Answers archive: Hurricane science and forecasting

Q: What's the difference between a hurricane and a typhoon?

A: They are two different names for the same kind of storm -- a tropical cyclone, which is a low-pressure area in which the central core is warmer than the surrounding atmosphere. They’re called hurricanes in the Atlantic Ocean and the Eastern Pacific Ocean, and typhoons in the Western Pacific. In addition, these storms are known as cyclones in the Indian Ocean and around the Coral Sea off northeastern Australia.

This USA TODAY resource page has links to more about hurricane science, while this page from the Atlantic Oceanographic and Meteorological Laboratory has more about the different names given to tropical cyclones.

(Answered by Doyle Rice, USA TODAY’s weather editor, April 10, 2007)

Q: What was the largest recorded hurricane in the Atlantic Basin in terms of wind field?

A: Since official weather records on this are not kept, it's difficult to come up with an answer. I did find two storms that had very large areas of tropical-storm-force winds (known as a storm’s “wind field”): Hurricane Cindy in 1999 had maximum tropical-storm-force winds 266 miles away from the center of the hurricane, while 1988’s Hurricane Gilbert had tropical-storm-force winds that were measured as far as 259 miles from its center. (Tropical-storm-force winds are 39 mph and above.)

Hurricane Cindy was a large storm that remained over the open Atlantic and did not strike land. I found the information about Cindy from

a paper published by the American Meteorological Society

(PDF) on the characteristics of intense hurricanes in the Atlantic between 1995 and 2003. The information about Gilbert came from a

chart reportedly made by the Hurricane Research Division

, which shows that Hurricane Gilbert had the largest wind field radii in their records. (The chart was linked off this

storm2K page

.)

One other item to mention: When storms are undergoing extratropical transition, the wind field expands greatly, and there may have been some transistioning storms in the North Atlantic with a larger wind field.

(Answered by meteorologist Dick Fletcher of WTSP-TV in St. Petersburg-Tampa, Fla., November 1, 2006)

Q: Does the size of a hurricane's eye have a relationship to its strength?

A: When forecasters are tracking a tropical cyclone, the eye typically becomes more well-defined and smaller as the system intensifies. However, depending on the overall size of the storm, there is a limit to how small the eye can become. Sometimes the eyewall becomes ragged or disappears completely, with a corresponding weakening of peak winds, only to be replaced by a new eyewall and restrengthening of the hurricane. This is called an eyewall replacement cycle.

While most hurricanes are about 300 miles in diameter, the overall size of the hurricane is also not necessarily indicative of its strength. Hurricane Andrew in 1992, a relatively small hurricane, packed quite a punch as it made landfall in southeast Florida as a Category 5 hurricane. Another good example of a small but potent storm was Hurricane Charley in 2004.

Want to learn more about hurricane science, check out the FAQ on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, October 10, 2006)

Q: Where do most hurricanes originate -- in the Atlantic off the African coast, in the Gulf of Mexico or in the Caribbean?

A: Most hurricanes begin in the Atlantic as a result of tropical waves that move westward off the African coast. Some form in the vicinity of the Cape Verde islands and are known as "Cape Verde" hurricanes. The Atlantic is the favored location for tropical cyclones that occur during the peak of hurricane season from mid-August through mid-October. During the early and late stages of hurricane season, which runs from June 1 through November 30, storms are typically less numerous and are more likely to originate in the Caribbean or Gulf of Mexico.

Learn more about Cape Verde hurricanes on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, October 5, 2006)

Q: A hurricane "picks up" saltwater over the ocean, but rains fresh water. Where does the salt go?

A: The process by which a hurricane "picks up" water over the ocean is called evaporation. That is, heating by the sun makes saltwater warm enough that molecules of water leave the liquid state and become water vapor in the atmosphere. The salts that are diluted in the water are left behind, making the remaining ocean water even saltier. The water vapor condenses to form clouds within the hurricane and eventually precipitates out of the storm as fresh rainwater.

For more on hurricanes, read this USA TODAY FAQ page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, September 4, 2006)

Q: What are the odds that a major hurricane will hit the U.S. coastline in 2006?

A: According to hurricane experts at Colorado State University, there is a 73% chance that a major hurricane (Category 3 or higher on the Saffir-Simpson scale of hurricane intensity) will hit somewhere along the U.S. coastline this year. This is well above the long-term average of 52%. The East Coast and peninsular Florida have a 64% probability of a major hurricane landfall in 2006, more than twice the average of 31%. The Gulf Coast has a 26% probability, just below the long-term average of 30%.

The updated forecast for the 2006 hurricane season complete with landfall probabilities can be found on this Colorado State resource page.

(Answered by Greg Seroka of USA TODAY’s weather staff, August 21, 2006)

Q: Why does wind shear tend to strengthen thunderstorms but weaken hurricanes?

A: Wind shear, changes in wind speed and direction with elevation, gives a thunderstorm its spin in the form of rotating updrafts called mesocyclones. Wind shear also helps to keep a thunderstorm's updraft separated from its downdraft, increasing the storm's strength and duration. A hurricane, on the other hand, is a large collection of thunderstorms and occurs on a much larger scale than do individual thunderstorms. A hurricane gets its spin from the rotation of the Earth rather than from wind shear. Hurricanes rapidly intensify when the thunderstorms form tightly around the center of circulation. Like pushing over a spinning figure skater, wind shear tends to disrupt the balance of the storm.

Read more about wind shear and hurricanes on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, August 7, 2006)

Q: Do hurricanes hit Central America?

A: Yes, Central America can be affected by hurricanes. Storms that form in the north Atlantic and eastern Pacific basins often hit or come close to the region. One of the worst storms was in October 1998, when catastrophic Hurricane Mitch killed more than 9,000 people, mostly in Honduras and Nicaragua.

South America, on the other hand, is rarely affected by tropical systems. Cooler water, high wind shear, and the lack of an Intertropical Convergence Zone -- a constant area of low pressure and thunderstorm activity along the Earth's equator -- in the south Atlantic and south eastern Pacific basins limit any tropical activity that may affect South America.

Check out FAQ's about hurricane science on this USA TODAY resource page.

(Answered by Greg Seroka of USA TODAY’s weather staff, July 19, 2006)

Q: What is a tropical wave?

A: Tropical waves, also known as easterly waves, are elongated low-pressure areas that cause cloudiness and thunderstorms. They form when warm air drifts off of Africa into the Atlantic Ocean, and they are usually pushed westward by prevailing winds. Tropical waves can often develop into tropical cyclones: 60% of all Atlantic tropical cyclones originate from tropical waves, and about 85% of Atlantic hurricanes of Category 3 or greater come from tropical waves.

About 60 tropical waves form off of the African coast each year. Sometimes, tropical waves can have tropical-storm force winds (39-73 mph) without being considered tropical storms, since tropical storms must have closed circulation.

Hurricane formation and its progression always begins with a tropical disturbance. Tropical disturbances can be tropical waves, but not always. The disturbance then develops into a tropical depression, which has a closed circulation. The system then forms into a tropical storm, before finally developing into a hurricane.

