Cyclones are generally most destructive and dangerous. In the case of cyclones, atmospheric pressure increases outwards from an area of low pressure in the centre. The result is that air from outlying areas rushes in towards the area of low pressure. These cyclones move forward in the Anticlockwise direction in the Northern Hemisphere but in Clockwise direction in the Southern Hemisphere. The cyclones are generally circular or egg-shaped or resembling V-sign at one end. On account of its impact on rainfall and temperature, a cyclone is generally of great significance. On the basis of location and physical features, cyclones are of two types- Tropical and Temperate Cyclones. Tropical cyclone will be specifically discussed in this article. The term cyclone used here will refer to Tropical cyclone. It is because in meteorological terminology, now temperate cyclone is generally referred to as an area of low pressure and the Tropical Cyclone as ‘Cyclone’ only.
Cyclones developed in the regions lying between Tropic of Cancer and Capricorn are called Tropical Cyclones. They develop from the ‘Warm Core’ of an extremely low-pressure area. Fast and high winds and torrential rains are associated with these thunderstorms. The cyclones have an important role to play in the general circulation of the atmosphere. According to an estimate, a fully developed cyclone transports in one hour, 350 million tonnes of hot humid air from low latitudes to mid latitudes. In its development cycle, a cyclone liberates as much energy as will be done by five lakh atomic bombs of the type dropped on Hiroshima(Japan) during Second World War. A cyclone recycles 200 million tonnes of sea water daily. These statistics give us an idea of the terrific force of cyclones.
Characteristics of a Tropical Cyclone
- They originate only over the sea and on reaching the land, they become weak and die out.
- The centre is characterized by extremely low pressure.
- No significant temperature variations.
- They are not always mobile. Sometimes, they become stationary over a particular area for several days and yield heavy rainfall.
- They are confined to a particular period of the year mainly during the summer season.
- These are very intense and destructive. The wind speed is about 180-400 km per hour.
- They generally advance in east to west direction.
How Tropical Cyclones Originate?
For the formation of Cyclone, certain atmospheric conditions are necessary:
- There must be adequate Coriolis Force without which the winds will not move in whirls. Between 5°N and 5°S of the Equator, the Coriolis Force is very low. As a result, most cyclones occur between 5° and 20° latitudes North and South of the Equator.
- An adequate oceanic area where the surface temperature is above 27°C. The cyclones get their humidity from these oceanic waters. After condensation of humid air, the release of latent heat takes place which becomes the source of energy for the cyclone. Sometimes, cyclones are formed in those oceans which have surface temperatures of 23°C or 24°C. But for this, the existence of very cold air currents in upper layers of the atmosphere is necessary.
- There should exist extremely low-pressure area or circulation of low-level cyclonic winds.
- At the height of 9 to 15 kilometers above the cyclone, there should be an area of high air pressure (anticyclone). This enables the rising air to move outward and low-pressure centre remains fixed at ground level.
The aforesaid conditions are likely to be found in those oceanic areas where Inter-Tropical Convergence Zone (ITCZ) lies 10° or more from the Equator. ITCZ is that zone which shifts with the movement of sun between Tropic of Cancer and Tropic of Capricorn.
Structure of a Tropical Cyclone
A fully developed cyclone has a central core of slow winds called ‘eye of the storm’. The diameter of this eye is between 30-50 km. The area of the eye of the cyclone has the lowest air pressure. Generally, this eye-area is cloud free as well as free of rainfall.
Around the cyclone are found thick walls of clouds spreading over an area of 50 km. Within these limits are found torrential rain bearing cumulus clouds. These clouds remain constantly shining with streaks of lightning and thunder. Within this area of the cyclone, torrential rainfall is caused. The velocity of wind is found to be the maximum in this area and ocean waves rise up to heights of 20 km.
Outside the centre of the storm up to about 50 km radial area, the speed of winds goes on decreasing rapidly. Rainfall also decreases here.
Movement of the Tropical Cyclones
In a cyclone, the winds move very fast from the outward high-pressure zone to low-pressure zone in the centre. The entire cyclone then moves forward from east to west at a speed of about 15 to 30 km per hour. After originating over the surface of an ocean, the cyclone may break on reaching the coast or may be weakened and turn into a centre of low-pressure area.
Different Areas, Different Names
Weather Phenomena associated with Tropical Cyclone
On arrival of the tropical cyclone, air pressure decreases, directions of wind change and the temperature begins to rise. Long rolls of cirrus clouds are seen in the sky. As the cyclone approaches, clouds become darker and come close to the surface. It starts to drizzle which soon changes into torrential rains with thunder and lightning. The visibility becomes zero. Such destructive conditions persist for a few hours only. The arrival of the centre or eye of the cyclone is marked by calm breezes, clear sky and low pressure at the centre. Such weather conditions persist for half an hour.
The weather suddenly changes with the arrival of the rear portion of the cyclone as the sky again becomes over cast, wind direction changes and pressure sharply goes up. There is heavy rainfall with thunder and lightning and the storm becomes very severe. This situation persists for several hours. Slowly, the cyclone weakens and weather becomes calm after the cyclone has passed off. The sea surface also becomes calm and clear weather sets in.
Areas of Occurrence
The tropical cyclones are limited to following six areas:
- Western Island Group, Gulf of Mexico, Carribean Sea.
- North Western Pacific Ocean, Philippines, China Sea including Japan.
- Bay of Bengal and Arabian Sea.
- Eastern Pacific Ocean ( Coast of Mexico and Central America)
- South Indian Ocean (Madagascar)
- South western Pacific Ocean (Samoa, Fiji Island and Australia)
Damage Caused by Cyclones
All along their path, the cyclones cause immense loss of life and property. Not only are temporary huts blown away, the whirlwinds with terrific force bring down even houses and places made of concrete, iron and stones. Whatever comes in their way- electric and telephone poles, trees or any structures- are devastated and broken down. Torrential rainfall follows and the consequent floods cause further damage and destruction. The terrific force of the cyclones gives rise to disturbances in ocean water known as Storm Surges. These surges rise like walls of water as much as 5 metres high and rush from the coast up to a distance of 10-15 km taking everything in their lap- houses, roads, whole coastal village or town. The cyclonic rainfall induced landslides prove more destructive.
To stop the cyclone is not within the capacity of man and only means of safety can be adopted:
- Close the doors and window and rush to safe places on getting alerted.
- As far as possible carry out repairs of your house to enable the building to withstand the cyclone. Houses can be strengthened to resist wind and flood damage. All elements holding the structure need to be properly anchored to resist the uplift or laying off of the objects. For example- avoid large overhangs for roofs and projections should be tied down.
- Coastal shelter belts should be planted. Rows of strongly rooted trees with needle like leaves are planted in the direction facing the wind. The trees in the first few rows should be provided fenced support to protect them from getting uprooted. These shelter belts grown along the coastline can help in mitigating the impact of strong cyclonic winds and checking the soil erosion.
- Store adequate food and drinking water. Prepare and keep emergency kits ready.
- After the cyclone, check up electricity connection, gas connection, etc, before putting to use. Do not venture into the water and be careful about snakes and other poisonous animals.
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