Hurricanes and Tropical Storms

When winds are particularly strong, they become known by different names. These superwinds are called:

• hurricanes in the Atlantic Ocean
• typhoons in the Pacific
• cyclones in the Indian Ocean

As we can see, these names depend on where the storms originate – although all three of these start over the sea. Tornadoes on the other hand form over land and are usually more violent.

However, in all cases, the high winds start in the same way. Large weather systems build up where there is a lot of warm moist air. This usually happens in autumn, after the long hot summers common in tropical areas. At this time, the ocean surface is warmed – along with the air above it. High winds cause the rapid evaporation of sea water, and the water vapour rises quickly to form clouds. As this happens, large amounts of heat are released into the atmosphere through the condensation process, and this provides the energy that fuels the whole system. The mixture of the heat and the water vapour often create violent thunderstorms. These storms may group together if the water is warm enough (usually over 24°C or 75°F) and begin to spin around as one large system. As they are pushed further across the ocean by the wind, they draw in more warm moist air, gathering energy all the time. They start to spin in ever-tighter circles, with their speed being increased all the time by the Earth’s rotation. When this occurs, the group of thunderstorms will develop into the tropical storms we have all heard of. This happens in 70% of all cases.

In the centre of the storm is a calm area of low pressure, initially up to 200 miles (300 km) across. As the wind speeds increase however, and the air is sucked inwards, this will develop into the "eye" of the hurricane – possibly only 30 miles (50km) wide. Here winds may be no more than 15 mph (25 kph), whereas winds around it can reach speeds of 220 mph (360 kph).

Spiralling air systems such as hurricanes can be enormous – possibly as much as 480 miles (800 km) across and 12 km or more high. Throughout this area, average wind speeds can be well above 100 mph (160 kph), so we can understand why they can be so devastating. This is particularly true in coastal areas, where huge waves caused by the wind can swamp the shore. When a tropical storm, such as a hurricane, passes over land or over cold seas, it loses its energy source and winds drop rapidly.

Since 1950, every hurricane and tropical storm has been given a name. This was designed to prevent confusion when producing weather forecasts and warnings. Originally names were taken from the international phonetic alphabet in use at that time, for example Able, Baker and Charlie. In 1953 however, it was decided to use girls’ names instead, two of the first being Carol and Edna.  These very quickly ran out, so boys’ names are now used as well. A list of names that will be used for hurricanes in 2000 can be seen here.

As we have discovered, hurricanes form over the sea.  There is another type of strong wind though, which forms over land and this is called the tornado.  We'll find out more about tornadoes in the next section.