The word cyclone owes its origin to India. In 1848, Sir Henry Piddington, President of the Marine Courts, Calcutta, coined the word cyclone, derived from the Greek word “cyclos” for coils of a snake.
Definition: The cyclones are irregular winds movements involving closed air circulation around a low atmospheric pressure centre. This closed circulation due to atmospheric disturbance over and above the earth’s surface. The cyclone is related to catastrophic and violent disturbances, such as heavy rain.
Importance of Cyclones:
1. The level of cyclone down inequalities of pressure and wind movement.
2. It plays a vital role in heat exchange between various latitudes.
3. In mid-latitude regions, precipitation occurs due to lifting the moist air from oceans and taking it into the surrounding landmasses.
4. In the northern hemisphere, the cyclonic winds move in anti-clockwise.
5. In the southern hemisphere, the cyclonic winds move clockwise.
6. Depending on their area of origin, cyclones can be either tropical or temperate/extra-tropical.
The tropical cyclones have a thermal origin and low-pressure system, which form over the warm tropical oceans. Conventional currents acquire a circular motion develop because of the Coriolis force generated by the earth’s rotation. Cyclones develop only at critical sea surface temperatures, which enable the ascent of air without mixing with the surrounding.
The quiet air, high temperature, and high saturated atmospheric conditions exist over the equatorial doldrums, mainly in the western margins of oceans, which have substantial moisture and carrying capacity due to the trade winds continuously replacing the saturated air. When the doldrums are the farthest from the equator, then circular motion is enhanced. At this movement, the air is overheated, and the sun is exactly over the equator. This source of energy for the development of a tropical cyclone is the latent heat of condensation.
Following favourable conditions required for the formation of Tropical Cyclones:
1. Source of latent heat: The tropical cyclones are formed over warm waters having a temperature of 26-degree Celsius or more. Adequate evaporation leads to the accumulation of moisture above sea’s surface, which in turn impart the necessary latent heat to supply the energy for the storm and later release in the process of cloud and rain formation.
The deep conventional currents within water do not stir and mix the cooler water below with, the warmer water near the surface.
2. Coriolis force: This force is zero at the equator, but it increases with latitude. The magnitude of Coriolis force is strong enough to help the development of a cyclone vortex. It also finds that about 65% of cyclonic activity occurs between 10 degrees and 20-degree latitude.
3. Low-level disturbances: It forms due to easterly wave disturbance in the Intra-Tropical Convergence Zone (ITCZ). Small local differences in the water and air temperature produce various low-pressure centres of small size. A weak cyclonic circulation develops around these areas.
Trade winds from both hemispheres meet along a line called the inter-tropical front. Temperature differences between theses air masses exist when the ITCZ is farthest from the equator. At this time, convergence of winds of different temperature and develop instability are the precondition for the origin and growth of the violet tropical storms.
4. Upper Air Distrubance: The upper tropospheric cyclone from the westerlies move deep into the tropical latitude regions which develops thunderstorms.
5. Wind Shear: Tropical cyclones develop when wind is uniform. Because of weak vertical wind shear which formation processes are limited to latitude equatorward of the subtropical jet stream.
Structure of Tropical Cyclone:
1. In the tropical cyclone, the eye at the centre of cyclone.
2. The width of this core varies from 10 to 50 km.
3. Temperatur is high in thses region due to the descending air currents which is heat up by compression.
Cumulonimbus clouds develop in the eye wall and maximum wind velocities. Thus, eye wall region observes the heaviest precipitation. Outside of the eye wall, two spiral bands are discovered. This bands are also called as rainbands or feeder bands. The annual zone is characterised by suppressed cloudiness, with high temperature and low humidity conditions. Source: Spectrum