumann 1993)

References: Neumann, C.J., B.R. Jarvinen, C.J. McAdie, and J.D. Elms (1993): Tropical Cyclones of the North Atlantic Ocean, 1871-1992, Prepared by the National Climatic Data Center, Asheville, NC, in cooperation with the NHC, Coral Gables, FL, 193pp.

The mean annual damage from hurricanes in the US is 9.5 billion dollars, when we adjust not only for inflation but for the increase in value of real goods in average households. Hurricane damage varies greatly from year to year, depending on the number and strength of hurricanes making landfall, but there does not seem to be a long-term trend in adjusted damage over the last century.

There is very little association between the physical size of a hurricane and its intensity. A big hurricane does not have to be an intense one and vice versa. The damage a hurricane can cause is a function of both its maximum sustained wind and the extent of the hurricane force winds. A broad, weak storm may cause as much damage as a small, strong one.

It is false to think that damage is linear with wind speed, that a 150-mph winds will cause twice the damage as a 75-mph winds. The relationship is exponential, and not linear. A category 5 storm could cause up to 250 times the damage of a category 1 hurricane of the same size.

Intensity Cases Median Damage Potential Damage * Tropical/Subtropical Storm 118 < $1,000,000 0 Hurricane Category 1 45 $33,000,000 1 Hurricane Category 2 29 $336,000,000 10 Hurricane Category 3 40 $1,412,000,000 50 Hurricane Category 4 10 $8,224,000,000 250 Hurricane Category 5 2 $5,973,000,000 500

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, are 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.

References: Weatherford, C. and W.M. Gray (1988): “Typhoon structure as revealed by aircraft reconnaissance. Part II: Structural variability” Mon. Wea. Rev., 116, pp.1044-1056

Pielke, Jr. R. A., and C. W. Landsea, 1998: “Normalized Atlantic hurricane damage 1925-1995” Wea. Forecasting, 13, pp.621-631

Hurricane forecasters estimate tropical cyclone strength from satellite using a method called the Dvorak technique. Vern Dvorak developed the scheme in the early 1970s using a pattern recognition decision tree (Dvorak 1975, 1984). Utilizing the current satellite picture of a tropical cyclone, one matches the image versus a number of possible pattern types: Curved band Pattern, Shear Pattern, Eye Pattern, Central Dense Overcast (CDO) Pattern, Embedded Center Pattern or Central Cold Cover Pattern. If infrared satellite imagery is available for Eye Patterns (generally the pattern seen for hurricanes, severe tropical cyclones and typhoons), then the scheme utilizes the difference between the temperature of the warm eye and the surrounding cold cloud tops. The larger the difference, the more intense the tropical cyclone is estimated to be.

From this one gets a “T-number” and a “Current Intensity (CI) Number”. CI numbers have been calibrated against aircraft measurements of tropical cyclones in the Northwest Pacific and Atlantic basins. On average, the CI numbers correspond to the following intensities:

Current Intensity Numbers CI Number Maximum Sustained

One Minute Winds

(kts) Central Pressure

(mb) Atlantic NW Pacific 0.0 <25 —- —- 0.5 25 —- —- 1.0 25 —- —- 1.5 25 —- —- 2.0 30 1009 1000 2.5 35 1005 997 3.0 45 1000 991 3.5 55 994 984 4.0 65 987 976 4.5 77 979 966 5.0 90 970 954 5.5 102 960 941 6.0 115 948 927 6.5 127 935 914 7.0 140 921 898 7.5 155 906 879 8.0 170 890 858

Note that this estimation of both maximum winds and central pressure assumes that the winds and pressures are always consistent. However, since the winds are really determined by the pressure gradient, small tropical cyclones (like the Atlantic’s Andrew in 1992, for example) can have stronger winds for a given central pressure than a larger tropical cyclone with the same central pressure. Thus caution is urged in not blindly forcing tropical cyclones to “fit” the above pressure- wind relationships. (The reason that lower pressures are given to the Northwest Pacific tropical cyclones in comparison to the higher pressures of the Atlantic basin tropical cyclones is because of the difference in the background climatology. The Northwest Pacific basin has a lower background sea level pressure field. Thus to sustain a given pressure gradient and thus the winds, the central pressure must accordingly be smaller in this basin.)