Learn more about tropical waves and hurricane formation through this USA TODAY interactive graphic.

(Answered by Greg Seroka of USA TODAY’s weather staff, July 6, 2006)

Q: Where do most Atlantic Ocean hurricanes originate?

A: The origins of many Atlantic hurricanes can be traced to tropical waves that develop over sub-Saharan Africa and move west into the eastern Atlantic. During the mid-August to mid-October peak of the hurricane season, which runs from June 1 to Nov. 30, tropical storms and hurricanes tend to develop in the eastern Atlantic, often near the Cape Verde Islands off the coast of Senegal. Early and late-season tropical storms and hurricanes are more likely to develop in the western Caribbean or even in the Gulf of Mexico.

Read more about the origins of tropical cyclones on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, April 6, 2006)

Q: Has a major hurricane ever hit Virginia or Maryland? Could it happen?

A: Yes, and yes. A major hurricane known as the "Chesapeake-Potomac Hurricane" passed over Norfolk, Virginia, on August 23, 1933, and moved up the Chesapeake Bay and Potomac River during the day. Winds in the Norfolk area were almost 90 mph and a storm surge of about 9-12 feet inundated Washington, D.C.,and many cities and towns along the shores of the bay and river.

The storm also created a breach in the Assateague peninsula — the Ocean City inlet — that remains to this day. A major hurricane such as this, though rare in Virginia and Maryland, could certainly happen again.

(Answered by Bob Ryan, chief meteorologist and leader of the five-person StormCenter4 team at NBC4, WRC-TV, in Washington, D.C., March 8, 2006.)

Q: Why do hurricanes strengthen over warm water and weaken over cooler water?

A: Hurricanes do not always strengthen over warmer water or weaken over cooler water. The atmospheric environment in which the hurricane is embedded has a profound effect on whether a hurricane weakens or strengthens. This is what makes forecasting hurricane intensity change so difficult.

However, sea water temperatures can cause changes in intensity. Cooler water puts less moisture and energy into the atmosphere, typically reducing its ability to make thunderstorms. Conversely, warmer water puts more moisture and energy into the atmosphere, making it more amenable to thunderstorm development.

Since thunderstorms are one important conduit for quickly transferring ocean energy into the atmosphere, their absence prevents strengthening and/or causes weakening.

Hence, cold water is unfavorable for hurricane development. In general, the ocean water temperature necessary to form a hurricane is around 80°F. But once a hurricane forms, it can persist in water below 80°F, sometimes for many days.

This USATODAY.com resource page shows how hurricanes develop.

(Answered by Steve Lyons, tropical weather expert at the Weather Channel, November 28, 2005)

Q: Were the Gulf of Mexico's water temperatures unusually warm this year? If so, is that why there were so many major hurricanes?

A: The Gulf of Mexico is shallower than the open ocean, so much like a backyard kiddie pool, it heats up quickly under the blazing summer sun. While preliminary data indicate that the Gulf water temperatures were slightly above normal this year, warm water is only one factor that can strengthen storms. In the cases of Katrina and Rita, and to a lesser extent, Wilma, these hurricanes strengthened rapidly in the Gulf not only because of warm Gulf waters, but also because of very favorable atmospheric conditions, such as low wind shear and upper-level high pressure.

A NOAA Climate Diagnostics Center map shows that sea-surface temperatures across much of the Gulf of Mexico were about 1°C above average between from early August to early November, 2005.

An article on global warming and its possible relation to Hurricane Katrina is on this

USA TODAY page

.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, November 10, 2005)

Q: I know typhoons and hurricanes are the same type of storm, but it seems like most typhoons are usually stronger than hurricanes. Is this true?

A: Hurricanes form in limited areas of the Atlantic and northeast Pacific, but typhoons in the northwest Pacific draw on the world’s largest pool of deep, warm seawater. The journal

Science

reported in September that, from 1990 to 2004, 41% of northwest Pacific typhoons and southwest Pacific tropical cyclones reached Category 4 or 5 strength, while only 25% of Atlantic hurricanes did.

This USATODAY.com resource page has more about typhoons.

(Answered by meteorologist Bob Henson, a writer at the University Corporation for Atmospheric Research and the author of The Rough Guide to Weather, November 7, 2005.)

Q: How is a hurricane's path predicted?

A: It should be recognized that, even for the experts at the National Hurricane Center, predicting a hurricane's path often comes with great uncertainty.

The forecast of a hurricane's path depends largely upon the accuracy of the predicted winds from computer forecast models. The speed and direction of these steering winds typically vary with altitude. Weak tropical cyclones tend to be steered more by lower-level winds, while upper-level winds usually influence the paths of stronger hurricanes.

So not only does an accurate path forecast depend on the accuracy of the computer models, but it is also tied into the forecast intensity of the storm as well.

Learn more about hurricane forecasting on this USA TODAY resource page.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, November 6, 2005)

Q: What is meant by the "Atlantic basin?"

A: It's a geographic term used to describe where Atlantic tropical storms and hurricanes form. The Atlantic basin includes a part of the North Atlantic Ocean, the Gulf of Mexico and the Caribbean Sea. There are

seven separate basins

in oceans around the world, each of which uses a different list of names chosen by the countries affected in that particular basin.

(Answered by Doyle Rice, USA TODAY's weather editor, November 3, 2005)

Q: Has a tropical storm or hurricane ever traveled from the Atlantic basin into the Pacific basin?

A: This has indeed occurred, most recently in 1996 when Atlantic Hurricane Cesar moved across Nicaragua and El Salvador and emerged in the northeast Pacific as Hurricane Douglas. Since 1949, storms have traveled from the Atlantic to the Pacific seven times. Two storms have moved from the Pacific into the Atlantic.

There is no record of a storm ever crossing from the Atlantic to the Pacific and back again into the Atlantic.

The Atlantic Oceanographic and Meteorological Laboratory maintains a complete list of these storms onthis FAQ page.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, October 16, 2005)

Q: Why isn't Hawaii prone to hurricanes?

A: While hurricanes are indeed rare in Hawaii, they are definitely possible. The most recent memorable hurricane to hit Hawaii was Iniki in 1992, a Category 4 central Pacific hurricane that killed seven people and injured about 100 on Kauai.

Estimated maximum sustained winds on Kauai were 140 mph with gusts as high as 175 mph, making Iniki the most powerful hurricane to strike the Hawaiian Islands in recent history. The 1992 seasonal summary from the Central Pacific Hurricane Center gives more details about this hurricane.

Typically storms that approach Hawaii from the east tend to weaken due to increased westerly wind shear as well as slightly cooler ocean temperatures.

Learn more the eastern Pacific hurricanes on this

USA TODAY resource page

.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, October 5, 2005)

Q: Why don't Pacific hurricanes hit the West Coast?

A: Winds in the tropics generally blow from east to west. This means that a west-northwest motion of a hurricane in the Atlantic basin will carry it toward the U.S. mainland. As they approach the U.S. the storms are maintained -- and sometimes strengthened -- by the warm water of the Gulf Stream off the Eastern Seaboard or warm water in the Gulf of Mexico.

