Hazards 6

Tropical storms are huge spinning storms with strong winds and torrential rain.

They develop over warm water. As warm, moist air rises and condenses, it releases energy that increases wind speed.

Scientists don't know exactly how they're formed, but they do know the conditions needed. These include:

• A disturbance near the sea-surface that triggers the storm.
• Sea water that's warm (above 27 degrees to at least 50m below the surface), so lots of water will evaporate.
• Convergence of air in the lower atmosphere - either within the ITCZ or along the boundary between warm and cold air masses. This forces warm air to rise.
• A location at least 5 degrees from the Equator. They don't form 0-5 degrees either side of the Equator because the Coriolis effect isn't strong enough to make them spin.

So tropica storms form in the tropics because the water there is warm enough.

They occur in the Caribbean Sea (hurricanes), in the Bay of Bengal (cyclones), in the China Sea (typhoons) and in Northern Australia.

Tropical storms lose strength when they move over land because their supply of warm, moist air is cut off.

They initially move westwards due to the easterly winds in the tropics.

They move away from the Equator because of the Coriolis effect.

Tropical storms are circular in shape, hundreds of kilometres wide and usually last 7-14 days. They spin anticlockwise in the northern hemisphere, and clockwise in the southern hemisphere.

At the centre of the storm is an area of very low pressure called the eye.

Rising air spirals around the eye in the eyewall, causing strong winds.

Near the top of the storm, there is an outflow of moisture-laden air, so cloud cover extends for a long distance either side of the eye

Storms are classified using the Saffir-Simpson Scale, which is based on wind speed. Category 5 is the strongest and 1 is the weakest.

The Saffir-Simpson Scale also estimates how much damage a storm of a given magnitude will do, from limited damage at Category 1…