Tropical cyclones have been the cause of a number of natural disasters. They are storms with large low-pressure centres and numerous thunderstorms that produce high winds and heavy rain. Generally they are known as hurricanes, but in the northwest pacific region they are known as typhoons. A hurricane is defined as having a wind speed in excess of 73 mph (117 kph), but maximum sustained winds in the strongest tropical cyclones have been estimated to reach 195 mph.
Cyclones form out in the ocean and there are distinct hurricane seasons in different parts of the world, lasting for between five and eight months. Special weather watches are kept during these times and in populated areas there are generally well-rehearsed plans that are put in place when a tropical cyclone approaches.
They often hit the coast with tremendous force causing significant damage, but having hit land, friction slows them down. While the winds will abate somewhat, heavy rains will continue and can cause serious flooding. Coastal storm surges can produce extensive flooding up to 25 miles (40 km) inland.
Hurricane Katrina struck the states of Louisiana and Mississippi in August 2008. 1,836 people died as a result and overall damage was estimated as exceeding $100 billion. Although this was America’s costliest natural disaster, the deadliest natural disaster in US history was the Galveston Hurricane of 1900 that killed between 6,000 and 12,000 people in Galveston.
Tornadoes can also be a lethal weather event. A tornado is a violent, dangerous rotating column of air that is in contact with the surface of the Earth and a cumulonimbus cloud. Because of their appearance they are often call twisters.
Most tornadoes are about 250 feet across, have wind speeds of up to 110 mph and blow themselves out after a few miles, but in extreme cases they can be 2 miles across, attain speeds of more than 300 mph and stay on the ground for considerable distances.
Although they can occur almost anywhere, the vast majority occur in the US, where the average is about 1,200 a year. They are most common in the spring, but they can occur at any time of the year. Worldwide the peak time for them is 5 pm, but one of deadliest in history, the Gainesville Tornado, occurred at 8.30 am local time.
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