Storms that form in the eastern Pacific ocean tend to move away from the Mexican coastline, as well as the U.S. mainland. The few that stray to the north occasionally make landfall in Mexico. Any further north and they run into the cooler ocean water off the California coast.

Learn more about tropical systems in the eastern Pacific on this

USA TODAY resource page

.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, September 26, 2005)

Q: I thought that weather systems move from west to east; however, why do hurricanes move to the west?

A: Not all weather systems move from west to east. With the exception of Florida, the continental USA is generally located within the mid-latitudes, which stretch from 30°N to 60°N. Mid-latitude weather systems in the Northern Hemisphere do indeed typically move from west to east. However, in the subtropics, where hurricanes typically form, most weather moves from east to west.

Learn more about global wind patterns on this

USA TODAY resource page

.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, September 18, 2005)

Q: What determines the different categories of a hurricane: wind speed or barometric pressure?

A: The Saffir-Simpson scale rates storms according to their intensity, as measured by the sustained wind speed of the storm. While wind speed is the determining factor in the scale, central air pressure and storm surge are also used to gauge the damage potential of the hurricane.

Check out this USA TODAY resource page for more on the history and application of the Saffir-Simpson scale.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, August 23, 2005)

Q: How are hurricane intensity and storm surge related?

A: Stronger, more intense hurricanes tend to produce a higher storm surge, which is the mound of water that builds up as a hurricane moves over water. Forecasting a hurricane's path is just as important as forecast intensity to determine the potential for storm surge. As the mound of water approaches the shore, the topography of the ocean floor and coast, along with the timing of local tides, will ultimately determine the size of the storm surge.

Rick Knabb of the National Hurricane Center discussed storm surge during a recent USATODAY.com chat: "An accurate storm surge forecast at any one point on the coastline is very difficult to make very far in advance. It requires a very accurate forecast of the hurricane's track, intensity and wind structure, as well as knowledge of the characteristics of the ocean floor and coastline.

"A small error in the track forecast can make a night-and-day difference in the surge experienced at any one point on the coast. This is why emergency managers evacuate fairly large areas in advance of a landfalling hurricane, to make sure everyone at risk, given the uncertainties involved, will be safe. It is always important to heed the advice of your local officials."

(Answered by Bob Swanson, USA TODAY's assistant weather editor, August 21, 2005)

Q: How does the jet stream affect the paths of hurricanes and has the jet stream moved north, thus causing or allowing such bad tropical weather the past few years?

A: The jet stream, particularly the subtropical jet stream, does impact the development of tropical systems. A strong subtropical jet tends to produce shear that weakens or destroys tropical systems. A weaker jet allows for development and active hurricane seasons.

It is true that the Atlantic has seen unusually active tropical seasons the past decade. The current hurricane season is on pace to be the ninth above-normal year in the past eleven years. The ingredients of warm ocean waters and relatively weak atmospheric wind shear have helped to increase tropical activity in recent years.

El Nino and La Nina tend to impact the location and strength of the jet stream, and therefore, impact hurricane season activity.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, August 16, 2005)

Q: If hurricanes draw their water from the sea, why isn’t the rain in hurricanes salt water?

A: When over the ocean, hurricanes draw much of their moisture and energy from the condensation of water vapor that evaporated from warm ocean water. Having undergone the evaporation process and, thereby, leaving its salt content in the ocean, the water is fresh. (Related graphic: What makes a storm a hurricane.)

However, the hurricane does cause some sea water to get into the lower atmosphere due to the strong winds taking the tops of waves and blowing them apart. Thus, the rain from hurricanes falls as drinkable water, with only slight salt content.

Other hurricane questions are answered on our

hurricane resources page

.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, August 7, 2005)

Q: Does the reported wind speed for a hurricane also include its forward wind speed?

A: A hurricane's sustained winds and wind gusts include the winds from the storm's rotation as well as the forward speed of the storm itself. For typical westward moving tropical systems spinning counterclockwise in the Northern Hemisphere, the forward speed adds to winds on the north side of the storm, while the forward speed subtracts from winds on the south side.

One excellent example of this addition of a storm's rotational and forward speeds is "The Long Island Express," the great hurricane of 1938. The 70 mph forward speed of the storm, the fastest known forward speed ever recorded, produced hurricane wind speeds that exceeded 180 mph in eastern Long Island and New England.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, July 20, 2005)

Q: What's the difference between a tropical cyclone and a subtropical cyclone?

A: A cyclone is defined as an area of low atmospheric pressure that has a closed circulation. Cyclones (or more commonly called low- pressure areas) are typically divided into tropical and extratropical cyclones, the difference being the source of energy for each type of system. Tropical cyclones form in warm, humid air masses found over warm ocean waters during the summer months. Extratropical cyclones, on the other hand, are those familiar mid-latitude storms that occur in all seasons and can form over land or water. Temperature contrasts are their major source of energy and they have fronts, or boundaries between warm and cold air.

Now that we have the basic differences between tropical and extratropical, we can tackle subtropical cyclones, non-frontal low pressure systems that have characteristics of both tropical and extratropical cyclones. Subtropical cyclones usually form out of extratropical storms over subtropical oceans, north of 23.5°N or south of 23.5°S latitude.

While the subtropical cyclone is initiated by a temperature contrast between air masses, thunderstorms near its center generate latent heat given off by the condensation of water vapor. This warms the storm's central core.

With a warm core established, meteorologists studying hybrid storms observed eye-type features, more typical of hurricanes, forming in many intensifying storms. They also determined the Northern Hemisphere version of these evolving systems exhibited a tight temperature contrast on their northern side, a feature exclusive to extratropical storms, not tropical storms. With these storms exhibiting characteristics of both tropical and extratropical cyclones, the term subtropical encapsulates the "hybrid" nature of these storms.

Check out a great USA TODAY article about bomb cyclones.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, July 17, 2005)

Q: Could Venezuela ever receive a direct hit from a hurricane?

A: While it is technically possible, no hurricane in recorded history has ever directly hit Venezuela. Most storms only wind up grazing the country's northern coast. Just like Hurricane Ivan in September 2004, the effects of which killed three Venezuelans, tropical storms have skirted the coast many times.

An unnamed tropical storm in 1933 was known to have killed about 25 people in Venezuela. In 1988, Tropical Storm Joan skirted the coast of Venezuela and Colombia as did Tropical Storm Bret in 1993. In 1996, Hurricane Cesar formed just off of Venezuela, and in 2000, Tropical Storm Joyce flirted with the Venezuelan coast.

There are several reasons that Venezuela is not a common target for Atlantic hurricanes. First of all, because northernmost South America (Venezuela and Colombia) is only at a latitude of about 12° north, theCoriolis force is still relatively weak and lacks the sufficient spin for tropical system development.

Storms that do form in these low latitudes are often steered toward Central America or the Gulf of Mexico by ocean currents and trade winds. Additionally, land masses inhibit the formation of tropical depressions and typically prevent them from intensifying into hurricanes.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, July 14, 2005)

Q: How does an African tropical wave turn into a tropical storm or hurricane?

