A cyclone is a huge strong wind system that blows around the centre of an intense low-pressure area. Cyclones are the local name of the Indian Ocean and the South Pacific Ocean. But in the Northwest Pacific Ocean, they are known as typhoons and in the Northeast Pacific Ocean and North Atlantic, they are known as hurricanes.
Since the cyclones form in the tropical region, they are also known as tropical storms, tropical revolving storms, or tropical cyclones. In the northern hemisphere, cyclone winds blow anticlockwise and they reverse in the southern hemisphere.
When and where do cyclones occur
Cyclones begin in tropical sea regions during the summer and tend to move initially westward, then eastward towards higher latitude. The regions of the world where tropical cyclones usually originate are:
- Tropical North Atlantic Ocean
- East of the Lesser Antilles and the Caribbean, east of 70°W during the months of July to October.
- North of the West Indies from June to October
- Western Caribbean during June and late-September to early November.
- The Gulf of Mexico from June to November
- Western North Pacific Ocean, including the Philippines, from May to November, but storms sometimes occur in all months.
- North Pacific off the West Coast of Central America, during the
- months of June to October.
- The Bay of Bengal and the Arabian Sea, from May to June and October to November.
- South Pacific Ocean, West of 140°W, from December to April.
- South Indian Ocean, from December to April.
- North-western Coast of Australia during the months of November to April
- West of 90° W from November to May.
- Tropical cyclones form over oceans of the world except in the South
- The Atlantic Ocean and in the South Eastern Pacific.
How do cyclones occur?
Cyclones develop over warm seas near the equator. Air heated by the sun rises very swiftly, which creates areas of very low pressure. As the warm air rises, it becomes loaded with moisture which condenses into massive thunder clouds. Surrounding air rushes in to fill the void that is left. But because of the constant turning of the earth on its axis, the air is bent inwards and then spiral upwards. The swirling winds rotate faster and faster, forming a huge circle that can be up to 500-1000 km across. At the centre of the storm is a calm, cloudless area called the eye – where there is no rain, and the winds are fair.
The development of a cycle of tropical cyclones can be divided into three stages:
- Formation and initial development
- Full maturity
Formation and initial development
Four atmospheric and oceanic conditions are necessary for the development of a cyclonic storm: –
- Warm sea temperature over 26 degrees centigrade to a depth of 60 meters which provides abundant water vapors in the air by evaporation.
- High relative humidity of the atmosphere to a height of above 7000 m facilitates condensation of water vapors into water droplets and clouds; releases heat energy thereby inducing a drop-in pressure.
- Atmospheric instability encourages the formation of massive vertical cumulus cloud convection with the condensation of rising air over.
- A location of at least 4-5 latitude degrees from the equator allows the influence of the forces due to the earth’s rotation to take effect wind circulation around low-pressure centres.
If the ocean and atmosphere environment continue to be favorable, the cyclone may continue to intensify as it moves pole-ward. The cloud system becomes more circular and develops a distinct eye. This is the shape that signifies the cyclone is at its most severe and dangerous stage. Approximately half of the cyclones of this form progress to full maturity.
A tropical cyclone begins to dissipate when the central pressure becomes filled up. The storm eye becomes distorted. High wind becomes weak. Severe weather becomes moderate. When the cyclone hits land, storm wind and storm surge may occur near the coastline, and heavy rain and flood may occur along the storm track in the land. Intense rain may last for weeks.
Classification of cyclones
Cyclones are classified according to their wind speed. However, the classification varies from region to region. In the United States, they are classified into five different categories based on their wind speed as measured on the Safire-Simpson scale (SS scale).
The types of cyclones are :
- Minimal damage cyclone with a storm surge of 4 to 5 feet.
- Moderate damage cyclone with a storm surge of 6 to 8 feet.
- Extensive damage cyclone with a storm surge of 9 to 12 feet.
- Extreme damage cyclone with a storm surge of 13 to 18 feet.
- Catastrophic damage cyclone with a storm surge of more than 18 feet.
Hazards associated with cyclones
There are three hazards associated with a cyclone, which cause destruction.
- Storm surge: A storm surge is an abnormal rise of sea level near the coast caused by a severe tropical cyclone; as a result, seawater inundates low lying areas of coastal regions drowning human beings and livestock, eroding agricultural land, beaches, and embankments, destroying vegetation and reducing soil fertility.
- Strong wind: The most destructive force of a cyclone comes from fierce winds. These winds are strong enough to easily topple fences, sheds, trees, power poles, and communication systems while hurling helpless people through the air. Many people are killed when the cyclone winds cause buildings and houses to collapse and completely blow away resulting in loss of life and property.
- Flood: Heavy and prolonged rains due to cyclones may cause floods and submergence of low lying areas causing loss of life and property. Floods and coastal inundation due to storm surges pollute drinking water sources causing an outbreak of epidemics. Long after a cyclone has passed, affecting road and rail transport by floodwaters. Water often becomes contaminated from dead animals or rotten food, and diseases like diarrhoea and other infections threaten people.
Destructive cyclones that occurred in India as well as it borders
1942 Bengal Cyclone: hit near the India/Bangladesh border, resulting in around 40,000 fatalities.
1960 East Pakistan I Cyclone: claiming 6,000 deaths this cyclone hit the eastern portion of Pakistan on October 10 what was known then as East Bengal State.
1963 East Pakistan II Cyclone: on May 23, the cyclone hit present-day Bangladesh (then known as East Pakistan province), causing Twenty-two thousand fatalities due to storm surge and flooding.
1965 Pakistani Bengali Cyclones: two cyclones that hit on May 11 and June 1 killed a total of 47,000 people.
1965 Pakistani Cyclone: hit near Karachi, Pakistan on December 15, causing about 10,000 casualties.
1970 Bhola Cyclone: killed between 300,000 to 500,000- people in East Pakistan (now Bangladesh).
1971 Orissa Cyclone: killed around 10,000- people in Cuttack, Orissa, India.
1977 Andhra Pradesh Cyclone: killed 10,000-people in Andhra Pradesh, India.
1985 Cyclone 01B: on May 25 in Bangladesh, Meghna River Delta a cyclone created a surge 15-20 feet high killing around 6,000 -10,000 people.
1988 Cyclone 04B: on November 26 the cyclone hit the Sundarbans part of Bangladesh. Heavy storm surge killed 2000-people (with 6000-missing). Besides, the storm caused1000 fatalities in Malaysia, Thailand, and western Indonesia.
1991 Bangladesh Cyclone: killed 138,000-people in the Chittagong region of Bangladesh.
1999 Orissa Cyclone: killed around 10,000-people in the Orissa state of India.
2007 Cyclone Sidr: struck Bangladesh on 15 November, and killed at least 3,500 people.
2008 Cyclone Nargis: struck the Ayeyarwady Delta in Myanmar killing over 130,000-people and devastating the country’s largest city, Yangon, making it Myanmar’s deadliest natural disaster in history, the costliest North Indian cyclone on record, and the second deadliest North Indian cyclone in recorded history.
Get Free “Winspire Digital Worksheets of worth Rs. 129 Value”
Please enter your details to get the monthly E - Booklet + absolutely premium content to expand your horizons.