Cyclone Asani

In News

• The India Meteorological Department (IMD) has predicted the formation of Cyclone Asani in the Bay of Bengal.

About

• Cyclone Asani is brewing in the southern Andaman Sea.
• It is on course to become a short-lived cyclone and will make landfall between Visakhapatnam in Andhra Pradesh and Bhubaneswar in Odisha.

What is 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:

Image Courtesy: Report 

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. 

Formation of Cyclone (Image Courtesy: NCERT)

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.

Development cycle of tropical cyclones

• The development cycle of tropical cyclones may be divided into three stages:
  • Formation and Initial Development Stage
  • Mature Tropical Cyclones
  • Modification and Decay
• Formation and Initial Development Stage:
  • The formation and initial development of a cyclonic storm depends upon various conditions. These are:
    • A warm sea (a temperature in excess of 26 degrees Celsius to a depth of 60 m) with abundant and turbulent transfer of water vapour to the overlying atmosphere by evaporation.
    • Atmospheric instability encourages formation of massive vertical cumulus clouds due to convection with condensation of rising air above ocean surface.
• Mature Tropical Cyclones:
  • When a tropical storm intensifies, the air rises in vigorous thunderstorms and tends to spread out horizontally at the tropopause level. 
  • Once air spreads out, a positive perturbation pressure at high levels is produced, which accelerates the downward motion of air due to convection. 
  • With the inducement of subsidence, air warms up by compression and a warm ‘Eye’ is generated. 
  • Generally, the ‘Eye’ of the storms has three basic shapes: 
    • Circular; 
    • Concentric; and 
    • Elliptical. 
  • The main physical feature of a mature tropical cyclone in the Indian Ocean is a concentric pattern of highly turbulent giant cumulus thundercloud bands.
• Modification and Decay:
  • A tropical cyclone begins to weaken in terms of its central low pressure, internal warmth and extremely high speeds, as soon as its source of warm moist air begins to ebb, or is abruptly cut off. 
  • This happens after its landfall or when it passes over cold waters.

Nomenclature

• The names are maintained and updated by an international committee of the World Meteorological Organisation (WMO).
• The tropical cyclones /hurricanes are named neither after any particular person, nor with any preference in alphabetical sequence.
• Regional Specialised Meteorological Centres (RSMCs) – They are responsible for monitoring and prediction of tropical cyclones over their respective regions. They are also responsible for naming the cyclones.
• There is a strict procedure to determine a list of tropical cyclone names in an ocean basin(s) by the Tropical Cyclone Regional Body responsible for that basin(s) at its annual/biennial meeting. 
• The importance for naming tropical cyclones: 
  • It helps to identify each individual tropical cyclone. 
  • It facilitates disaster risk awareness, preparedness, management and reduction. 
  • Local and international media become focused on the tropical cyclone.
  • It removes confusion where there are multiple cyclonic systems over a region.
  • Warnings reach a much wider audience very rapidly, if a name is associated with it.
• Also, a cyclone is known by different names in different parts of the world as:

Source: DTE