Hurricanes

Since this is Hurricanes season in the USA (Hurricane Ike is on the way to hit my place this weekend. So I’m back from work early today), thought of gathering some information about Hurricanes and putting it together for our visitors. In this article, I have explained the following about hurricanes.

What is a Hurricane?
Hurricane Structure
How Hurricanes Form and Die
Hurricane Classification and Extremes
Naming Hurricanes and Tropical Storms (This will be more interesting)
Hurricane Preparedness tips

What is a Hurricane?
Originally derived from the word huracán from the Caribbean Taino Amerindian language (and first adopted into Spanish), hurricane most commonly denotes a tropical cyclone. The term is most often used for cyclones occurring in the Atlantic and eastern Pacific Oceans. (Reference Wikipedia).

A hurricane is a powerful, rotating storm that forms over warm oceans near the Equator. Another name for a hurricane is a tropical cyclone. Hurricanes have strong, rotating winds (at least 74 miles per hour or 119 kilometers per hour), a huge amount of rain, low air pressure, thunder and lightning. The cyclonic winds of a hurricane rotate in a counterclockwise direction around a central, calm eye.

If this type of storm forms in the western Pacific Ocean, it is called a typhoon.

Hurricanes often travel from the ocean to the coast and on to land, where the wind, rain, and huge waves can cause extensive destruction.
Generally, when a hurricane moves over land (or over cold ocean waters) the storm begins to weaken and quickly dies down because the storm is fueled by warm water.
On average, there are about 100 tropical cyclones worldwide each year; 12 of these form in the Atlantic Ocean, 15 form in the eastern Pacfic Ocean and the rest are in other areas.
Hurricane season is the time when most Atlantic Ocean hurricanes occur; it is from June 1 until November 30. In the eastern Pacific Ocean, hurricane season is from May 15 until November 30.

Hurricane Structure
Hurricane winds blow in a counterclockwise spiral around the calm, roughly circular center called the eye. In the eye, which is roughly 20 to 30 miles wide, it is relatively calm and there is little or no rain.
The eye is the warmest part of the storm. Surrounding the eye is the eyewall, a wall of thunder clouds. The eyewall has the most rain and the strongest winds of the storm, gusting up to 225 mph (360 km/h) in severe storms.

The smaller the eye, the stronger the winds. The winds spiral in a counterclockwise (in the northern hemisphere) direction into the storm's low-pressure center. Long bands of rain clouds appear to spiral inward to the eyewall -- these are called spiral rainbands.

Hurricanes can be hundreds of miles across. In addition to rotating with wind speeds of at least 74 mph, a hurricane travels relatively slowly across the ocean or land, usually at about 20 to 25 mph. If you are facing in the direction that the hurricane is traveling, the right side generally has the fastest winds, and the left side usually has the most rain.

How Hurricanes Form and Die:
The birth of a hurricane requires at least three conditions.

First, the ocean waters must be warm enough at the surface to put enough heat and moisture into the overlying atmosphere to provide the potential fuel for the thermodynamic engine that a hurricane becomes.

Second, atmospheric moisture from sea water evaporation must combine with that heat and energy to form the powerful engine needed to propel a hurricane.

Third, a wind pattern must be near the ocean surface to spirals air inward. Bands of thunderstorms form, allowing the air to warm further and rise higher into the atmosphere. If the winds at these higher levels are relatively light, this structure can remain intact and grow stronger: the beginnings of a hurricane!

Often, the feature that triggers the development of a hurricane is some pre-existing weather disturbance in the tropical circulation. For example, some of the largest and most destructive hurricanes originate from weather disturbances that form as squall lines over Western Africa and subsequently move westward off the coast and over warm water, where they gradually intensify into hurricanes.

Hurricane winds in the northern hemisphere circulate in a counterclockwise motion around the hurricane's center or "eye," while hurricane winds in the southern hemisphere circulate clockwise.

The eye of a hurricane is relatively calm. It is generally 20 to 30 miles wide (the hurricane istself may extend outward 400 miles). The most violent activity takes place in the area immediately around the eye, called the "eyewall". At the top of the eyewall (up to 50,000 feet), most of the air is propelled outward, increasing the air's upward motion. Some of the air, however, moves inward and sinks into the eye, creating a cloud-free area.

