The generic name tropical storm is applied to typhoons, cyclones and hurricanes. These storms form over tropical waters.
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 meters.
The necessity for warm seas is what determines the storm season, towards the end of summer and start of autumn. At this time the water has been warmed all summer, but the air is beginning to cool.
For a storm to form the air over the sea must cool sufficiently quickly to allow 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. The mid-troposphere (five kilometers above the earth) must also be moist to allow a column to form.
A storm will not form even with these conditions unless certain other ones 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 center of the storm needs to be at least five hundred kilometers (three hundred miles) from the equator. This allows the Coriolis effect to maintain the necessary low pressure at the center 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.
The center of the storm can vary in size from thirty to one hundred kilometers across and is known as the eye. Inside the eye there are no clouds and little wind. The highest wind speeds are experienced in eye wall.
A fully developed tropical storm has a clear eye area. Winds spiral up the eye wall throwing wind and moisture out at the top, leading to the classic satellite image we see.
Until wind speeds in the system exceed thirty-nine miles per hour the system is known as a tropical depression, at higher than this the depression is classed as a tropical storm. As wind speeds pick up the category of the storm is upgraded, starting at force one and rising to five for speeds in excess of one hundred and seventy five miles per hour.
As long as the storm continues to travel over the warm seas required to feed moisture and energy into the atmosphere the storm will continue to grow in size and force. Once the storm hits cooler water or land it will collapse, releasing all the stored moisture as rain. The storing of energy over water is the reason that storms crossing the Atlantic from the African coast will have more force than those forming in the Caribbean.
The El Nino effect is important in predicting the frequency of storms. During an El Nino event the eastern pacific warms up and storms become more frequent in that region and activity in the Caribbean decrease. Storms will still occur, just less frequently.
An understanding of how these storms form is the first step towards perhaps being able to divert or prevent them.