A: Once the wave moves over the ocean, showers and thunderstorms often intensify. When this occurs, the surface air pressure drops and the wave becomes a strong low-pressure area. Sometimes the wave is so strong that it exits Africa as a closed low-pressure area, and then intensifies. Given enough time, and with persistent showers and thunderstorms, the wave transforms into a well-organized tropical depression, then a tropical storm and finally a hurricane.

Our tropical cyclone glossary has more on this, as do our graphics on a typical hurricane's life story and onwhy hurricanes form over warm oceans.

(Answered by Steve Lyons, tropical weather expert at the Weather Channel, June 30, 2005)

Q: Could it ever snow during a hurricane, say if a late-season hurricane meets up with true arctic air? Has it ever happened?

A: Hurricanes are driven by warm, moist air. When cold air intrudes, hurricanes quickly becomeextratropical lows, with a different structure built on temperature contrasts. Over a foot of snow fell in parts of New England on October 10, 1804, on the edge of a storm believed to have once been a hurricane.

Weather histories have reported other snowfalls related to systems believed to have had tropical origins, much like the “snow hurricane” of 1804. However, it is difficult to verify if actual tropical storms or hurricanes were involved. Nor’easters were sometimes confused with hurricanes before the 20th century, because of their very high winds they bring and the mild air that often sweeps inland near the coastline in places such as Boston.

Our resource page on hurricane history has links to information about past hurricanes.

(Answered by meteorologist Bob Henson, a writer at the University Corporation for Atmospheric Researchand the author of The Rough Guide to Weather, June 28, 2005.)

Q: What causes African tropical waves (that later turn into hurricanes) to form?

A: Tropical waves are thought to be formed by instability in a low-level jet stream of east winds over tropical West Africa (known as the African easterly jet stream). Undulations in these east winds result in waves in the easterly flow and are called “easterly” or “tropical” waves. They are tied to the monsoon trough of low pressure that is a regular summer climate feature over this region of Africa.

Our tropical cyclone glossary has more on this, as do our graphics on a typical hurricane's life story and onwhy hurricanes form over warm oceans.

(Answered by Steve Lyons, tropical weather expert at the Weather Channel, June 26, 2005)

Q: Do hurricanes in the Atlantic basin relate to those in the eastern Pacific basin?

A: Hurricanes that form in the Atlantic Ocean don't have any direct relation or cause for hurricanes forming in the eastern Pacific Ocean. However, during an

El Niño

year, when the eastern Pacific water is unusually warm, there tend to be more eastern Pacific hurricanes and fewer Atlantic hurricanes.

Our page on understanding hurricanes has much more information; see our page on eastern Pacific hurricanes to see how these storms form and develop.

(Answered by Bob Ryan, chief meteorologist at NBC4, WRC-TV, in Washington, D.C., May 3, 2005.)

Q: What’s the difference between a typhoon, cyclone and hurricane?

A: They are different names for the same type of storm, collectively known as tropical cyclones. Where they form determines what they’re called. They’re called

hurricanes

in the Atlantic Basin and the Pacific Ocean east of the International Date Line; typhoons in the north Pacific Ocean west of the date line; and cyclones in the Indian Ocean and in the Coral Sea off northeastern Australia.

More information can be found in our tropical cyclone glossary.

(Answered by April Chan, USATODAY.com weather intern, April 26, 2005)

Q: What are the dates that Florida is most likely to be hit by hurricanes?

A: Hurricanes can hit Florida during any month of the Atlantic hurricane season, which runs from June through November. Odds of a strike are higher, however, from late August through October. Since 1900, 29 hurricanes have hit Florida in September and 20 have hit in October, as compared with only three in July.

See our hurricane climatology page for detailed information when and where hurricanes are most likely to hit in different parts of the world, with a focus on the USA.

(Answered by Chris Landsea, research meteorologist with NOAA's Hurricane Research Division in Miami, April 25, 2005)

Q: When is the hurricane season in the Atlantic Ocean?

A: According to the National Hurricane Center, the official Atlantic and Caribbean hurricane season each year runs from June 1 to November 30, when 97% of all tropical storms and hurricanes occur. Most hurricanes, however, develop between mid-August and mid-October.

More information about when and where hurricanes hit can be found on our hurricane climatology page.

(Answered by April Chan, USATODAY.com weather intern, April 20, 2005)

Q: What determines the different categories of hurricanes?

A: The United States uses the Saffir-Simpson hurricane intensity scale for hurricanes in the Atlantic and North Pacific basins. The five categories are: 1 (minimal); 2 (moderate); 3 (extensive); 4 (extreme); and 5 (catastrophic). The scale was invented to communicate the storm's danger, describing how much damage it could do once it hits.

The different categories are based upon measurements of the hurricane's wind speed taken from airplanes or other sources before the storm makes landfall. This is different from the Fujita scale for tornadoes, which is based upon after-the-fact damage surveys.

The scale is named after Herbert Saffir, a consulting engineer in Coral Gables, Fla., and Robert Simpson, who was director of the National Hurricane Center from 1967 through 1973.

The Atlantic Oceanographic and Meteorological Laboratory has more information about the Saffir-Simpson scale.

(Answered by Doyle Rice, USATODAY.com assistant weather editor, March 22, 2005)

Q: My students want to know: What were the strongest winds ever recorded in a hurricane?

A strong hurricane's fastest winds either blow away or damage wind instruments, which is why the fastest, ground-level winds are estimates. Today's estimates are usually based on measurements from hurricane hunter airplanes.

The fastest such estimated winds are the 190 mph winds in Hurricane Camille (1969) and Hurricane Allen.

Such estimates become pretty complicated and in recent years, scientists have changed their method of making estimates based on airplane measurements because research showed them that the old methods were probably underestimating wind speeds.

This is why in 2002, the National Oceanic and Atmospheric Administration decided that Hurricane Andrew was really a Category 5 hurricane when it hit Florida in August 1992, not a Category 4 storm as believed at the time. Our story on Science led to Hurricane Andrew's reclassification has more on this.

(Answered by Jack Williams, USATODAY.com weather editor, Dec. 27, 2004)

Q: Why don't certain areas in Asia, like Singapore, have typhoons, tornadoes or other disastrous weather?

A: Singapore is only 1.36 degrees of latitude north of the equator, which puts it about as deep into the tropics as you can go. This means that it's far away from outbreaks of cold air that can help stir up severe thunderstorms and tornadoes when they move into areas where it's warm.

Tropical cyclones, such as typhoons, do not need cold air. In fact, they form over warm oceans and cold (either water or air) is fatal to them. (Related: What makes a storm a hurricane)

But, tropical cyclones generally do not form within about five or so degrees of latitude from equator because the Coriolis effect, caused by Earth's rotation, is not strong enough to create the swirl of wind and thunderstorms around a center that tropical cyclones need. (Related: Understanding the Coriolis force)

This is the usual case, but as people in Singapore learned on Dec. 27, 2001, the weather doesn't always obey the rules.

A tropical storm formed that day just east of Singapore over the South China Sea and grew into 75 mph Typhoon Vamei. It crossed the Malay Peninsula just north of Singapore, disrupting traffic at Singapore's airport.

