Cyclone Sitrang

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    • Recently, Cyclone Sitrang made landfall in southern Bangladesh taking the death toll to 35 and millions other remained without power in the country.
      • It is the first tropical cyclone of the post-monsoon season of 2022. It also helped Delhi record its cleanest post-Diwali air since 2015. 

    About Cyclone Sitrang 

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    • The name Sitrang has been given by Thailand.  
      • The last October cyclone in the Bay of Bengal was Titli in 2018.
    • It developed in the Bay of Bengal before turning north toward Bangladesh’s vast coast. The maximum wind speed was 88 kmh. 
    • A red alert indicating heavy to very heavy and extremely heavy rainfall under the influence of cyclone was issued for Assam, Meghalaya, Mizoram, and Tripura.
    • The IMD is one of the world’s six RMSCs mandated to provide cyclone advisories and alerts to 13 member countries Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, United Arab Emirates and Yemen.  
    • Affected regions: 
      • Andaman and Nicobar Islands, Odisha, West Bengal and Bangladesh, particularly the coastal districts. 

    Why storms in October? 

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    • The months of October-November and May-June see storms of severe intensity develop in the North Indian Ocean comprising the Bay of Bengal and the Arabian Sea.
    • In the past 131 years, October saw 61 storms develop in the Bay of Bengal.
      • The east coast, notably Odisha, has faced many of its severest storms in October, including the Super Cyclone of 1999.
    • After the withdrawal of the Southwest monsoon, there is a rise in ocean heating, this leads to rise in sea surface temperature over the Bay of Bengal.
    • The atmospheric moisture availability over the ocean region, too, is higher.
      • So, when remnant systems from the South China Sea reach the Bay of Bengal, they get conducive conditions, aiding the formation and intensification of cyclones in October.
    • In some years, ocean-atmospheric factors hinder this phenomenon.
      • In 2020, weak La Nina conditions along the equatorial Pacific Ocean prevented a cyclonic formation near India’s coasts. 

    Cyclones in the Arabian Sea

    • In comparison with the Bay of Bengal, only 32 storms have developed in the Arabian Sea in October since 1891. 
    • Climatologically too, the IMD states that of the five storms formed in the North Indian Ocean in a calendar year, four are in the Bay of Bengal and one in Arabian Sea.  

    What is a Cyclone?

    • A cyclone is any low-pressure area with winds spiralling inwards and is caused by atmospheric disturbances around a low-pressure area distinguished by swift and often destructive air circulation. 
    • The air circulates inward in an anticlockwise direction in the Northern hemisphere and clockwise in the Southern hemisphere.
    • The amount of pressure drop in the center and the rate at which it increases outwards gives the intensity of the cyclones and the strength of winds. 
    • Eye: The centre of a cyclone is a calm area. It is called the eye of the storm. The diameter of the eye varies from 10 to 30 km. It is a region free of clouds and has light winds.
    • High-speed winds: Around this calm and clear eye, there is a cloud region of about 150 km in size. In this region there are high-speed winds (150–250 km/h) and thick clouds with heavy rain. Away from this region the wind speed gradually decreases.
    • A large cyclone is a violently rotating mass of air in the atmosphere, 10 to 15 km high. 
    • The criteria followed by the India Meteorological Department (IMD) to classify the low pressure systems in the Bay of Bengal and in the Arabian Sea as adopted by the World Meteorological Organisation (W.M.O.) are given in the following Table:

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    Types of Cyclones

    • Tropical Cyclone: Cyclones that develop in the regions between the Tropics of Capricorn and Cancer are called tropical cyclones. Tropical cyclones are large-scale weather systems developing over tropical or subtropical waters, where they get organised into surface wind circulation.
    • Extra tropical Cyclone (also called Temperate Cyclone): They occur in temperate zones and high latitude regions. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.

    Formation of Cyclones

    • Before cloud formation, water takes up heat from the atmosphere to change into vapour. When water vapour changes back to liquid form as raindrops, this heat is released to the atmosphere. 
    • The heat released to the atmosphere warms the air around. The air tends to rise and causes a drop in pressure. More air rushes to the centre of the storm. This cycle is repeated. 
    • The chain of events ends with the formation of a very low-pressure system with very high-speed winds revolving around it. It is this weather condition that is called a cyclone. 

    Conditions favouring the formation and intensification of tropical cyclone storms

    • Sea surface with a temperature higher than 27° C,
    • Coriolis force,
    • Small differences in the vertical wind speed,
    • Weak- low-pressure area.

    Source: TH

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