How Do Tropical Storms Form

The generic name tropical storm is applied to typhoons, cyclones and hurricanes. These storms form over tropical waters in the Atlantic, Pacific and Indian Oceans.

In order for a tropical storm to form several different factors must come together. The water must be over 28C (80F) and deep enough to provide the energy needed to power the storm. The depth required is unknown but the warmer the water the less the depth that is required as more energy is held in the water: the minimum depth required appears to be fifty metres.

The storm season is towards the end of summer and start of autumn when the ocean waters are warmest but the air is beginning to cool. For a storm to form the air over the sea must cool quickly allowing heat to be rapidly released into the air along with moisture. The moist air rises more quickly than it can be replaced from below, creating an area of low pressure. If the mid-troposphere is also moist (five kilometres above the earth) a column of warm moist air is formed. Air moves towards the area of low pressure to fill it.

A storm will not form even now unless certain other conditions are met. Some sort of surface or near surface disturbance is necessary to start the winds circling inwards to fill the area of low pressure, but there must be no winds crossing the area, which would disturb the initial formation by disturbing the vertical air convection necessary.

The centre of the storm needs to be more than five hundred kilometres (three hundred miles) from the equator. This allows the Corriolis effect to maintain the necessary low pressure at the centre of the storm, and determines the direction of spin of the winds. South of the equator the winds spin in a clockwise manner, and north in an anticlockwise manner. Until wind speeds in the system exceed thirty-nine miles per hour the system is known as a tropical depression, at higher speeds than this the depression is classed as a tropical storm.