Hurricanes

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 kilometres per hour), a huge amount of rain, low air pressure, thunder and lightning. The cyclonic winds of a hurricane rotate in an anti-clockwise 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 fuelled 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 Pacific Ocean and the rest are in other areas.
The weather symbol for a hurricane is: 



How Hurricanes Form and Die


Hurricanes need four conditions to form:
  • low air pressure
  • warm temperatures
  • moist ocean air
  • tropics winds (near the equator).


Hurricanes form in the tropics, over warm ocean water (over 80ºF or 27ºC). The Hurricane Season is between June and November. These powerful storms are fuelled by the heat energy that is released when water vapour 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. This 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 reached speeds from 39 to 73 miles per hour, it is called a tropical storm.
  • 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 Structure



Hurricane winds blow in an anti-clockwise spiral around the calm, roughly circular centre 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 thunderclouds. 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.

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.

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