You can learn more by going to the USATODAY.com story, Scientists dissect rare typhoon near Equator. Links from this story explain how a tropical cyclone formed so near the equator.

Answered by Jack Williams, USATODAY.com weather editor, Nov. 1, 2004)

Q: What determines whether a hurricane spins clockwise or counterclockwise?

Q: Do all hurricanes turn in a counter clock wise direction? If so, why?

The Earth's rotation causes winds in large storms, such as hurricanes, to go counterclockwise - looking down on the storm - around the storm's center in the Northern Hemisphere and clockwise in Southern Hemisphere.

This is called the Coriolis, for the French scientist Gustave-Gaspard Coriolis, who figured out how it works in 1835.

The Coriolis force affects all large storms, and also ocean currents but not short-lived events such as water going down a drain.

If you go to the USATODAY.com Understanding the Coriolis force page, you'll learn more about how this works, and why anyone who tells you that water flows in different directions down drains north and south of the equator is wrong.

(Answered by Jack Williams, USATODAY.com weather editor, Oct. 19, 2004)

Q: When a hurricane's eye comes on shore, how strong are the winds behind the eye? I am assuming that they are less than the winds in front of the eye. In other words, when the eye comes on land is the worst of the winds over at that point?

A: People who were caught in the 1926 Miami hurricane - most had never been in a hurricane - discovered that an idea like yours can be fatal. When the eye of the Category 4 storm came ashore a little after sunrise on Sept. 18, 1926 many people thought the hurricane was over and went outside.

When the other side of the storm arrived, one anemometer measured the wind going from less than 10 mph to a gust of 132 mph in two minutes. The available reports say "many people" died when the other side of the storm hit, but no exact figures are available. Newspaper accounts referred to "the second storm."

It's hard to believe today when many Florida newspapers, such as the Miami Herald, have writers and editors who are very knowledgeable about hurricanes, than few people in Miami in 1926 even realized that a hurricane consists of winds going around a calm eye, with almost equally strong winds on all sides of the eye.

You can learn a lot more by going to the USATODAY.com Understanding hurricane science and technologypage.

(Answered by Jack Williams, USATODAY.com weather editor, Oct. 14, 2004)

Q: Is lightning found in hurricanes? If not, why not?

A: Lightning sometimes occurs in hurricanes, but not nearly as much as in thunderstorms on land. For more on this, see the USATODAY.com Answers archive: Lights and lightning in hurricanes.

(Answered by Jack Williams, USATODAY.com Weather Editor, Sept. 26, 2003)

Q: What caused Ivan to reverse track?

Q: Can you tell me how Ivan is now said to be back in the Gulf of Mexico? Our local TV weather people say remnants are threatening the Texas-Louisiana coast, but don't tell how they got there. I thought Ivan pretty well dissipated in Pennsylvania.

A: Ivan didn't reverse its track. I've seen one track map that makes it look like Ivan somehow "jumped" in a straight line from over the Appalachians back over the Gulf of Mexico. I'm pretty sure this was drawn by a computer with no human involved.

It didn't do anything like that. My Ivan comes back to life over Gulf of Mexico story briefly describes what happened.

I guess this shows, if you want to understand what the weather is up to, come to USATODAY.com.

(Answered by Jack Williams, USATODAY.com weather editor, Sept. 7, 2004)

Q: Do hurricanes lose strength more quickly over cool water or warm land?

A: Your question brings up an important point - hurricanes need both moisture and heat energy from a warm ocean to strengthen and maintain strength.

If a hurricane passes over colder water, it is likely to weaken considerably. However, if a hurricane moves over land, regardless of the land's temperature, friction plus the loss of ocean moisture will cause it to weaken much more rapidly.

If you go to the USATODAY.com Hurricane page you'll find links to a lot of information.

(Answered by Josh Larson, USATODAY.com Weather Team, June 14, 2004)

Q: When hurricanes move over land and produce tornadoes how come with the energy and strength of the storm there are no F 4 or F 5 tornadoes? Is it due to the lack of cold dry air since hurricanes are warm core storms?

A: You've hit on a key part of the answer. The kind of thunderstorms known as supercells spin out the strongest tornadoes, the F-4 and F-5 storms. A USATODAY.com graphic shows the key things going on in asupercell.

For supercells to form, winds at different levels of the atmosphere need to be coming from different directions, and a large contrast in air temperature at the ground (hot or at least warm) and aloft (cold) is needed. As you said, hurricanes are warm-core storms; that is, they are made of warm air with the warmest air at the center. When cold air aloft is pulled into a hurricane it weakens the storm.

Another way to look at it. A supercell, in effects, concentrates the energy available in the atmosphere. Other, weaker, thunderstorms are often in the neighborhood of a supercell, but it's the big kid on the block. A hurricane's bands of thunderstorms are made up of weaker cells, all pretty much the same size, except maybe for a couple of storms in the eyewall.

Our tornado and hurricane pages will take you to more information.

(Answered by Jack Williams, USATODAY.com weather editor, June 9, 2004)

Q: What is the worst side to be on when there is a hurricane? I have heard there is one side that is worse than the other.

A: The right side of a hurricane usually has the fastest winds, while the left side often has the heaviest rain. (Most people describe hurricane severity by wind speed not rain amounts, even though the number one cause of death in a hurricane is fresh water flooding.)

A hurricane is a whirling mass of thunderstorms that spins counterclockwise. As a hurricane moves, the wind on its right side blows in the direction of the storm's motion. That means a hurricane packing 100 mph winds and moving at 20 mph has a combined wind of 120 mph on that side of the storm. It also means that the forward motion of 20 mph takes away from the opposing wind on the left side of the storm: 100 mph - 20 mph = 80 mph. In this example, that's a 40 mph difference in wind from one side of the storm to the other. The forward motion has no effect on the wind in the front or rear part of the hurricane.

While storm surge generally occurs in the center of a hurricane, higher waves and the onshore wind in the right side of the storm can worsen the surge there. Also, tornadoes are most likely in the right-front part of a hurricane, which makes the right side that much worse.

Learn more about hurricanes by going to USATODAY.com's Resources: Hurricanes and tropical cyclonespage.

(Answered by Chris Cappella, USATODAY.com Weather Team, Aug. 14, 2003)

Q: How do hurricanes in the USA and Bangladesh differ?

A: The meteorology of both kinds of storms is the same; they are both tropical cyclones that form over warm oceans. One difference, but it's not a basic one, is that tropical cyclones are called "hurricanes" if they are over the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, or the Pacific Ocean east of the International Date Line. If they are over the North Pacific west of the Date Line, they are "typhoons." Everywhere else, including the Indian Ocean, they are called "cyclones." Since the Bay of Bengal is part of the Indian Ocean Basin, the storms that hit Bangladesh are "cyclones.

A USATODAY.com graphic and text that shows a typical Northern Hemisphere hurricane also applies to the storms that hit Bangladesh since it's in the Northern Hemisphere. Southern Hemisphere tropical cyclones are the same except that their winds circle the storm's center in a clockwise direction, while winds in a Northern Hemisphere storms are counterclockwise - or as those who speak British instead of American English say, "anticlockwise."

