Hurricanes are intense tropical storms that form in warm moist air. However, they don’t appear out of nowhere. A storm must go through several stages of development before it can be classed as a hurricane force storm.
A hurricane’s beginning can be found in the thunderstorm. Warm air rises quickly, causing towering clouds, heavy rainfall and intense low pressure.
When several thunderstorms group together and are subject to the right atmospheric conditions they become a tropical depression. At this point wind speeds are in the range of 23 to 39 mph. As the thunder storms release heat, this warms the disturbance. This causes the air density inside the storm to lower, which in turn leads to a dropping of the air pressure.
Wind speeds increase as cooler air rushes underneath the rising warm air. As this wind is subject to the Coriolis force, the storm begins to rotate.
Once a tropical depression intensifies, with wind speeds of 40 – 73 mph, it becomes a tropical storm. It is at this stage that it will be assigned a name by the meteorological service. The storm will become more organized and will begin to take on the circular shape of a hurricane on satellite pictures. Tropical storms do not always develop into hurricanes, but can cause their own problems, usually due to the heavy rain fall they bring with them.
As the air pressure continues to drop, and wind speeds reach and exceed 74mph, a tropical storm officially becomes a hurricane. At this point the intense rotation of the winds will cause a central eye to form at the centre of the storm, which will be clearly visible from satellite images. The walls of the eye carry the highest winds and rainfall.
Once a hurricane is formed, it takes more energy from the warm ocean water to become stronger. A hurricane will strengthen if there is a supply of warm, moist air to feed it.
This is why most hurricanes form around the equator. Under the right conditions, a hurricane can sustain itself for several weeks, however once they make landfall they lose their energy very quickly.
As a hurricane continues to form and grow, its intensity is measured on the Saffir-Simpson scale, which gives hurricanes a rating between 1 and 5.
A Category 1 hurricane will have wind speeds between 74 – 95mph, and can cause a storm surge 5 feet high. It will generally cause some damage to trees and unanchored mobile homes.
A Category 2 hurricane will have wind speeds between 96 – 110mph, and can cause a storm surge between 6 – 8 feet high. It generally causes major damage to mobile homes, roofs of fixed buildings, and can blow trees down.
A Category 3 hurricane will have wind speeds between 111 – 130mph, and can cause a storm surge between 9 – 12 feet high. It can destroy mobile homes, blow down large trees and damage small buildings.
A Category 4 hurricane will have wind speeds between 131 – 155mph, and can cause a storm surge between 13 – 18 feet high. This will completely destroy mobile homes, and other small structures, and will generally cause wide spread flooding.
A Category 5 hurricane will have wind speeds greater than 155mph, and can cause a storm surge over 18 feet high. The majority of buildings will be at risk of significant damage or destruction, alongside wide spread flooding due to storm surge.
http://spaceplace.nasa.gov/en/kids/goes/hurricanes/
http://www.cotf.edu/ete/modules/sevweath/swhoware.html
