Anatomy of a hurricane

Hurricane season comes every June through November. Though Americans dread the storms, eastern Asia and many other areas of the world depend on hurricanes to bring much of their needed rainfall.

In the eastern Pacific, the storms are known as "typhoons" (probably from Cantonese Chinese toi fung, "great wind"), but are "cyclones" in the Indian Ocean (from Greek kuklon, "rotating"). Australians call them "willy-willys", wrote Frederick K. Lutgens and Edward J. Tarbuck in The Atmosphere ( Prentice-Hall, 1979). Our word "hurricane" comes from Carib huracan.

In the Encyclopedia of Earth and Physical Science (Marshall Cavendish, 1998), Shelly Fennel listed the North Atlantic’s most hurricane-prone areas: the Cape Verde Islands, the West Indes (east and north sections), the north and southwest Caribbean, and the Gulf of Mexico. Hurricanes spring up on either side of the equator, but the Coriolis effect keeps them from forming within 5 degrees of that line Caused by the earth’s movement, this effect knocks oncoming winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere (it’s also responsible for the spiraling winds that drive the storms).

Up until a few years ago,  hurricanes didn’t rage in the South Atlantic, because those waters were simply too cool (the ocean surface must be above 82º F for a hurricane to form). But in March of 2004, the first-ever hurricane on record formed in that area of the ocean, striking Brazil (see http://www.usatoday.com/weather/news/2004-03-28-brazil-storm_x.htm) 

Hurricanes arise when forces disturb the speed and direction of tropical trade winds. The most common disturbance is the "tropical wave", a low-pressure zone that travels westward over tropical waters. Other culprits are the low-pressure doldrums that shift up and down the equator as the seasons change, and "midlatitude weather disturbances" drawn to the equator from subtropical seas.

According to The New Book of Popular Science (Grolier, 1996), "a hurricane will develop from one of (these) disturbances when its upper-level winds push away the air that was previously lying over the top of the weather system. This results in a central area of low pressure, the eye, around which the hurricane will develop." Temperatures within this "eye" are relatively high.

Lutgens and Tarbuck described a hurricane as "a heat engine", each one generating power approaching the "amount of electricity consumed in the United States over a 6-month period". Because a storm needs a continual supply of moisture-laden air, it draws vast amounts of vapor rising from the warm surface of the ocean up through its center.

The storm will intensify if it can pump air out at its top faster than the water surface can replace it. These masses of vapor condense in the high altitudes, releasing heavy precipitation and the heat energy that makes the clouds swirl faster and faster. The resulting cumulonimbus (or thick, towering rain-bearing) clouds make up the "doughnut-shaped" core of the hurricane. Long bands of other cumulonimbus clouds spin like a pinwheel around this "axle" at the eye (a typical hurricane system spans about 300 miles in diameter; the eye is usually around 12 miles across, with clouds 8 miles high). The eye wall is the ring of the thunderstorms surrounding the eye of a hurricane; these storms bring the most severe wind, rain and turbulence.

North Atlantic trade winds tend to move newly-formed hurricanes from east to west at around 15 miles per hour. "Then, almost without exception," wrote Lutgens and Tarbuck, "hurricanes move (north-) poleward and are deflected into the westerlies, which increase their forward motion up to a maximum of 100 kilometers (62 miles) per hour." They often zigzag off the coast as we make helpless guesses about where landfall may be.

Of course, all storms are not created equal. The Saffir/Simpson Hurricane Scale ranks hurricanes into five strength categories:

Category 1: Minimal damage (Winds 74-98 mph/Storm surge 4-5 feet);
Category 2: Moderate damage (Winds 96-110 mph/Storm surge 6-8 feet);
Category 3: Extensive damage (Winds 111-130 mph/Storm surge 9-12 feet);
Category 4: Extreme damage (Winds 131-155 mph/Storm surge 13-18 feet);
Category 5: Catastrophic damage (Winds 155 mph or more/Storm surge higher than 18 feet).
 

A storm will begin to die as soon as the air supply stops moving. That’s the main reason why hurricanes always weaken when they hit land or move over colder waters. The "drag" of the land surface also helps bring Goliath down, forcing the hurricane to collapse into its own low-pressure center.

HOW HURRICANES ARE NAMED

Though people have been naming hurricanes for centuries, meteorologists once identified storms by their latitude-longitude numbers, a practice that Chase's Calendar of Events called "cumbersome" and "subject to error." The current system of personal names is really just an A-W phonetic alphabet that is easy to remember, quick to communicate and which clearly identifies each new storm. The first names issued by the Miami-Dade County-based hurricane center in 1953 were women's names, and men's names joined the list in 1979. Atlantic and Eastern Pacific name lists are rotated year by year (weather officials will use the 2002 list again in 2008, with new names to replace those - like Hugo and Andrew - that have gone to major, "killer" storms).

2008 HURRICANE NAMES

Arthur
Bertha
Cristobal
Dolly
Edouard
Fay
Gustav
Hanna
Ike
Josephine
Kyle
Laura
Marco
Nana
Omar
Paloma
Rene
Sally
Teddy
Vicky
Wilfred

NOTE: This article is the intellectual property of the author, denseatoms, and appears on the Beaufort County (SC) Library web site at http://www.beaufortcountylibrary.org/rooms/documents/html/hurricanes.htm. It is reproduced here, with permission, for informational purposes.

PHOTO CREDIT: "Hurricane Katrina Satellite Image" http://flickr.com/photos/gisuser/40719712/, under Creative Commons license Attribution 2.0 Generic. This permission in no way implies endorsement of denseatoms or the use of the work.