Hurricane Formation

Information on hurricane formation and what they are called in different parts of the world.

Hurricanes occur throughout the Atlantic and Pacific Oceans and go by several different names, depending on the region in which they occur.

In the Atlantic and Eastern Pacific, they are simply "hurricanes". In the South Pacific, Australians refer to them as "cyclones", and in other Pacific regions, they are known as "typhoons."

For simplified purposes, all storms of this nature, which spin around a center or an eye, are often referred to generically as "tropical cyclones".

The Atlantic hurricane season is from June 1 to November 30. The Eastern Pacific hurricane season is from May 15 to November 30.

All hurricanes form from a cluster of thunderstorms called a tropical wave. In the Atlantic, most tropical waves originate off the coast of Africa or in the Gulf of Mexico. In the Pacific, these waves originate off the coast of Central America and Mexico. However, these waves can develop in all the oceans of the tropics.

When these waves being to organize themselves into a swirl, they become tropical depressions. These depressions carry top sustained winds, wind lasting more than one minute, of 35 miles per hour.

Less than 10 per cent of tropical weather disturbances grow into tropical storms because it is relatively rare for several factors to be just right for their development.

First, the ocean water should be above 80 degrees Fahrenheit, which provides for the proper amount of water to evaporate. The warm water should also be a couple hundred feet deep because storms stir up the water, bringing some colder water to the surface.

Next, the winds need to come together at the surface, almost like colliding into each other. The air needs to be unstable so that these colliding winds will rise, like in a chimney.

The air several thousand feet up needs to be humid. This humidity, vapor, will supply a lot of the energy needed for the storm's development. The upper level winds should be traveling in the same direction and close together so that they do not tear up the upper level clouds of the storm.

Tropical depressions can grow into tropical storms in a matter or hours or even days, depending on the atmospheric conditions surrounding the depression.

The tropical depression is deemed a tropical storm when its maximum sustained winds exceed 40 mph. With many tropical storms, we begin to see an eye wall in development as well as a near-definite center of circulation, the eye. When the tropical cyclone reaches tropical storm status, it is given a name. Storms are given names so that it is easier to talk about them and refer to them. The old system was based on using coordinates, which was very confusing because it is not unusual for there to be two or even three hurricanes developing in an ocean.

With favorable conditions: warm water, open sky and away from land, the tropical storm will grow into a hurricane. It gains this status when its maximum sustained winds exceed 74 mph. At this wind velocity, it is also deemed a category one hurricane, a minimal hurricane. At this stage, the eye will become a bit more defined and an eye wall, a wall of thunderstorms on the westward side of the eye, begins to develop.

Hurricane intensity is ranked according to the Saffir-Simpson Scale. A hurricane with winds of 75-95 mph is a Category 1, 96-110 mph is a Category 2 storm, 111-130 mph winds make it a Category 3, 131-155 mph storms are Category 4, and storms with winds greater than 155 mph are Category 5 hurricanes. Hurricanes category 3 and higher are considered major hurricanes.

If the Category 1 hurricane is in open water and other conditions are favorable, it will develop within a matter of short days into full-blown hurricane with winds exceeding 100 mph.

Major hurricanes with winds above 130 mph are rare. They have strong eye walls, which are intense and violent cloud bands that contain the highest winds of the storm.

The other components of the storm are rainfall and the storm surge.

Rainfall is difficult to predict at times because while a hurricane may spend nearly a week developing out in the ocean, it may make landfall and not dump much rain at all.

The high waves brought on shore by a storm are the storm surge. Some surges can be taller than 20 feet and overflow bays and rivers. They are the deadliest part of any hurricane, and even the most minimal of hurricanes produce a storm surge.

Tropical cyclones throughout the world, in general, travel from East to West.

Once Atlantic hurricanes are relatively close to entering the Caribbean Sea, they usually change onto a path of due west, west northwest, northwest or north northwest. In other words, they will either sweep across the Caribbean Sea, head into the Gulf of Mexico, brush either side of Florida, brush the Eastern sea board of the U.S. or just veer into the North Atlantic. These are not the only paths they take, but these are the most common.

Hurricanes in the Eastern Pacific will usually either veer into the colder waters off the coast of California then veer into the North Pacific or they will stay on a more westward path.

In the South Pacific, they usually travel in a west-northwest direction, often striking the northern region of Australia and the islands of Indonesia.

In the Western Pacific Ocean, the systems travel in a west-northwest direction and veer into Japan or other parts of Eastern Asia.

Hurricanes of the South Indian Ocean turn in the direction of Madagascar or take a more west-southwest approach.

Once a hurricane makes landfall on a continent or large island, its development usually stops and the storm weakens. However, if it makes landfall on a small island or small cluster of islands, the opposite could occur. Because of the warmer and shallower water surrounding most small islands, the hurricane may intensify greatly. This is often the case with Atlantic hurricanes that strike the Bahamas. These hurricanes often intensify slightly when they are over the warm water of the Bahama islands, which are very close to South Florida.

When they strike a large land mass, the hurricane will fizzle into a cluster of thunderstorms or rain showers.

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