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The Formation of Tropical Storms

Tropical storms (hurricanes, typhoons, cyclones) are low pressure hazards.

Tropical storms are seasonal. In the northern hemisphere the hurricane season is usually from August-November. In the southern hemisphere the hurricane season is usually between March-May.

Solar energy is need to heat the sea’s temperature to above 26/27oC. This is why hurricanes are a tropical system. A lot of energy is needed to heat to this temperature because water loses heat so easily (a high specific heat capacity). Because the SHC of water is so high it needs high amounts of energy a long period of time - the whole summer - to reach temperatures above 26/27oC.

The heat from the warm oceans is passed into the air above by conduction. This warms the atmosphere and because this warm air begins to rise, forms an area of low pressure. The air is not only hot, it is also wet. This means the air is drawn in – air moves from areas of high pressure to areas of low pressure creating wind. As the air rises the moisture will condense and form cumulo nimbus clouds.

As condensation takes place, heat is transferred. The cloud therefore gets hotter and the air is heated around it. So the more moisture there is, the more heat there is. It is what is known as a positive feedback system.

Once the average wind speed reaches 39 mph it is classed as a storm. Once the winds reach 75 mph, it is classed as a hurricane.

Turbulence begins inside the cloud. As the pressure drops, the storm grows. The storm is then affected by the Coriolis force so it starts to revolve and suck in wind from all around it. As it spins nothing can get into the middle so it develops an ‘eye’, a calm area of low pressure. It is not a static storm, it moves with the prevailing winds (in the northern hemisphere it moves NW).

The storm slows down when it reaches land because it begins to lose energy. There is no water and the rain from the storm cools down the land. If it moves over an ocean cooler than 26/27oC then the hurricane will also die out because there is less energy available due to the cooler water. Hurricanes cannot cross the equator because there is no Coriolis force here. The storm will therefore not revolve, will probably stop moving and will be driven away from the equator.

The 3 hazards associated with hurricanes are storm surges, winds and extreme flooding from rainfall.