The errors for using the above Dvorak technique in comparison to aircraft measurements taken in the Northwest Pacific average 10 mb with a standard deviation of 9 mb (Martin and Gray 1993). Atlantic tropical cyclone estimates likely have similar errors. Thus an Atlantic hurricane that is given a CI number of 4.5 (winds of 77 kt and pressure of 979 mb) could in reality be anywhere from winds of 60 to 90 kt and pressures of 989 to 969 mb. These would be typical ranges to be expected; errors could be worse. However, in the absence of other observations, the Dvorak technique does at least provide a consistent estimate of what the true intensity is.

While the Dvorak technique was calibrated for the Atlantic and Northwest Pacific basin because of the aircraft reconnaissance data ground truth, the technique has also been quite useful in other basins that have limited observational platforms. However, at some point it would be preferable to re-derive the Dvorak technique to calibrate tropical cyclones with available data in the other basins.

Lastly, while the Dvorak technique is primarily designed to provide estimates of the current intensity of the storm, a 24 h forecast of the intensity can be obtained also by extrapolating the trend of the CI number. Whether this methodology provides skillful forecasts is unknown.

References: Dvorak, V.F., 1975: “Tropical cyclone intensity analysis and forecasting from satellite imagery” Mon. Wea. Rev., 103, pp.420-430

Dvorak, V.F., 1984: “Tropical cyclone intensity analysis using satellite data” NOAA Tech. Rep. NESDIS 11, 47pp

Fitzpatrick, P.J., J.A. Knaff, C.W. Landsea, and S.V. Finley (1995): “A systematic bias in the Aviation model’s forecast of the Atlantic tropical upper tropospheric trough: Implications for tropical cyclone forecasting” Wea. Forecasting, 10, pp.433-446

Martin, J.D., and W.M. Gray (1993): “Tropical cyclone observation and forecasting with and without aircraft reconnaissance” Wea. Forecasting, 8, pp.519-532

The U.S. Government once supported research into methods of hurricane modification, known as Project STORMFURY.

It was an ambitious experimental program of research on hurricane modification carried out between 1962 and 1983. The proposed modification technique involved artificial stimulation of convection outside the eyewall through seeding with silver iodide. The invigorated convection, it was argued, would compete with the original eyewall, lead to the reformation of the eyewall at larger radius, and thus, through partial conservation of angular momentum, produce a decrease in the strongest winds.

Since a hurricane’s destructive potential increases rapidly as its strongest winds become stronger, a reduction as small as 10% would have been worthwhile. Modification was attempted in four hurricanes on eight different days. On four of these days, the winds decreased by between 10 and 30%, The lack of response on the other days was interpreted to be the result of faulty execution of the seeding or of poorly selected subjects.

These promising results came into question in the mid-1980s because observations in unmodified hurricanes indicated:

That cloud seeding had little prospect of success because hurricanes contained too much natural ice and too little supercooled water.

That the positive results inferred from the seeding experiments in the 1960s stemmed from inability to discriminate between the expected results of human intervention and the natural behavior of hurricanes.

For a couple decades NOAA and its predecessor tried to weaken hurricanes by dropping silver iodide – a substance that serves as an effective ice nuclei – into the rainbands of the storms. During the STORMFURY years, scientists seeded clouds in Hurricanes Esther (1961), Beulah (1963), Debbie (1969), and Ginger (1971). The experiments took place over the open Atlantic far from land. The STORMFURY seeding targeted convective clouds just outside the hurricane’s eyewall in an attempt to form a new ring of clouds that, hopefully, would compete with the natural circulation of the storm and weaken it. The idea was that the silver iodide would enhance the thunderstorms of a rainband by causing the supercooled water to freeze, thus liberating the latent heat of fusion and helping a rainband to grow at the expense of the eyewall. With a weakened convergence to the eyewall, the strong inner core winds would also weaken quite a bit. For cloud seeding to be successful, the clouds must contain sufficient supercooled water (water that has remained liquid at temperatures below the freezing point, 0°C/32°F). Neat idea, but in the end it had a fatal flaw. Observations made in the 1980s showed that most hurricanes don’t have enough supercooled water for STORMFURY seeding to work – the buoyancy in hurricane convection is fairly small and the updrafts correspondingly small compared to the type one would observe in mid-latitude continental super or multicells.