Our map showing where the world's tropical cyclones form illustrates one difference between storms that hit the USA and those that hit Bangladesh. As you can see, some storms that hit the USA have almost the full width of the Atlantic Ocean to gain strength, which means that now and then a Category 5 storm with winds faster than 155 mph hits places around the Atlantic Basin, including the USA.

Even though Indian Ocean tropical cyclones don't have the room, or time, to grow as strong as Atlantic hurricanes, they have killed many more people.

Bob Sheets and I discuss this in our book, Hurricane Watch: Forecasting the Deadliest Storms on Earth.While our book focuses on Atlantic Basin hurricanes, we take a brief look at tropical cyclones around the world. We note that tropical cyclones are especially deadly in Bangladesh because the normal tidal range in the Bay of Bengal is as much as 18 feet, which means that a storm arriving at high tide pushes Storm surge far inland across the "low-lying Ganges Delta, where more than three million people live in extreme poverty." During the twentieth century, Bay of Bengal cyclones were the deadliest in the world, killing an estimated 600,000 people from drowning alone, and maybe 1.5 million people when you include deaths from famine and disease the storms caused.

Fortunately, Bangladesh has been improving its forecasting and warning services, and the government has built concrete shelters atop mounds that are higher than surge levels. Bob and I note in our book that these efforts are saving lives.

Answered by Jack Williams, USATODAY.com weather editor, April 26, 2003)

Q: We are a group of French students, studying cyclones. We have heard about the Stormfurry Project. Could you send us some information about it or any contacts that would provide us with help? If possible,any information provided in French would be welcomed. Actually, anything about means of destroying cyclones could help us.

A: For readers who don't know, Project Stormfury was the U.S. government program that operated for almost 20 years, ending in 1982, that searched for a way to weaken hurricanes. If the scientists involved had found a way, it would have worked to weaken tropical cyclones in other parts of the world, not just hurricanes over the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and northern Pacific east of the International Date Line.

It's a fascinating story, which is why when Bob Sheets and I wrote our book, Hurricane Watch: Forecasting the Deadliest Storms on Earth, we included an entire chapter on Stormfury. Bob was the last director of the project. In addition, I talked at length with Bob and Joanne Simpson, who came up with the theory on how cloud seeding might weaken hurricanes. Joanne headed Stormfury for a time, and Bob was head of the USA's first hurricane research project and also headed the National Hurricane Center in the late 1960s.

In brief, the final conclusion of Stormfury was that while cloud seeding might have some effect on hurricane strength, at least for a while, it's impossible to say whether a storm weakened because it was seeded or because it would have weakened anyway. Hurricanes, and other tropical cyclones, naturally gain and lose strength.

Scientists who study tropical cyclones don't see any way to destroy or change the paths of these storms and I know of no research into this topic that's now going on. The general opinion now is that any money available for hurricane research is much better spent of improving understanding of tropical cyclones with the goal of improving forecasts.

The USATODAY.com Stormfury attempted to weaken hurricanes story is a brief introduction to the topic. We also have an FAQ on trying to weaken or destroy storms. The U.S. National Oceanic and Atmospheric Administration has a brief article on Stormfury on its Web site, which includes a diagram illustrating the Simpsons' Stormfury hypothesis. This page has a reference to a 1985 article in the Bulletin of the American Meteorological Society, which is one of the few overviews of the project that I found in print while we were writing Hurricane Watch. This is a good overview of the science behind Stormfury, but Bob and Joanne Simpson disagree with some of the article's conclusions. NOAA's Hurricane Research Division (HRD) also has some information on Stormfury in its Web history of the HRD.

Unfortunately, I don't speak or read French and am not at all familiar with what might be available in French on Stormfury. But, given the small amount of information that's available in English, I fear that even less is available in French.

Still, it might be worth your time to go to the Meteo France Web site to see if you can find any information on your nation's tropical cyclone forecasting and research. Since France's national weather service handles tropical cyclone forecasting for places such as Martinque in the Caribbean, the organization has an interest in tropical cyclones, although not as great as the USA, where hurricanes are one of the major weather threats to parts of the country where large numbers of people live.

I hope this helps.

(Answered by Jack Williams, USATODAY.com weather editor, March 7, 2003)

Q: How far north can tropical systems go before dissipating in the North Atlantic?

A: Believe it or not, former hurricanes and tropical storms have traversed the Atlantic and crashed ashore in the Europe. But, they are no longer classified as "tropical" in structure by the time they get there. The cold water of the North Atlantic changes them into extratropical storms – the same storms that daily affect the areas outside of the tropics. (Learn more about the differences in these storms.)

The reason: Hurricanes and tropical storms need warm water to survive and usually quickly die when they reach the cold North Atlantic. The North Atlantic goes from warm quickly to cold because of the cold Labrador Current between 50° and 55° north latitude – the same general latitude as the British Isles, and slightly farther north than the Canadian Maritime province of Newfoundland.

If a hurricane or tropical storm manages to follow the Gulf Stream — a warm current that traverses the Atlantic from the Straits of Florida to the British Isles — it might remain tropical a bit longer than usual. But then upper-level winds in the atmosphere that blow stronger at higher latitudes will usually kill any tropical systems moving into higher latitudes by toppling their thunderstorms with wind shear – winds blowing much faster aloft than near the ground. Learn more in a USATODAY.com interactive graphic that explainshow wind shear weakens hurricanes.

Even though a former tropical storm or hurricane might not be classified as tropical system by the time it reaches Europe it can still do hurricane-like damage with wind gusts reaching 100 mph or more in a few storms. Some — but not all — of the British Isles' worst storms were former hurricanes.

You can read more about the many recent storms that have hit the British Isles by going to USATODAY.com's Weather front page and typing the following into the search engine you'll find at the bottom of the navigation links on the left side of the page: storm AND Britain. Don't use quotation marks, but be sure to capitalize the word AND between storm and Britain.

(Answered by Chris Cappella, USATODAY.com Weather Team, Feb. 11, 2003)

Q: Has there ever been any type of tropical development in the Mediterranean Sea?

A: In 1947, 1969, 1982, 1983 and 1995 storms were spotted over the Mediterranean Sea that seem to have at least some characteristics of tropical storms, such as an eye. The January 1995 storm was the best observed of these, both with satellites and the German weather ship Meteor measured sustained winds of 83 mph.

Jack Beven, a forecaster at the National Hurricane Center, has information on the 1995 storm on his personal Web site. He notes that surface and upper air temperatures and water temperature were much colder than you would expect in a tropical system. He says that some scientists think it was more like a "polar low," an extratropical cyclone, that forms over cold oceans, but sometimes develop an eye.

(Answered by Jack Williams, USATODAY.com weather editor, Oct. 28, 2002)

Q: I was trying to figure out why the powers that be felt so sure that Lili would move to the north, away from Galveston. It seemed to have been headed northeast for a while. I heard something about an upper-level trough. Are these things predictable and consistent?

