Why do you think we are having more severe hurricane seasons?
Hurricanes
The strong rotating storms called hurricanes in the USA and elsewhere in the tropics are called typhoons in south-east Asia and cyclones in the Indian Ocean. Tropical storms are only upgraded to hurricane status if they sustain wind speeds above 73mph (force 12 on the Beaufort Scale). There are about 50 hurricanes a year. A hurricane that hit Galveston, Texas in 1900 was responsible for the deadliest natural disaster in U.S. history, killing 6,000 to 10,000 people. Hopefully with modern warnings and evacuations this record will never be beaten. Hurricane Andrew provided the costliest disaster in 1992.
For a hurricane to form, the surface temperature of the sea needs to be above 26.5°C (79.7°F), so rising sea temperatures due to global warming may well mean more and stronger hurricanes. Hurricanes derive their power from the heat energy released when water vapour condenses. The warm air above the ocean rises rapidly as the sea heats it, and begins to rotate. In the centre is the 'eye', an area of low pressure, which is calm in contrast to the rapidly spinning winds around it. Without the warmth of the oceans to continue fuelling them, hurricanes tend to start to fizzle out once they have hit the mainland, but not before wreaking havoc. When they do hit inland regions, or regions off the normal path, or head in a different direction to forecast, they can catch people out (Hurricane Forecasters Relying on Public).
Hurricanes can be 400 miles wide and 10 miles high, and can cross 3,000 miles in a week or longer before they run out of energy by making landfall or passing over colder water. The Gulf of Mexico has a thin layer of warm water on the surface that keeps storms going until churning in the basin brings up colder waters from underneath. If a storm passes over a warm current or eddy, it picks up strength. Hurricane seasons tend to run in cycles, with 1970-94 being quiet in the Atlantic, and 1995-2004 being particularly active. There is a natural cycle in the Atlantic's deep current circulation salinity and temperature that lasts 40-60 years; there were 4 named storms by July 7 1959, and by mid July in 1900. Because of this cycle, it is difficult to find agreement about whether the current severe storm seasons are due to global warming or not, and it may not be obvious for 20 years or more.
The National Hurricane Center uses the Saffir-Simpson damage potential scale to rate hurricanes for potential or actual damage. Category 1 has sustained wind speeds of 74-95 mph, category 2 is 96-110 mph, category 3 is 111-130 mph , category 4 is 131-155 mph , and category 5 is greater than 155 mph. Hurricane Gilbert, which devastated Jamaica in 1998, reached 160mph. A 10 percent increase in wind speeds leads to a 20 percent increase in destructive force.
During El Nino years the waters of the eastern tropical Pacific are warmer than average, but because global wind patterns are changed, this actually prevents as many hurricanes forming, rather than fuelling more or stronger ones. When the sea surface temperatures of the Atlantic exceed 81°F (27°C or so), as in 2005, storms are more likely.
In the USA the season runs from June 1 to Nov. 30. 2005 has already broken records started in 1851, however, by having four named storms by early July: Arlene, Bret, Cindy and Dennis, and Emily was not far behind. Dennis was also the earliest Category 4 hurricane in the Caribbean, and perhaps in the U.S. Such early big storms means that a bad hurricane season is likely. This is bad news for those who experienced the "once in a lifetime kind of a year" in August and September of 2004 when five hurricanes and four tropical storms hit the Atlantic and Caribbean basins. The average is 9.6 named storms and 5.9 hurricanes in a season.
Apart from the damage from the strength of the winds, hurricanes can cause huge waves and massive flooding due to torrential rain which often passes over an area only slowly (perhaps 20-25 mph). Storm surges of up to 4m can also flood low-lying areas of the coast (Flooding Surprises Village Far From Dennis). These occur because the eye of the storm is at very low pressure, and the sea rises 1cm for every millibar of pressure, and can be pushed along by the strong winds.
The names prepared for tropical storms originating in the Atlantic in 2005 are: Arlene, Bret, Cindy, Dennis, Emily, Franklin, Gert, Harvey, Irene, Jose, Katrina, Lee, Maria, Nate, Ophelia, Philippe, Rita, Stan, Tammy, Vince, and Wilma. Storms in the Pacific have a different set of names. Once in a while a Pacific storm will head "the wrong way" (east rather than west), as Adrian did this year. If Adrian had crossed the land and regrouped in the Atlantic, it would have switched names and genders and become Arlene. This happens rarely. The names of some of the most severs hurricanes, such as Andrew, will never be re-used.
Emily Weakens After Striking Grenada,
Jamaica a hurricane target once again
Hurricane Emily headed towards Jamaica, the Cayman Islands and Mexico's Yucatan Peninsula, weakening slightly to a category 3. The hurricane is forecast to hit the mainland between Tuxpan, Mexico, and Galveston, Texas. Jamaica was hit by hurricane Dennis.
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