In addition, it was found that unseeded hurricanes form natural outer eyewalls just as the STORMFURY scientists expected seeded ones to do. This phenomenon makes it almost impossible to separate the effect (if any) of seeding from natural changes. The few times that they did seed and saw a reduction in intensity was undoubtedly due to what is now called “concentric eyewall cycles.” Thus nature accomplishes what NOAA had hoped to do artificially. No wonder the first few experiments were thought to be successes. Because the results of seeding experiments were so inconclusive, STORMFURY was discontinued. A special committee of the National Academy of Sciences concluded that a more complete understanding of the physical processes taking place in hurricanes was needed before any additional modification experiments. The primary focus of NOAA’s Hurricane Research Division today is better physical understanding of hurricanes and improvement of forecasts. To learn about the STORMFURY project as it was called, read Willoughby et al. (1985).

Reference: Willoughby, H.E., D.P. Jorgensen, R.A. Black, and S.L. Rosenthal (1985): “Project STORMFURY: A scientific chronicle 1962-1983” Bull. Amer. Meteor. Soc., 66, cover and pp.505-514

There have been numerous techniques that have been considered over the years to modify hurricanes: seeding clouds with dry ice or silver iodide, reducing evaporation from the ocean surface with thin-layers of polymers, cooling the ocean with cryogenic material or icebergs, changing the radiational balance in the hurricane environment by absorption of sunlight with carbon black, flying jets clockwise in the eyewall to reverse the flow, exploding the hurricane apart with hydrogen bombs, and blowing the storm away from land with giant fans, etc. As carefully reasoned as some of these suggestions are, they all share the same shortcoming: They fail to appreciate the size and power of tropical cyclones. For example, when Hurricane Andrew struck South Florida in 1992, the eye and eyewall devastated a swath 20 miles wide. The heat energy released around the eye was 5,000 times the combined heat and electrical power generation of the Turkey Point nuclear power plant over which the eye passed. The kinetic energy of the wind at any instant was equivalent to that released by a nuclear warhead.

Human beings are used to dealing with chemically complex biological systems or artificial mechanical systems that embody a small amount (by geophysical standards) of high-grade energy. Because hurricanes are chemically simple –air and water vapor – introduction of catalysts is unpromising. The energy involved in atmospheric dynamics is primarily low-grade heat energy, but the amount of it is immense in terms of human experience.

Attacking weak tropical waves or depressions before they have a chance to grow into hurricanes isn’t promising either. About 80 of these disturbances form every year in the Atlantic basin, but only about 5 become hurricanes in a typical year. There is no way to tell in advance which ones will develop. If the energy released in a tropical disturbance were only 10% of that released in a hurricane, it is still a lot of power. The hurricane police would need to dim the whole world’s lights many times a year.

Maybe the time will come when men and women can travel at nearly the speed of light to the stars, and we will then have enough energy for brute-force intervention in hurricane dynamics.

Until then, perhaps the best solution is not to try to alter or destroy the tropical cyclones, but just learn to co-exist with them. Since we know that coastal regions are vulnerable to the storms, building codes that can have houses stand up to the force of the tropical cyclones need to be enforced. The people that choose to live in these locations should be willing to shoulder a fair portion of the costs in terms of property insurance – not exorbitant rates, but ones which truly reflect the risk of living in a vulnerable region. In addition, efforts to educate the public on effective preparedness needs to continue. Helping other nations in their mitigation efforts can also result in saving countless lives. Finally, we need to continue in our efforts to better understand and observe hurricanes in order to more accurately predict their development, intensification, and track.

References: Simpson, R.H. and J. Simpson (1966): “Why experiment on tropical hurricanes ?” Trans. New York Acad. Sci., 28, pp.1045-1062

Gray, W.M., W.M. Frank, M.L. Corrin, C.A. Stokes (1976): “Weather modification by carbon dust absorption of solar energy” J. Appl. Meteor., 15, pp.355-386

Gray, W.M., W.M. Frank, M.L. Corrin, C.A. Stokes, 1976: Weather Modification by Carbon Dust Absorption of Solar Energy, J. of Appl. Meteor., 15 4, pp. 355-386.

