|Demand of the question|
Introduction. Differentiate storm and cyclone.
Body. Discuss various conditions for cyclone formation in tropical region.
Conclusion. Way forward.
A storm is an atmospheric depression with wind speeds below a cyclone. This atmospheric depression can be violent and is associated with unusual weather. Storms are associated with periods of strong often damaging winds, heavy flood-producing rainfall, thunder and lightning, heavy snowfall or blizzard conditions. A cyclone is another name for a low-pressure area. Tropical cyclones are revolving storms that begin in the tropics. Storms can be isolated events such as thunderstorms or they may be associated with weather fronts and many other situations. The major difference between the two is intensity, and speed of winds.
Conditions Favourable for Cyclone Formation:
- Good Source of Latent Heat: Ocean waters having temperatures of 27° C or more is the source of moisture which feeds the storm. The condensation of moisture releases enough latent heat of condensation to drive the storm.
- Presence of the Coriolis force enough to create a cyclonic vortex: The Coriolis force is zero at the equator (no cyclones at equator because of zero Coriolis Force) but it increases with latitude. Coriolis force at 5° latitude is significant enough to create a storm. About 65% of cyclonic activity occurs between 10° and 20° latitude.
- Small variations in the vertical wind speed: Tropical cyclones develop when the wind is uniform. Because of weak vertical wind shear (differences between wind speeds at different heights), cyclone formation processes are limited to latitude equator ward of the subtropical jet stream. In the temperate regions, wind shear is high due to westerlies and this inhibits convective cyclone formation.
- Low pressure area: A pre-existing weak low-pressure area or low-level-cyclonic circulation is must for cyclone formation in tropics.
- Humidity Factor: High humidity (around 50% to 60%) is required in the mid-troposphere, since the presence of moist air leads to the formation of cumulonimbus cloud. Such conditions exist over the equatorial doldrums, especially in western margins of oceans (this is because of east to west movement of ocean currents), which have great moisture, carrying capacity because the trade winds continuously replace the saturated air.
- Upper divergence above the sea level system: A well developed divergence in the upper layers of the atmosphere is necessary so that the rising air currents within the cyclone continue to be pumped out and a low pressure maintained at the centre.
- Low-level Disturbances: Low-level disturbance in the form of easterly wave disturbances in the Inter-Tropical Convergence Zone (ITCZ) should pre-exist.
- Temperature contrast between air masses: Trade winds from both the hemispheres meet along inter-tropical front. Temperature contrasts between these air masses must exist when the ITCZ is farthest, from the equator. Thus, the convergence of these air masses of different temperatures and the resulting instability are the prerequisites for the origin and growth of violent tropical storms.
The post-monsoon period sees a higher number of cyclones than the pre-monsoon period. This is because summers and pre-monsoons see dry and hot air moving from north-western India towards the Bay. This blocks the rise of air from the water, and the subsequent formation of clouds, preventing cyclone-friendly conditions. But the absence of this air movement in the post-monsoon phase increases the chances of cyclones.