tropical cyclones

What is a tropical cyclone?

A tropical cyclone is a storm system characterised by a lowpressure centre, which produces strong winds and flooding rain. A tropical cyclone feeds on heat released by the condensation of moist air. The latent heat gets converted into kinetic energy and feeds the strong winds emerging out of it.

Cyclonic storms have counterclockwise rotation in the Northern Hemisphere and clockwise in the Southern Hemisphere. Developed over warm water bodies — ocean and seas — they lose their strength once they move over land. They also help in the global atmospheric circulation mechanism by carrying heat and energy away from tropics towards temperate latitudes.

What are the different types of tropical cyclones?

Tropical cyclones are formed in eight basins — Northern Atlantic, Northeastern Pacific, North Central Pacific, Northwestern Pacific, Northern Indian Ocean, Southwestern Indian Ocean, South and Southwestern Pacific and Southeastern Indian Ocean.

Each basin has a different naming system. In the North Atlantic Ocean, Northwest Pacific Ocean east of the International Date Line and South Pacific Ocean, they are called hurricanes. Typhoon is the name given to a tropical cyclone formed in the Northwest Pacific Ocean west of the dateline.

In the southwest Pacific Ocean and southeast Indian Ocean, it’s called a severe tropical cyclone. Similarly, tropical cyclones in the north Indian Ocean and southwest Indian Ocean are called severe cyclonic storm and tropical cyclone respectively.

What is a storm surge?

A storm surge is an offshore rise of water caused by the low-pressure system of a tropical cyclone. During the cyclone, high-speed winds start pushing on the ocean’s surface which piles the water up higher than sea level. The low-pressure centre of the cyclone adds to the surge and the combined effect causes flooding.

Why are cyclones named?

Tropical cyclones are named to provide ease of communication between forecasters and the public. Apart from this, they can often last a week or longer and the same basin can have more than one cyclone, hence giving a name reduces confusion. Naming of cyclones started in early 20th century when an Australian forecaster named the cyclone after politicians whom he disliked. Now, cyclones are given names contributed by member nations of the World Meteorological Organisation. The new names include those of men, women, flowers and so on. In the North Atlantic and Northeastern Pacific, feminine and masculine names are alternated in alphabetic order during a given season.

What is the process of naming cyclones?

The regional body responsible for monitoring a tropical cyclone in a particular basin makes a list of cyclone names for the particular basin. There are five such bodies which keep 10 pre-designated lists of cyclone names. The names are proposed by the member countries. For instance, the names of cyclones in the northern Indian Ocean are contributed by Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand.

The super cyclone of 1999

• On October 29, 1999, a super cyclone with a wind speed of 300 mph had struck Odisha, making it probably the greatest cyclonic disaster ever recorded in the last century. 
• It was first detected when it was at its low pressure stage over the gulf of Siam by the IMD cyclone surveillance system on the morning of October 24, five days before it made landfall.
• Winds of up to 260 kph raged for over 36 hours.
• Coastal districts of Balasore, Bhadrak, Kendrapara, Jagatsinghpur, Puri and Ganjam were forced to evacuate their homes.
• Landfall point:Between Ersama and Balikuda in Jagatsinghpur district (southwest of Paradip)
• Time of landfall 10.30 am, October 29, 1999
• Eye of storm: Paradip
• High wind speed : The wind speed of the super cyclone was so high that the anemometer, a device used for measuring wind speed, at the IMD office and at Paradip had failed to record it.
• Three days of torrential rain : The super cyclone centred over coastal areas of Odisha for three days was accompanied by torrential rain as a tidal surge of about 7 to 10 metre that swept more than 20 km inland. 
• Diameter of cyclone: 200 km 
• Originated from 1999 super cyclone had originated from about 550 km east of the Andaman Islands as a depression Storm Surge While the impending storm Phailin may cause a storm surge of about 1.5- 2 metre this time, the state witnessed it at 7 -10 metre in 1999. 
• Districts and towns affected : The storm in 1999 led to 45 cm to 95 cm of rainfall and affected 14 coastal districts, 28 coastal towns and two major cities of Bhubaneswar and Cuttack. 
• Death toll : While the official death toll then was 9,885 people, unofficial sources estimated the toll to be above 50,000. An estimated 1,500 children were orphaned. Of the total casualty, Jagatsinghpur district alone had accounted for 8,119 people. 
• Affected people : At least 13 million people, including 3.3 million children, 5 million women and nearly 3.5 million elderly people were affected in 1999. 
• Injured people : The storm had left 7,505 people injured  
• Livestock lost : 3,15,886 head of cattle 
• `Roof snatched : 16,50,086 houses damaged, 23,129 houses washed away, 7,46,337 houses fully destroyed and 8,80,620 houses partially damaged