Woodcock, A.H., D.C. Blanchard, C.G.H. Rooth, 1963: Salt-Induced Convection and Clouds, J. of Atmos. Sci., 20, 2, pp. 159-169.

Blanchard, D.C., A.H. Woodcock, 1980: The Production, Concentration, and Vertical Distribution of the Sea-salt Aerosol, Ann. NY Acad. Sci., 338, 1, p. 330-347.

In the Atlantic basin (Atlantic Ocean, Gulf of Mexico, and Caribbean Sea) and in the eastern and central Pacific, as required, hurricane reconnaissance is carried out by two government agencies, the U.S. Air Force Reserves’ 53rd Weather Reconnaissance Squadron and NOAA’s Aircraft Operations Center (AOC). The U.S. Navy stopped flying hurricanes in 1974.

The 53rd WRS is based at Keesler AFB in Mississippi and maintains a fleet of ten WC-130 planes. These cargo airframes have been modified to carry weather instruments to measure wind, pressure, temperature and dew point as well as drop instrumented sondes and make other observations.

AOC is presently based at Linder Airfield in Lakeland, Florida and among its fleet of planes has two P-3 Orions, originally made as Navy sub hunters, but modified to include three radars as well as a suite of meteorological instruments and dropsonde capability. Starting in 1996 AOC added to its fleet a Gulfstream IV jet that is able to make observations from much higher altitudes (up to 45,000 feet).

The USAF planes are the workhorses of the hurricane hunting effort. They are often deployed to a forward base, such as Antigua, and carry out most of the reconnaissance of developing waves and depressions. Their mission in these situations is to look for signs of a closed circulation and any strengthening or organizing that the storm might be showing. This information is relayed by satellite to the hurricane specialists who evaluate this information along with data from other platforms.

The NOAA planes are more highly instrumented and are primarily used for scientific research on storms, but they may also be called upon for reconnaissance of mature hurricanes when they are threatening landfall, especially on U.S. territory.

The planes carry between six to fifteen people, both the flight crew and the weather crew. Flight crews consist of an aircraft commander, co-pilot, flight engineer, navigator, and electrical and data technicians. The weather crew might consist of a flight meteorologist, lead project scientist, cloud physicist, radar scientist, and dropsonde quality scientist.

The primary purpose of reconnaissance is to track the center of circulation, these are the co-ordinates that the National Hurricane Center issues, and to measure the maximum winds. But the crews are also evaluating the storm’s size, structure, and development and this information is also relayed to hurricane specialists via satellite link. Most of this data, which is critical in determining the hurricane’s threat, cannot be obtained from satellite.

The purposes of research are more varied. Onboard scientists direct the aircraft to those parts of the storm of interest, which might not be near the eye of the hurricane. Experiments might be planned to examine the outer rainbands or the hurricane’s interaction with the environment.

The NOAA G-IV jet usually does NOT penetrate the hurricane eye, but is assigned to fly synoptic scale patterns AROUND the storm, deploying dropsondes along the way, in order to profile the environmental flow that is moving the hurricane. In certain circumstances, a USAF WC-130 will also be assigned to fly a similar pattern in coordination with the G-IV to increase the coverage of this synoptic flow mission.

Whatever the mission’s purpose, information from all of these flights are shared via satellite with land-based forecasters to keep them current on the storm’s status. Radar and probe data are sent in real-time to be ingested into a variety of computer forecast models to ensure the best quality forecast.

The most incredible sight that I’ve ever seen is in the middle of a strong hurricane. One might not believe this, but most hurricane flights are fairly boring. They last 10 hours, there are clouds above you and clouds below – so all you see is gray, and you don’t feel the winds swirling around the hurricane.

