TEMP 19.1°C Ι DEW POINT 17.4°C Ι FEELS LIKE 20.0°C Ι HUMIDITY 90% Ι PRESSURE 998.7mb Ι WIND 5.4mph 180° S
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Published on: February 5, 2012, by: STW Editor

Hurricanes

Hurricanes, cyclones, tropical storms, typhoons….these are all the same thing, but are named differently depending where on earth they are formed.

A storm that is created in the North Atlantic Ocean, the Northeast Pacific Ocean (east of the dateline) or the South Pacific Ocean (east of 160E) will be called a Hurricane.  The same kind of storm that forms in the Northwest Pacific Ocean, west of the dateline, will be called a Typhoon.  A Tropical Cyclone will form in the Southwest Indian Ocean.  These kind of storms can cause catastrophic damage if they reach land.

How do they form?

In the case of a hurricane, the storm begins life in tropical regions. They form there because they need warm water of at least 26°C, high humidity with moist air, light winds, and very warm surface temperatures. Summer and the early fall months are perfect for hurricanes to brew up in the oceans around us. Most of the Atlantic hurricanes brew up on the coast of Africa. For that the northern hemisphere hurricane season is considered through the months of June and November. 

The first sign of a hurricane is a cluster of thunderstorms over tropical oceans. After the cluster of thunderstorms arrives they will break away and become better organized. It can take anywhere from hours to several days for a thunderstorm to actually turn into a hurricane.

Three things must happen for a hurricane to form though. A continuous evaporation and condensation cycle must take place, patterns of winds that are characterized by the converging winds, and a difference in air pressure between the surface and high altitude.  Warm and moist air from the ocean will begin to rise at rapid rates. As this warm air rises the water vapor condenses to form dark storm clouds and droplets of rain. Surfaces pressures begin to decrease as water vapor condenses and releases latent heat into areas where the tropical disturbance is located. (Latent heat is heat energy that is released during the phase change of water vapor.) This latent heat causes the air to become less dense. The warm air then rises; as it rises it becomes cooler and expands. That triggers more condensation and releases more latent heat, which allows more air to rise. A chain reaction is now in place. The exchange of the heat from the surface creates a pattern of wind that moves around the center.

Then converging winds—(these are winds moving in different directions that run into each other) at the surface collide and then push warm air downward and the moist air upward. The rising air backs up the air that is already rising from the surface. This makes windspeeds in the storm to increase.  In the meantime, strong winds that are blowing at high speeds and at high altitudes help to remove the hot rising air from the storms center. But if there are wind shears (Wind shears are when directions of wind and speed differ.) the storm will weaken.

If there are aren’t any wind shears the storm’s air in the upper atmosphere will rise to higher and higher pressures. Therefore the air cycle and hurricanes growth will get bigger and bigger. There you have your hurricane. Before a storm is classed as a hurricane it may first become a tropical depression,Winds near the center are constantly between 20 and 34 knots (23 – 39 mph).  Once maximum sustained winds are between 35-64 knots (39-73 mph), it becomes a tropical storm. It is at this time that it is assigned a name. During this time, the storm itself becomes more organized and begins to become more circular in shape — resembling a hurricane.

Hurricane strength is based on wind speeds, this is known as the Saffir Simpson Scale.  

Category One

No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs. Also, some coastal road flooding and minor pier damage. Hurricanes Allison of 1995 and Danny of 1997 were Category One hurricanes at peak intensity.

Category Two

Some roofing material, door, and window damage of buildings. Considerable damage to shrubbery and trees with some trees blown down. Considerable damage to mobile homes, poorly constructed signs, and piers. Coastal and low-lying escape routes flood 2-4 hours before arrival of the hurricane center. Small craft in unprotected anchorages break moorings. Hurricane Bertha of 1996 was a Category Two hurricane when it hit the North Carolina coast, while Hurricane Marilyn of 1995 was a Category Two Hurricane when it passed through the Virgin Islands.

Category Three

Some structural damage to small residences and utility buildings with a minor amount of curtainwall failures. Damage to shrubbery and trees with foliage blown off trees and large tress blown down. Mobile homes and poorly constructed signs are destroyed. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the hurricane center. Flooding near the coast destroys smaller structures with larger structures damaged by battering of floating debris. Terrain continuously lower than 5 ft above mean sea level may be flooded inland 8 miles (13 km) or more. Evacuation of low-lying residences with several blocks of the shoreline may be required. Hurricanes Roxanne of 1995 and Fran of 1996 were Category Three hurricanes at landfall on the Yucatan Peninsula of Mexco and in North Carolina, respectively.

Category Four

More extensive curtainwall failures with some complete roof structure failures on small residences. Shrubs, trees, and all signs are blown down. Complete destruction of mobile homes. Extensive damage to doors and windows. Low-lying escape routes may be cut by rising water 3-5 hours before arrival of the hurricane center. Major damage to lower floors of structures near the shore. Terrain lower than 10 ft above sea level may be flooded requiring massive evacuation of residential areas as far inland as 6 miles (10 km). Hurricane Luis of 1995 was a Category Four hurricane while moving over the Leeward Islands. Hurricanes Felix and Opal of 1995 also reached Catgeory Four status at peak intensity.

Category Five

Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. All shrubs, trees, and signs blown down. Complete destructon of mobile homes. Severe and extensive window and door damage. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the hurricane center. Major damage to lower floors of all structures located less than 15 ft above sea level and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within 5-10 miles (8-16 km) of the shoreline may be required. There were no Category Five hurricanes in 1995, 1996, or 1997. Hurricane Gilbert of 1988 was a Category Five hurricane at peak intensity and is the strongest Atlantic tropical cyclone of record.

All tropical storm/Hurricanes names are chosen from a list selected by the World Meteorological Organization. The Atlantic and the Pacific are assigned six lists of names each, 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.

Often when an unusually destructive hurricane hits, that hurricane’s name is retired and never used again. Since 1954, forty names have been retired.

Atlantic storms often start life off the coast of Africa, it’s quite amazing that a storm can travel from there, to come close (ish) to the shores of Britain.  Although we do not experience hurricanes as such, as they have lost their characteristics before they reach us, we often experience hurricane force winds from the remnants of these storms.

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