This satellite image provided by NOAA shows Hurricane Felix as it moves inland over the countries of Central America at 14:15 EDT Tuesday, Sept 4, 2007. Felix roared ashore before dawn as a Category 5 storm along Nicaragua's remote northeast corner _ an isolated, swampy jungle where people get around mainly by canoe. But later Tuesday evening the National Hurricane Center said Felix had been downgraded to a tropical storm, with sustained winds near 96 km/h. Country borders drawn on the image were added by the source.
ADARSAT image of Hurricane Alma in the Pacific taken on May 30, 2002. (Courtesy Canada Centre for Remote Sensing).What are tropical storms?
Severe storms spawned in the tropics are known by different names in different parts of the world: hurricanes in the Atlantic and east Pacific; typhoons in the northwest Pacific and cyclones in the southwest Pacific and Indian Ocean. These storms originate over tropical waters, close to the equator. If the atmosphere is calm and the water is warmer than about 27 C, evaporation forces large amounts of moisture into the air, creating a low-pressure system. When this water vapour condenses, it releases heat that powers the circular winds that characterize these storms.
Rainfall in the developing storm releases more heat, triggering a convection process that pulls more moisture-laden air up through the centre of the system. The storm grows via this feedback mechanism. The strongest winds are found immediately outside the centre, or "eye," of the hurricane at ground level.
Every one of these systems begins as a tropical depression, a system of thunderstorms with an overall circular motion and maximum sustained winds less than 62 km/h. When a storm becomes severe enough and the winds pick up to more than 62 km/h, it is designated a tropical storm. When the winds reach 119 km/h, the system is called a hurricane, typhoon or cyclone.
An average of 10 tropical storms develop over the Atlantic Ocean each year, of which about six become severe enough to be called hurricanes. Besides strong winds and heavy rain, these hurricanes also create a "storm surge," a massive wave beneath the centre of the storm. In the eye of the hurricane, air is sucked upward faster than it can rush in at the bottom. This lowers the atmospheric pressure under the eye of the storm; as a result, the eye tries to pull at the ocean itself, creating a bulge of water as much as six metres high that moves together with the storm.
Hurricane season officially runs from June 1 to November 30, but the most intense storms mainly occur between mid–August and mid–October.
How are tropical storms named?
A tropical storm is given a name if its winds reach a speed of 62 kilometres per hour. An international committee has drawn up a list of 126 names - half male and half female - which are repeated after a six-year cycle. There are 21 names on the list for a given year, with each name corresponding to a letter of the alphabet. The letters Q, U, X, Y and Z are skipped.
If the list of names is exhausted for a given year, which may occur in 2005 with the formation of tropical storm Wilma in October, subsequent storms are named after the letters of the Greek alphabet: Alpha, Beta, Gamma and so on.
However, if a hurricane causes extensive damage, its name is retired from use. So far, more than 50 names have been retired, from Hazel in 1954 to Juan in 2003.
The word "hurricane" itself comes from the Carib Indian word "hurican," which referred to the tribe's god of evil. This may in turn derive from the name of a Mayan god who created the world with his breath, blowing on the oceans to create dry land.
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