Cyclones are also termed atmospheric disturbances. When the velocity of winds increases to such an extent that they attain strong force, a cyclonic storm is formed. Cyclones are the centres of low pressure. They are surrounded by closed isobars having increasing pressure outward.
The wind circulation is from outside towards the central low pressure. They rotate anti-clockwise in the Northern hemisphere and clockwise in the southern hemisphere.
Conditions required for the origin of tropical cyclones:
- Presence of warm Ocean surface with a temperature of 27 degrees Celsius or more. The warm surface is the source of thermal convection and strong hot and humid currents. It is because of this factor that tropical cyclones are characteristically developed over the east coast of continents under the influence of warm ocean currents.
- The presence of ITCZ (Inter Tropical Convergence Zone) supports the intensification of low pressure and augment the supply of moisture due to the inflow of trade winds.
- Coriolis force helps in deflection of strong influence wind to generate circulatory system. In the absence of Coriolis force tropical cyclones do not originate near the equator.
- Presence of high moisture in the atmosphere as it becomes the source of latent heat of vaporization which is the basic source of energy for a tropical cyclone.
- Minimum vertical wind shear to support the unrestricted flow of air currents upwards.
- Divergence of air in the upper troposphere.