A: History has shown that hurricanes are rarely consistent. But, hurricane forecasters are finding that with more ways of gathering observations over remote areas such as the vast oceans, accurate predictions of hurricane tracks can be made. Three-day forecasts still have an error in position of about 250 miles, but that was 315 miles a decade ago, and steadily decreasing.

For instance, as early as the night of Sept. 30 National Hurricane Center predictions showed Hurricane Lili would curve north in the Gulf of Mexico and strike Louisiana on the morning of Oct. 3, 2002. As you mentioned, there was a trough of low pressure moving into the central part of the USA. Winds ahead of the trough were forecast to increase out of the southwest, which would help make Lili's turn. But the timing of the trough's arrival was late enough in Lili's journey across the Gulf to begin the turn when the storm was nearly ashore. Had the trough slowed down or not arrived at all, Lili likely would have continued northwest and hit Texas. Learn more about how troughs can deflect hurricanes in a USATODAY.com graphic.

Computer models of the atmosphere that are used to forecast weather had a good handle on the trough's progression. Other computer models working to forecast Hurricane Lili also performed exceptionally well, Sim Aberson of the National Oceanic and Atmospheric Administration's Hurricane Research Division says.

What helped was repeated measurements of the atmosphere surrounding Lili at an altitude where steering winds from various weather systems would have an effect on the hurricane's future motion. The measurements were made using a NOAA Gulfstream IV jet – one of the trio of hurricane hunter airplanes NOAA flies to study hurricanes. They were fed into the models adding crucial information about how the weather was evolving over the Gulf of Mexico as Lili arrived from its journey over the Caribbean. With the additional data, forecasters were able to give Louisiana three days notice of an impending landfall. Lili's growing size and intensity kept Houston and New Orleans under the threat of at least Lili's fringe effects.

But, Lili's rapid intensification and sudden weakening also showed forecasting hurricane intensity is largely an educated guessing game. Read more in an online story: Scientists don't yet know why Lili suddenly collapsed.

Learn more about the hurricane hunters, and about forecasting hurricanes using computer models.

You mention that Lili was moving northeast for awhile. Likely this appeared true in a satellite loop. But NHC never said Lili moved northeast until after landfall. Possibly what the satellite loop showed was a wobble in the location of the center with respect to its shield of overlying, dense and cold cloud tops. Wobbles in the eyes of intense hurricanes near landfall are common, and do not indicate a sudden change in direction.

(Answered by Chris Cappella, USATODAY.com Weather Team, Oct. 6, 2002)

Q: Why do hurricanes travel from east to west across the Atlantic while all of the other weather we get in the USA comes from the west and moves east?

A: Hurricanes are born and spend their early lives in the tropics, where the general flow of the winds is from east to west. Winds at all levels of the atmosphere push hurricanes along. When a hurricane moves far enough north, the general west-to-east middle latitude winds begin pushing it from west to east. If the storm turns toward the north soon enough,it will stay over the Atlantic Ocean and begin moving east before it hits the USA.

A USATODAY.com graphic shows how different steering winds caused one hurricane to hit the USA and another miss in 1996.

I haven't found any Web sites that do a good job of explaining global winds patterns; why tropical winds are from the east while middle latitude winds are from the west. I do cover this topic in detail, however, in the

USA TODAY Weather Book

.

(Answered by Jack Williams, USATODAY.com weather editor, June 4, 2002)

Q: I live in São Paulo in southeastern Brazil and I would like to know why hurricanes do not develop in the Southern Atlantic Ocean, affecting the Brazilian coast, knowing that the Atlantic Ocean has warm waters and strong winds that could provoke hurricanes in the middle of the ocean?

A: Hurricanes need warm water and a light-wind environment to form. While the Atlantic Ocean south of the equator does not warm quite as much as the Atlantic Ocean north of the equator, it's still plenty warm in spots to grow hurricanes. But, few ever have a chance because high-altitude winds blow strongly too close to the equator in the South Atlantic Ocean, inhibiting storm formation or ripping apart fledgling storms.

The question below has a much more detailed answer to this question.

(Answered by Chris Cappella, USATODAY.com Weather Team, Feb. 5, 2003)

Q: Why are there few or no hurricanes or tropical storms in the South Atlantic?

A: Only one hurricane is on record as occurring in the South Atlantic. This was the storm that his Brazil about 520 miles south of Rio de Janeiro in March 2004. Here are the USATODAY.com stories on this hurricane:

• First South Atlantic hurricane hits Brazil
• Forecasters tracking first-ever South Atlantic hurricane

Before this storm, the U.S. National Hurricane Center did spot in satellite images what was either a strong tropical depression or a weak tropical storm (it's hard to tell the difference using only satellite images) that formed off the African Coast in April 1991. It lasted about five days.

This is a question that I talked about with Bill Gray, the hurricane researcher who has become well known for his hurricane season forecasts, when I was working with Bob Sheets on our book, Hurricane Watch: Forecasting the Deadliest Storms on Earth. In the mid 1960s Gray did an extensive study of global tropical cyclones and continues to research tropical cyclones around the world. He was among the first to come up with the conditions needed for tropical cyclones to form. Gray says that the water of the South Atlantic is warm enough, but that strong upper-level winds from the west are closer to the equator than in the Northern Hemisphere. This is one of the effects of the Southern Hemisphere having a lot less land than the Northern Hemisphere.

These strong upper level winds create "wind shear" that can keep a disturbance from growing into a storm or can rip apart a storm. A USATODAY.com graphic on Wind shear weakens hurricanes has more on this. This graphic is one of the many graphics and pages of text you'll find by going to our Understanding hurricane science, technology page.

Also, Gray told me, the Intertropical Convergence Zone (ITCZ), the area where the trade winds converge to create bands of thunderstorms across the tropics, mostly stays north of the equator in the Atlantic in both the northern and southern hemisphere summers. This is in contrast to the western Pacific and southern Indian oceans where the ITZ dips south of the equator during the Southern Hemisphere summer. ITCZ thunderstorms are one source of the disturbances that can grow into tropical cyclones. A graphic on page 43 of the second edition of the USA TODAY Weather Book shows how the word's major pressure patterns, winds and ITCZ differ in January and July.

(Originally Answered by Jack Williams, USATODAY.com weather editor, June 4, 2002 and updated on June 22, 2004)

Q: How much energy is generated daily by a major (category 3 or more) hurricane? I have heard that a major storm (like Andrew) releases as much energy as a nuclear explosion, daily.

A: It's a whole lot more than that. The figure you usually see is that the latent heat energy released in a hurricane in one day could be equivalent of the energy released by 400, twenty-megaton hydrogen bombs. Or, if that energy could be captured and turned into electricity with no losses, it could supply the electrical needs of all of the United States for about six months. I've never seen anyone who's worked out this figure for different categories of hurricanes, but this very general one. If anyone is interested I think I have an article in my files at home with a reference to the 1950s or early 1960s scientific article the figure comes from. Anyway, the amount of energy involved is one reason why the idea of using nuclear bombs to destroy hurricanes is not likely to work. A USATODAY.com FAQ on trying to weaken or destroy hurricanes has more on this.

(Answered by Jack Williams, USATODAY.com Weather Editor, 10-21-99)

Q: What is the hurricane's deadly weapon: storm surge or the high winds?

