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Atahualpa
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Hawthrone
Mar 1999
time: 06:18
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I just read that to form a hurricane the sea would need to be at least 26°C warm and a lot of condensation need to occur.
The wikipedia article says:
quote:
Structurally, a tropical cyclone is a large, rotating area of clouds, wind and thunderstorm activity. The primary energy source of a tropical cyclone is the release of the heat of condensation from water vapor condensing at high altitudes. Because of this, a tropical cyclone can be thought of as a giant vertical heat engine.
The ingredients for a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist long enough, they can combine to produce the violent winds, incredible waves, torrential rains, and floods associated with this phenomenon.
This use of condensation as a driving force is the primary difference which distinguishes tropical cyclones from other meteorological phenomena. Mid-latitude cyclones, for example, draw their energy mostly from pre-existing temperature gradients in the atmosphere. In order to continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the atmospheric moisture needed. The evaporation of this moisture is driven by the high winds and reduced atmospheric pressure present in the storm, resulting in a sustaining cycle. As a result, when a tropical cyclone passes over land, its strength will diminish rapidly.
A hurricane can be viewed as a localized, runaway greenhouse effect. The driving force, the source of the energy, is the sun. The sun warms the water and leads to increased evaporation. Water vapor is an effective greenhouse gas. The heating of the oceanic water is never quite uniform. When locally there is increased temperature, there will be increased evaporation providing a positive feedback mechanism. As the heated air expands and rises, the air pressure is lowered. The air cannot rise indefinitely and will spread at higher altitudes, where the vapor will condense and form clouds. The clouds shield the underlaying water by reflecting the solar radiation. This enhances the localizing effect. The Coriolis effect provides for the circular motion. |
This makes me wonder, since with all the talk about global warming the number of hurricanes would have to increase.
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shawnmmcc
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I do aviation weather for a living, and hurricanes have a tremendous impact on it. Just a small change in the surface temperatures of the Carribean and the Atlantic spawning grounds for hurricanes can make a tremendous change in both their severity, frequency, and the length of the season. It is having an effect, don't take just my word on it or the people in the Bush adminstration (the question is not whether there is global warming, or if it is going to have substantial effects on humans - the question is how much of it is due to human factors, and are those human factors primarily greenhouse gases).
quote:
Four named storms in early July set record
The Associated Press
Arlene, Bret, Cindy and now Dennis. Storm hunters don't expect to be hunched over their radar screens and dispatching chase aircraft until Labor Day. But 2005 is no normal year.
Martin Nelson, the lead forecaster at the National Hurricane Center in Miami, says this is the first time the Atlantic hurricane season had four named storms this early since record-keeping began in 1851. The season runs from June 1 to Nov. 30.
The first three storms never grew beyond tropical storms that dumped rain and cut utilities from Louisiana to the Carolinas. Dennis got its name on July 5 and two days later it had morphed into a Category 4 monster with winds reaching 150 mph. It also is the earliest occurrence of a Category 4 hurricane in the Caribbean, and possibly the U.S., meteorologists say.
Having killed 20 people in Haiti and Cuba, now Dennis has set its sights on the U.S. Gulf Coast. Forecasters predict it will regain strength over warm waters of the Gulf of Mexico and the storm will hit the mainland anywhere from Florida to Louisiana on Sunday.
Researchers have several ideas why this hurricane season is beginning so ferociously, but they say one thing appears likely.
"If you get these really early big storms," says senior research meteorologist Hugh Willoughby of Florida International University in Miami, "that means it is likely to be an active season."
That's just what 65 million Americans living on the Gulf and Atlantic coasts don't want to hear.
Hurricanes are among nature's most fearsome events. A storm can span 400 miles and tower 10 miles high. Inhaling energy from warm seawater, it might churn for a week or more across 3,000 miles before it collapses.
Some people still are rebuilding from last year when five hurricanes and four tropical storms pounded the Atlantic and Caribbean basins in August and September. At least four of the storms caused hundreds of deaths and billions of dollars in damages. Scientists called it a "once in a lifetime kind of a year."
Did they speak too soon? Maybe.
That's because the meteorological conditions that spawned last season's destruction are persisting in the Atlantic and Caribbean basins this year, and possibly for decades to come.
Forecaster William Gray at Colorado State University has upped his 2005 Atlantic hurricane forecast three times since December, beginning with 11 named storms, then 13, then 15. Now he is saying the number of named storms will be "significantly above" the long-term average of 9.6 named storms and 5.9 hurricanes. At least four storms may blow up into major hurricanes like Dennis, nearly twice as many as normal.
The chance of a major hurricane making landfall somewhere on the East Coast, including the Florida peninsula, is nearly twice as high as in an average year, Gray says. For the Gulf coast from Pensacola, Fla. to Brownsville, Texas, the risk is about one-third higher.
Gray and others base their judgments on several measurements of atmospheric and ocean conditions worldwide.
Quiet hurricane seasons coincide with El Nino conditions in the Pacific. When Pacific water temperatures rise, it changes global wind patterns. High in the atmosphere, wind shear knocks down storms that arise in the Atlantic, preventing many from reaching wind speeds of at least 74 mph.
But in stormy years like 2005, Atlantic sea surface temperatures are warming above 81 degrees. Without much wind shear, humid westerly winds from Africa's bulge grow stronger. The warmer ocean heats the air in a rising column, creating a center of moist low pressure.
Trade winds rush in toward this depression. Combined with the planet's rotation, they spin clouds counterclockwise around this steamy core, or "eye" of the storm.
Conditions in the Gulf of Mexico can perpetuate these storms over days and hundreds of miles.
Normally, the Gulf consists of a thin layer of warm water that rides atop a foundation of cold seawater. When storms cross into the basin, the winds churn these layers and the colder water pulls the plug on the storm's motor.
But sometimes, the large Loop Current spawns deep pockets of warm water called eddies that move east-to-west and cover up to 20% across the Gulf. If a hurricane happens to pass over one of these eddies, it acts like a shot of espresso and re-caffeinates the storm.
Louisiana state climatologist Barry Keim says surface temperatures in the Gulf are at least 81 degrees, adding to the dangerous conditions.
"This is going to be a big, bad storm," Keim said of Hurricane Dennis. "The fuel for the storm is the energy of the evaporation off the Gulf surface. Warmer water means more fuel to feed the system."
Experts believe the current hurricane surge is part of an obvious storm cycle.
Roughly from 1970-94, Atlantic hurricane activity in the United States was relatively mild.
But 1995-2004 is the most active 10 consecutive hurricane seasons on record, Gray says. The cycle of heightened activity could last another 20 years or more.
The trend is believed to be a consequence of natural salinity and temperature changes in the Atlantic's deep current circulation that shift back and forth every 40-60 years.
Keim says the last year there were this many named storms early in the storm season was 1959, with the fourth named on July 7, 1959. In 1900, there were four storms by mid-July, but only one made landfall, Willoughby said. |
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All times are GMT. The time now is 05:18.
Apolyton Time is 00:18. |
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10-07-2005 22:38
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