The following are the necessary requirements established for tropical cyclone formation:
Sufficiently large ocean areas with a surface temperature of more than 26°C or 27°C that air lifted from the lowest atmospheric layers and expanded moist adiabatically remain considerably warmer than the surrounding undisturbed atmosphere at least up to a level of about 40,000 feet.
Initial disturbances from which storms later developed may be detected within 5° of latitude of the equator, but these disturbances do not intensify into typhoons or hurricanes until they are more than 5° of latitude from the equator (since the value of the coriolis parameter should be larger than a certain minimum value.
Weak vertical wind shear in the basic current so in those areas of small mean zonal-wind shear are also areas of active storm formation.
A pre-existing low level disturbance over a warm ocean area and a region of upper-level divergence or outflow above the surface disturbance (though not all these areas of organized convective activity develop into tropical cyclones or greater intensity).
Several theories have been formulated on the formation of tropical cyclones. These are the "convective theory" and the "frontal" or "counter-current" theory.
According to the convective theory, a large mass of air becomes convectively unstable and moist compared with its surroundings, which results in an upward motion of air. The air from the surroundings tend toward the low pressure area formed, so that, a cyclonic circulation is formed. The combined effects of the earth's rotation and the centrifugal force, retards the movements of air towards the center causing further pressure fall. The process continues until a vigorous cyclonic wind system is developed. Likewise, the outward flow of air from the center at high levels also makes the pressure lower.
The frontal theory indicates that many tropical cyclones form along the front between the trade winds and the equatorial air in the doldrums . Winds develop along this front and when conditions are favorable, forms into tropical cyclones. The convergence of the two air masses results in the upward motions which in addition to the deflective effect of the earth's rotation, centrifugal force, and divergence at the upper levels results in allow pressure area with a spiral circulation toward the center.
Likewise, as already listed above, tropical cyclones develop over sea surfaces having at least 26°C. Though these heat sources are not sufficient to start a hurricane going, the heat of condensation supports the process once started. Tropical cyclones are also generated in disturbances along the intertropical convergence zone, on traverse waves or under superimposed upper disturbances. But the upper divergence must exceed low-level convergence in order to cause surface pressures to decrease (which is called deepening).
In general, therefore, development of a tropical cyclone takes place when there is proper combination of circulation, divergence and convergence which is maintained over a considerable period of time on a proper scale.