If the sea surface temperature is greater than 26 degrees centigrade (generally found south of latitude 30 North and north of latitude 30 South).

If the Coriolis Force causes winds to deviate to the right in the Northern hemisphere and to the left in the Southern hemisphere.

Winds blowing from high pressure to low pressure in the Northern hemisphere will end up rotating counter-clockwise or cyclically around the low for this reason. The Coriolis Force is strongest at the poles and is zero at the Equator. Hurricanes therefore cannot form at the Equator. They must form at least ten degrees north or south of this line in order to have enough Coriolis Force to rotate.

If there is an unstable atmosphere.

The atmosphere is unstable when hot, or lighter air, lies below cold, or heavier air. This can happen when the atmosphere is heated from below, in the instance of warm sea surface temperatures for example. The hot surface air will want to rise and the cold air above it will want to sink. The hot surface air will expand and cool as it rises, and the water vapour in it will condense, forming clouds. If the air is hot enough to start with, it will rise very high up in the atmosphere and thunderstorms will develop.

Thunderstorms commonly develop along the Inter-Tropical Convergence Zone (ITCZ). The ITCZ is the zone which receives the maximum solar radiation at any given time of the year. The ITCZ lies north of the Equator in the Northern hemisphere summer, and south of the Equator in the Northern hemisphere winter.

If the atmosphere is moist.

In order for thunderstorms and hurricanes to develop, rising hot air must contain water vapour. That is, the air must have a high relative humidity count. As the air rises, this water vapour will condense into clouds. If the air is dry, no clouds will form.

When water vapour condenses into liquid water (cloud droplets), it releases its latent heat of evaporation -- the energy it took for it to change from liquid to vapour in the first place. This latent heat adds `fuel' to thunderstorms and hurricanes by warming the rising air even more, and causing it to ascend to even greater heights.

If there is no wind shear.

When the winds high in the atmosphere are much stronger than the surface winds, there exists a strong wind shear. Strong wind shear exists, for example, with the Jet Stream. The strong winds aloft will tend to blow the tops off thunderstorms and hurricanes and prevent their development. Air cannot rise straight up. It is blown aside before it reaches any great altitude.

If there are `triggers' present.

In order for an ordinary thunderstorm or group of thunderstorms to develop into a hurricane, a `trigger is needed'. In the North Atlantic, this trigger is the extra instability imparted to the atmosphere by the dry Sahara winds blowing over the eastern tropical Atlantic.

Dry air is heavier than moist air. When the easterly trade winds carry dry Saharan air over the moist Atlantic air near the African coast, the atmosphere becomes extra unstable. The surface air will rise very fast and very high because it is hotter and moister than the air above it. Hurricanes therefore, tend to form near the west coast of Africa.

Weather charts can show meteorologists exactly where a hurricane or tropical storm is. Although the 1997 hurricane season is going to be a bad one, according to the experts, this chart, taken off the Internet and supplied by Internet Bermuda Limited, shows that there are no hurricanes or storms threatening Bermuda or any other part of the Americas.

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