Hurricanes

The strongest tropical storms are called hurricanes, typhoons or tropical cyclones. The different names all mean the same thing, but are used in different parts of the world. If these huge storms start in the Atlantic off the west coast of Africa, they are called hurricanes.

In an average year over a dozen hurricanes form over the Atlantic Ocean and head westwards towards the Caribbean, the east coast of Central America and the southern USA (Florida in particular). Hurricanes may last as long as a month, and although they travel very slowly, usually about 24 km/h (15 mph) – wind speeds can reach over 120 mk/h (75 mph).

Map showing route of hurricane Mitch, 1998

How hurricanes form

Hurricanes need a lot of heat to form (a sea surface temperature of at least 26 degrees), which is why they usually occur over tropical seas. They also need to be between 5 and 20 degrees north or south of the equator. It works like this:

• Rising warm air rises fast, causing towering clouds, heavy rainfall, and intense low pressure.
• The low pressure sucks in air, causing very strong winds which spiral - anti-clockwise in the northern hemisphere - around the centre of the low, at speeds of around 120 km/h (75 mph).
• Seen from above hurricanes are huge circular bodies of thick cloud around 450 km (300miles) wide. The cloud brings heavy rain, thunder and lightning.
• In the centre is the eye of the hurricane, about 45 km across (30miles) across. Often there will be no cloud in the eye. Seen from below it will seem calmer, with a circle of blue sky above. The eye is formed because this is the only part of the hurricane where air is sinking.
• In the northern hemisphere, the prevailing easterly tropical winds tend to steer hurricanes toward land - although their course is unpredictable. As they move inshore their power gradually reduces, because their energy comes from sucking up moist sea air.

Cross section diagram of a hurricane

Effects of tropical storms

The intense winds of tropical storms can destroy whole communities, buildings and communication networks. As well as their own destructive energy, the winds generate abnormally high waves and tidal surges, which cause flooding in coastal areas. The most destructive elements are the high seas and the flooding that accompany the storms.

MEDCsMEDC: A More Economically Developed Country [MEDC] has high levels of development, based on economic indicators, such as gross domestic product (the country's income). More of the countries in the northern hemisphere are MEDCs, eg UK, USA, Canada, Europe. More of the countries in the southern hemisphere are Less Economically Developed Countries [LEDCs]. are better placed to reduce the effects of tropical storms, because they have more financial resources and educational and technological know-how to help deal with them. This means they are better able to observe and predict storm behaviour, and to invest in infrastructure, which can withstand storms - as well as having more resources for repairing the damage caused.