But what does get interesting is flying through the hurricane’s rainbands and the eyewall, which can get a bit turbulent. The eyewall is a donut-like ring of thunderstorms that surround the calm eye. The winds within the eyewall can reach as much as 200 mph [325 km/hr] at the flight level, but you can’t feel these aboard the plane. But what makes flying through the eyewall exhilarating and at times somewhat scary, are the turbulent updrafts and downdrafts that one hits. Those flying in the plane definitely feel these wind currents (they sometimes makes us reach for the air-sickness bags). These vertical winds may reach up to 50 mph [80 km/hr] either up or down, but are actually much weaker in general than what one would encounter flying through a continental supercell thunderstorm. But once the plane gets into the calm eye of a hurricane like Andrew or Gilbert, it is a place of powerful beauty: sunshine streams into the windows of the plane from a perfect circle of blue sky directly above the plane, surrounding the plane on all sides is the blackness of the eyewall’s thunderstorms.

Directly below the plane peeking through the low clouds one can see the violent ocean with waves sometimes 60 feet high [20 m] crashing into one another. The partial vacuum of the hurricane’s eye (where one tenth of the atmosphere is gone) is like nothing else on earth. I would much rather experience a hurricane this way – from the safety of a plane – than being on the ground and having the hurricane’s full fury hit without protection.

The USAFR 53rd Hurricane Hunters have a ‘cyber flight’ through a hurricane. Visit the page here.

Hurricanes form both in the Atlantic basin (i.e. the Atlantic Ocean, Gulf of Mexico and Caribbean Sea) to the east of the continental U.S. and in the Northeast Pacific basin to the west of the U.S. However, the ones in the Northeast Pacific almost never hit the continental U.S., while the ones in the Atlantic basin strike the U.S. mainland just less than twice a year on average. There are two main reasons. The first is that hurricanes tend to move toward the west-northwest after they form in the tropical and subtropical latitudes. In the Atlantic, such a motion often brings the hurricane into the vicinity of the U.S. east coast. In the Northeast Pacific, a west-northwest track takes those hurricanes farther off-shore, well away from the U.S. west coast.

In addition to the general track, a second factor is the difference in water temperatures along the U.S. east and west coasts. Along the U.S. east coast, the Gulf Stream provides a source of warm (> 80°F or 26.5°C) waters to help maintain the hurricane. However, along the U.S. west coast, the ocean temperatures rarely get above the lower 70s, even in the midst of summer. Such relatively cool temperatures are not energetic enough to sustain a hurricane’s strength. So for the occasional Northeast Pacific hurricane that does track back toward the U.S. west coast, the cooler waters can quickly reduce the strength of the storm. You may have remnants of such storms move over the Southwestern United States bringing heavy rainfall.

Recently Chenoweth and Landsea (2004), re-discovered that a hurricane struck San Diego, California on October 2, 1858. Unprecedented damage was done in the city and was described as the severest gale ever felt to that date nor has it been matched or exceeded in severity since. The hurricane force winds at San Diego are the first and only documented instance of winds of this strength from a tropical cyclone in the recorded history of the state. While climate records are incomplete, 1858 may have been an El Niño year, which would have allowed the hurricane to maintain intensity as it moved north along warmer than usual waters. Today if a Category 1 hurricane made a direct landfall in either San Diego or Los Angeles, damage from such a storm would likely be few to several hundred million dollars. The re-discovery of this storm is relevant to climate change issues and the insurance/emergency management communities risk assessment of rare and extreme events in the region.

Reference: Chenoweth, M., and C.W. Lansea (2004): “The San Diego hurricane of October 2, 1858” Bull. Amer. Meteor. Soc., 85, pp.1689-1697

The vast majority of Atlantic activity takes place during August-September-October, the climatological peak months of the hurricane season. The overall number of named storms (hurricanes) occurring in June and July (JJ) correlates at an insignificant r = +0.13 (+0.02) versus the whole season activity. In fact, there is a slight negative relationship between early season storms (hurricanes) versus late season – August through November – r = -0.28 (-0.35). Thus, the overall early season activity, be it very active or quite calm, has little bearing on the season as a whole. These correlations are based on the years 1944-1994.

A significant number of pre-season (April-May) and early season (JJ) storms are hybrid systems (neither fully tropical nor midlatitude lows). So their formation mechanisms are very different from fully tropical systems that form in the Main Development Region (MDR). So conditions favoring hybrid storm formation can be very different from those favoring tropical cyclone formation.