Hurricanes form in the tropics, over warm ocean water (over 80ºF or 27ºC) and at latitudes between 8° and 20°, Hurricanes form mostly from June through November (hurricane season). These powerful storms are fueled by the heat energy that is released when water vapor condenses (turns into liquid water -- rain).

A hurricane goes through many stages as it develops:

It starts as a tropical wave, a westward-moving area of low air pressure.
As the warm, moist air over the ocean rises in the low air pressure area, cold air from above replaces it. This produces strong gusty winds, heavy rain and thunderclouds that is called a tropical disturbance.
As the air pressure drops and there are sustained winds up to 38 miles per hour, it is called a tropical depression.
When the cyclonic winds have sustained speeds from 39 to 73 miles per hour, it is called a tropical storm (storms are given names when they begin to have winds of this speed).
The storm becomes a hurricane when there are sustained winds of over 73 miles per hour.

The End of a Storm: When a hurricane travels over land or cold water, its energy source (warm water) is gone and the storm weakens, quickly dying.

Hurricane Classification and Extremes

Hurricanes are classified into five categories based on current maximum wind speed. This rating scale is called the Saffir-Simpson Hurricane Scale, named for Herb Saffir and Robert Simpson, who developed it.

Category 1 -- Winds 74-95 mph
Category 2 -- Winds 96-110 mph
Category 3 -- Winds 111-130 mph
Category 4 -- Winds 131-155 mph
Category 5 -- Winds over 155 mph (these are VERY rare)

Naming Tropical Storms and Hurricanes
Since a hurricane can last for a week or more, and there can be more than one storm at a time, weather forecasters give each storm a name so there is no confusion when talking about a particular storm.

Hurricanes names are chosen from a list selected by the World Meteorological Organization. The Atlantic is assigned six lists of names, with one list used each year. Every sixth year, the first list begins again. Each name on the list starts with a different letter, for example, the name of the very first hurricane of the season starts with the letter A, the next starts with the letter B, and so on. The letters "Q", "U", "X", "Y" and "Z", however, are not used. Women's and men's names are alternated. The name lists are made up by meteorologists at the World Meteorological Organization.

In the active hurricane season of 2005, Greek letters began to be used to name tropical storms after the letters of the alphabet had been used.
There are different name lists for Atlantic and eastern Pacific tropical storms. Storms are named as soon as the winds are 39 mph or more. The names of very destructive storms (like Andrew, Camille, Hugo, and Katrina) are retired (they are never used again). Since 1954, forty names have been retired. In 1996 Hurricane Luis was retired.

The History of Naming Tropical Storms
Until late in the 1940s, hurricanes were not officially named (hurricane forecasting was then in its infancy). Only the most severe hurricanes were given names, and they were often named for the place they did the most damage (such as the Galveston Hurricane of 1900) or the time they hit (such as the Labor Day Hurricane of 1935).

US meteorologists working in the Pacific Ocean began naming tropical cyclones during World War 2, when they often had to track multiple storms. They gave each storm a name in order to distinguish the cyclones from each other more quickly than referring to each storm by its position.

The first US named hurricane (unofficially named) was George, which hit in 1947. The next one given a name was Hurricane Bess (named for the First Lady of the USA, Bess Truman, in 1949). Various naming conventions were used until the use of women's names was adopted in 1953; the names used that year were: Alice, Barbara, Carol, Dolly, Edna, Florence, Gilda, Hazel, Irene, Jill, Katherine, Lucy, Mabel, Norma, Orpha, Patsy, Queen, Rachel, Susie, Tina, Una, Vicky, and Wallis.

Between 1953 and 1979, only women's names were used to name tropical storms. Since 1979, men's and women's names are alternated as names.

Hurricane Preparedness tips
Folks in the coastal areas, please visit following links and be prepared.

http://www.hhs.gov/news/facts/hurricane.html

http://www.nhc.noaa.gov/HAW2/english/action.shtml

http://www.nws.noaa.gov/nwr/

Closely monitor radio, TV, NOAA weather radio for official bulletins.

Thanks,
Ravindra Reddy
Please send in your suggestions and comments to ursravindra@gmail.com

Published On: August 03 2008
Published By: Ravindra Reddy