A: Hurricane safety materials often say that storm surge is the biggest killer in hurricanes, but that hasn't been true in the United States for almost 30 years. Historically, surge has been the big killer and it certainly has the potential to kill more people than hurricane winds. In 1969 Hurricane Camille killed 256 people. About half of those deaths were from storm surge when Camille came ashore in Mississippi. Most of the other half were killed when the storm's remnants caused flash floods in Virginia. Camille's surge toll would have been much worse if not for a last-minute report from an Air Force hurricane hunter airplane that the storm had strengthened and a prediction from a new storm surge computer model that it would cause major storm surge. The National Hurricane Center stressed the surge danger in its forecasts and local emergency management officials took the warnings seriously and evacuated thousands of people at the last minute. Since Camille storm surge has killed only a half dozen people in the United States because the combination of storm surge forecasts and large evacuations has been getting people out of the way of surge.

This doesn't mean we don't have to worry about surge deaths any longer. As coastal population grows there is a chance that a forecast won't be made in time to get everyone out of the way of the surge from a major hurricane. The Florida Keys and New Orleans are two major areas of concern because a quickly intensifying storm might not allow time to get everyone out of the way of storm surge.

The big killer in the USA and also in cases such as Hurricane Mitch last year in Central America has been "fresh water" flooding; that is floods caused by a storm's heavy rain falling inland. Mud slides caused by heavy rain are also a big killer, especially in the parts of the Caribbean, Central America and Mexico with mountains.

Of course wind is a danger mainly because of the debris it tosses around. As many victims of Hurricane Andrew's winds learned in 1992, following the tornado safety rules of hunkering down in a small room away from windows can save your life even when the storm is taking the roof off your house and throwing pieces of other people's houses through your windows. Many who rebuilt after Andrew have included in-home tornado shelters. This is a good idea for anyone near the U.S. Gulf of Mexico or Atlantic coasts whose home is away from potential storm surge flood areas. You wouldn't want to be drowned by surge in your otherwise safe, home tornado shelter. You'll find links to a lot of information about Camille and other past hurricanes on the USATODAY.com hurricane history page. (7-6-99)

Q: I am a scientist myself. I have a question. Water temperature decreases with the depth of the ocean till a minimum of 4 degrees Celsius about 2,000 feet below. Why not pump it up to cool the surface before a hurricane arrives?

A: At first glance this might seem like a good idea because hurricanes sometimes weaken themselves by staying in place and churning up cool water. A USATODAY.com graphic shows how this happens. As it shows, you wouldn't have to pump water up from the bottom of the ocean.

But, trying to weaken a hurricane by cooling the water runs into three kinds of problems:

• The size of the area that would have to be cooled,
• Getting the cool water to the correct place,
• The amount of energy (which costs money) needed to pump up enough water.

A hurricane or tropical storm is 100 or more miles across. Of course the strongest convection that supplies a good share of the storm's energy is around the eye, maybe an area only 20 or 30 miles across. But, even if you could pump up water in this area, the storm would most likely redevelop over the warmer water outside the area of cold water. This would be similar to the ways storms regain strength after weakening as they travel over the Island of Hispaniola. If you wanted to use ships, and it would take many, to pump up the water you'd have to move them into the right part of the ocean. But, even today's improved hurricane track forecasts are not good enough to ensure you'd always get the ships to the right place in time.

You might think about installing pumps in the sea for miles around some place, such as St. Maartin, and turning them on when a storm approaches. But, for the cost of doing that, you could probably build a houses, apartment buildings and hotels for everyone on St. Maartin, including tourists, that would stand up to a hurricane with only minor damage. In general, those who study hurricanes are convinced that the best investments are in improved forecasts, ways of evacuating people from dangerous areas, and building much stronger houses and other structures that even relatively weak hurricanes wouldn't destroy.

A USATODAY.com FAQ has more information on trying to weaken or destroy hurricanes. (5-31-99)

Q: What force keeps a hurricane or its winds going around for so long? What happens in layman terms, please. My father, 87, wants to know. Thanks.

Q: How do hurricanes get started?

A: When water vapor in the air (this is the invisible form of water as a gas that you feel as humidity) condenses to form cloud drops or turns directly into ice it releases that heat that it picked up when it evaporated. This is called "latent" heat. Hurricanes form, grow and stay alive over warm oceans with very humid air in a fairly deep layer over the ocean. Air in the center of the storm is rising, which makes the air cool. As the air cools, the water vapor condenses, which releases heat. This warms up the air, making it rise just as a hot-air balloon rises. As the air rises, more air flows in to replace it, causing the winds. The Earth's rotation makes the winds spiral into the storm's center. If you click here, you'll go to one of our graphics that explains more about this.

Obviously, for a hurricane to get started, you need to have warm ocean water and plenty of warm, humid air over that ocean. You also need to have winds at all levels of the atmosphere coming from pretty much the same direction at about the same speeds to avoid wind shear that can rip apart a storm. When these and other factors are present, a tropical depression and form and grow into a hurricane. A USA TODAY graphic shows the life cycle of a typical hurricane. (9-27-98)

Q: On Friday and Saturday there were four hurricanes in the Atlantic. Two of them are relatively close to each other. Have two hurricanes ever collided? If so, what happens? I imagine they would both be dissipated.

A: You're close to being right. Storms don't actually collide, their winds begin interfering with each other. Often one of the storms will dissipate. But, sometimes the two storms will do a kind of dance around each other. Click here for more on this. (9-27-98)

Q: Can you tell me why is the center of a hurricane is not windy or rainy but is sunny?

A: A hurricane's winds are spiraling toward the storm's center, that is they are going in a circle around and toward the center. As the wind nears a hurricane's center it goes faster and faster. It's somewhat like being in a car trying to go around a sharp curve as high speed, if you don't have a seat belt on you slide away from the center of the curve. If you try to go too fast, the car itself will skid away from the center of the curve. In a hurricane, this "centrifugal force" is pushing the wind away from the center, air pressure is pushing it toward the center. The resulting balance of forces forms the eye, a mostly calm areas with winds roaring around it in a circle. The eye is often cloud-free because no wind is blowing into it with humidity needed to form clouds. Hurricane eyes are very unusual places. Our hurricane science index has more about how hurricanes work. (12-2-96)

Q: The Oregon coast often has winds in excess of 100 mph. Why are these storms not reported as hurricanes like those on the east coast?

A: The strong winds that hit the Pacific Coast are from extratropical storms, not hurricanes, which are tropical cyclones. While 100 mph winds are strong for an extratropical storm, a 100 mph hurricane is relatively weak compared with what hurricanes can do. The much greater destruction hurricanes bring led to the establishment of the National Hurricane Center and the other special attention given to these storms. The term "hurricane force winds" causes confusion. While any wind 75 mph or faster is "hurricane force," such winds don't always come from hurricanes. USA TODAY online has a file with more information on eastern Pacific tropical and extratropical storms, including one storm that began life as a tropical storm but was extratropical when it hit Oregon with 100 mph plus winds. (9-16-96)