As shown in (Goldenberg 2000), if one looks only at the June-July Atlantic tropical storms and hurricanes occurring south of 22°N and east of 77°W (the eastern portion of the MDR for Atlantic hurricanes), there is a strong association with activity for the remainder of the year. According to the data from 1944-1999, total overall Atlantic activity for years that had a tropical storm or hurricane form in this region during JJ have been at least average and often above average. So it could be said that a JJ storm in this region is pretty much a “sufficient” (though not “necessary”) condition for a year to produce at least average activity. (I.e., Not all years with average to above-average total overall activity have had a JJ storm in that region, but almost all years with that type of JJ storm produce average to above-average activity.) The formation of a storm in this region during June-July is taken into account when the August updates for the Bill Gray and NOAA seasonal forecasts are issued.

Recent research describes two distinct types of Atlantic climate drivers: 1) Internal variability is caused by natural processes within the atmosphere and ocean climate system. 2) External variability is caused by forces outside of the atmosphere/ocean climate system.

Examples of natural internal forces are oceanic oscillations such as ENSO, meridional overturning circulation, and Saharan dust storms that blow mineral dust over the tropical Atlantic. The effects of the El Nino/Southern Oscillation are discussed in another section in detail.

Examples of external climate forcing agents are solar variability, cosmic radiation changes, and air pollution such as industrial particulate and sulfur emissions.

The Atlantic meridional overturning circulation, which transports ocean heat from the tropics to higher latitudes and can cause substantial climate swings in the Atlantic region and beyond as this circulation increases or decreases.

Saharan dust storms have a similar effect on the Atlantic climate as the dust blows westward in the trade-winds off the African continent and blocks sunlight from reaching the ocean surface. Saharan dust storms are strongly seasonal, but can also exhibit multi-decadal swings that can cause similar swings in Atlantic ocean temperatures.

Our sun has 11-year and 22-year cycles in sunspot and magnetic activity, which affects the solar wind and Earth’s magnetic field. It may also exhibit longer scale variability in its output. Along with changes in comic ray activity, this may alter Earth’s cloud cover in subtle ways and drive changes in ocean heat content.

Volcanic eruptions cause a transient cooling of ocean temperatures as they tend to block some of the incoming sunlight from reaching the surface. These natural eruptions tend to occur randomly and don’t exhibit any clear multi-decadal swings.

Finally, there is human-caused particulate and sulfate air pollution, which tends to block incoming sunlight similarly to volcanic eruptions and mineral dust. Human-caused sulfate pollution over the Atlantic exhibits a pronounced variability over time. Prior to the various Clean Air Acts and Amendments instituted by the United States and European countries in the 1970s, industrial sulfate emissions were much less regulated and air quality had become progressively worse. As the concentration of sulfate pollution over the Atlantic Ocean increased from the 1940s through 1970s, a cooling effect was noted as the pollution blocked incoming sunlight. According to some studies, as sulfate pollution concentrations decreased during and after the 1970s, the offsetting cooling effect is believed to have been reduced.

Hurricanes: Their Nature and Impacts on Society

An excellent introductory text into hurricanes (and tropical cyclones in general), this book by R.A. Pielke, Jr. and R.A. Pielke, Sr. provides the basics on the physical mechanisms of hurricanes without getting into any mathematical rigor. The book also discusses hurricane policy, vulnerability and societal responses and ends with an in-depth look at Hurricane Andrew’s forecast, impact and response. Roger A. Pielke, Jr. is a Sociologist at the Environmental and Societal Impacts Group at the National Center for Atmospheric Research in Boulder, Colorado, USA. Roger A. Pielke, Sr. is a Professor of Atmospheric Science at Colorado State University (USA).

John Wiley & Sons, Chichester, UK, 1997, 279 pp.

Meteorology Today for Scientists and Engineers

This paperback book is designed to accompany C. Donald Ahrens’ introductory book “Meteorology Today.” For a concise mathematical description of hurricanes that has NO calculus and NO differential equations, then I would suggest obtaining a copy of this book by Rolland B. Stull

West Publ. Co., Minneapolis/St. Paul, MN, 2000, 385 pp.

Chapter 16 Hurricanes p.289-304.

Global Perspectives on Tropical Cyclones: From Science to Mitigation

edited by Johnny C. L. Chan and Jeffrey D. Kepert

This book is a completely rewritten, updated and expanded new edition of the original Global Perspectives on Tropical Cyclones published in 1995. It presents a comprehensive review of the state of science and forecasting of tropical cyclones together with the application of this science to disaster mitigation, hence the tag: From Science to Mitigation.Since the previous volume, enormous progress in understanding tropical cyclones has been achieved. These advances range from the theoretical through to ever more sophisticated computer modeling, all underpinned by a vast and growing range of observations from airborne, space and ocean observation platforms. The growth in observational capability is reflected by the inclusion of three new chapters on this topic. The chapter on the effects of climate change on tropical cyclone activity is also new, and appropriate given the recent intense debate on this issue. The advances in the understanding of tropical cyclones which have led to significant improvements in forecasting track, intensity, rainfall and storm surge, are reviewed in detail over three chapters. For the first time, a chapter on seasonal prediction is included. The book concludes with an important chapter on disaster mitigation, which is timely given the enormous loss of life in recent tropical cyclone disasters.

World Scientific, 2010, 448 pp.ISBN: 978-981-4293-47-1 or 978-981-4293-48-8 (ebook).

Global Guide to Tropical Cyclone Forecasting

For the tropical cyclone forecaster and also of general interest for anyone in the field and those with a non-technical interest in the field, the loose-leaf book Global Guide to Tropical Cyclone Forecasting(1993) by G.J. Holland (ed.), World Meteorological Organization, WMO/TD-No. 560, Report No. TCP-31 is a must get.

North Carolina’s Hurricane History, Florida’s Hurricane History

These two books are an amazing documentaries of the hurricanes which have struck the states of North Carolina and Florida from 1526 until 1996 and 1546-1995, respectively. The author Jay Barnes – Director of the North Carolina Aquarium – tells the stories of the hurricanes and their effects upon the people of the state in an easily readable style with numerous photographs.

University of North Carolina Press, Chapel Hill, NC, 1998, 330pp.

Atlantic Hurricanes

A classic book describing tropical cyclones primarily of the Atlantic basin, but also covering the physical understanding of tropical cyclone genesis, motion, and intensity change at the time. Written in 1960, by Gordon E. Dunn and Banner I. Miller, this book provides insight into the knowledge of tropical cyclones as of the late 1950s. It is interesting to observe that much of what we know was well understood at this pre-satellite era. Gordon E. Dunn was the Director of the National Hurricane Center and Banner I. Miller was a research meteorologist with the National Hurricane Research Project.

Louisiana State Press, 1960, 326pp (revision 1964)

Hurricanes, Their Nature and History

Before Dunn and Miller’s book, Ivan Ray Tannehill came out with an authoritative reference on the history, structure, climatology, historical tracks, and forecasting techniques of Atlantic hurricanes as was known by the mid-1930s. This is one of the first compilations of yearly tracks of Atlantic storms – he provides tracks of memorable tropical cyclones all the way back to the 1700s and shows all the storm tracks yearly from 1901 onward. The first edition came out in 1938 and the book went through at least nine editions (my book was published in 1956). Mr. Tannehill was engaged in hurricane forecasting for over 20 years and also lead the Division of Synoptic Reports and Forecasts of the U.S. Weather Bureau.

Princeton University Press, 1956, 308 pp.

Into the Hurricane

(Published in Britain as “The Devil’s Music”)

Author Pete Davies spent the summer of 1999 looking at Atlantic hurricanes, traveling to Honduras to see the aftermath of Hurricane Mitch, and flying on research missions with NOAA’s Hurricane Research Division. He explores the science of why the storms occur and how to predict them, and recounts the impacts of Hurricane Floyd.

Henry Holt and Company. 2000, 264 pp., ISBN: 0-8050-6574-1.

The Divine Wind

(translated into Chinese) Hurricanes are presented in verse, art, history, and science in this all-encompassing book of the science and culture of hurricanes. Author Kerry Emanuel discusses hurricane forecasting, historical events and human impacts. The book includes many artworks, figures, and photographs, plus a description of flying into hurricanes.

Oxford University Press, 2005, 296 pp.,ISBN-10: 0195149416.

A Global View of Tropical Cyclones

(A revised version of this book is Global Perspectives on Tropical Cyclones listed above.)

A very thorough book dealing with the technical issues of tropical cyclones for the state of the science in the mid-1980s by Elsberry, Holland, Frank, Jarrell, and Southern.

University of Chicago Press, 1987,195 pp.

The Hurricane

(1997 revision titled “Hurricanes: Their Nature and Impacts on Society” by Pielke and Pielke is listed above.)

A very good introductory text into hurricanes (and tropical cyclones in general), this book by R.A. Pielke provides the basics on the physical mechanisms of hurricanes without getting into any mathematical rigor. This first version is just 100 pages of text with another 120 pages devoted toward all of the tracks of Atlantic hurricanes from 1871-1989. Roger A. Pielke is a professor of Atmospheric Science at Colorado State University.

Routledge Publishing, New York, 1990, 279 pp. (revision 1997)

Hurricanes

An introductory text book for young readers on hurricanes by Sally Lee.

Franklin Watts Publishing, New York, 1993, 63 pp.

Cyclone Tracy, Picking up the Pieces

Twenty years after Cyclone Tracy, this book recreates, by interviews with survivors, the events during and after the cyclone that nearly destroyed Darwin, Australia by B. Bunbury

Fremantle Arts Centre Press, South Fremantle, Australia, 1994, 148 pp.

Beware the Hurricane!

This book tells “the story of the cyclonic tropical storms that have struck Bermuda and the Islanders’ folk-lore regarding them” by Terry Tucker.

The Island Press Limited, Bermuda, 1995, 180 pp.

Florida Hurricanes and Tropical Storms, Revised Edition

This recent book provides a historical perspective of Florida Hurricanes extending from 1871 to 1996 by J.M. Williams and I. W. Duedall

Florida Sea Grant College Program, University of Florida Press, Gainesville, FL, 1997, 146 pp.

Hurricanes of the North Atlantic

This book by J. B. Elsner and A. B. Kara focuses on the statistics and variability of Atlantic hurricanes as well as detailed discussions on how hurricanes impact the insurance industry and how integrated assessments can be made regarding these storms. The book provides very valuable information on hurricane frequencies, intensities and return periods that are not easily available elsewhere. Also sections are devoted on the development of seasonal (and longer) hurricane forecast models and their performance.

Oxford University Press, New York/Oxford, 1999, 488 pp.

Natural Disasters – Hurricanes

This reference book by P. J. Fitzpatrick provides a very useful compilation of a wide range of topics on Atlantic hurricanes. Of particular interest is the chronology of advances in the science and forecasting of hurricanes along with biographical sketches of researchers and forecasters prominent in the field. This book is an excellent resource in answering questions on many issues in the field.

ABC-CLIO, Santa Barbara, CA, 1999, 286 pp.

Tropical Cyclones of the North Atlantic Ocean, 1851-2006

Researchers and those who follow Atlantic hurricanes should all have a copy of the atlas. Previous versions:

Tropical Cyclones of the North Atlantic Ocean, 1871-1998

Tropical Cyclones of the North Atlantic Ocean, 1871-1992

Tropical Cyclones of the North Atlantic Ocean, 1871-1986

Tropical Cyclones of the North Atlantic Ocean, 1871-1980

Tropical Cyclones of the North Atlantic Ocean, 1871-1977

Tropical Cyclones of the North Atlantic Ocean, 1871-1963

North Atlantic Tropical Cyclones, 1886-1958

National Climatic Data Center, Asheville, NC, in cooperation with the Tropical Prediction Center/National Hurricane Center, Miami, FL, 2006, 238 pp.

Hurricanes and Florida Agriculture

Dr. John A. Attaway, former Scientific Research Director of the Florida Department of Citrus, wrote this well-researched history and litany of the impacts that hurricanes have had upon agriculture in Florida.

Florida Science Source, Inc., Lake Alfred, FL, 1999, 444 pp.