Key Takeaways
About Cyclones
- Cyclone, any large system of winds that circulates about a centre of low atmospheric pressure
- In the Northern Hemisphere, air flows counterclockwise, while in the Southern Hemisphere, it circulates clockwise.
How Are Cyclones Formed?
- Warm, moist air over the ocean rises due to less density. This air rises up and away from the ocean surface, leaving less air near the surface, creating a low-pressure zone.
- Due to the surrounding high-pressure areas, air flows into this low-pressure area and eventually warms up, forming a cycle.
- Now, this cycle makes the warm air above the ocean rise and cool the water below to generate clouds. With the constant heating and evaporating process, the entire cloud and wind system spins and grows.
- With more speed, cyclone eye formation takes place in the centre. This zone signifies the lowest air pressure and is calm and clear. The high-pressure air from above flows down into this region.
Classification of Cyclones
- Cyclones are classified as extra tropical cyclones (also called temperate cyclones); and tropical cyclones.
Tropical Cyclones
- The term ‘Tropical Cyclone’ is used by the World Meteorological Organisation to describe weather systems with winds greater than ‘Gale Force’ (minimum of 63 km per hour).
- A tropical cyclone, an intense circular storm that originates over warm tropical oceans and is characterised by low atmospheric pressure, high winds, and heavy rain.
- These cyclones are one of the world’s most destructive natural disasters.
- Favourable Condition for tropical cyclone
- A large area of the sea surface with a temperature above 27° C.
- The Coriolis force is present.
- The vertical wind speed varies slightly.
- A weak low-pressure area or low-level-cyclonic circulation that already existed.
- Above the sea level system, there is a higher divergence.
- Formation
- The development cycle of tropical cyclones may be divided into three stages:
- 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 Stage
- The air rises in powerful thunderstorms as a tropical storm intensifies, and it tends to spread out horizontally at the tropopause level. When air spreads out, a positive pressure is created at high elevations, speeding up the downward migration of air due to convection.
- When subsidence is induced, air warms up due to compression resulting in a warm ‘Eye’ (low pressure centre). A m ature tropical cyclone in the Indian Ocean has a concentric pattern of highly turbulent large cumulus thundercloud bands as its principal physical feature.
- Modification and Decay
- As soon as its source of warm moist air begins to ebb or is abruptly cut off, a tropical cyclone begins to weaken in terms of core low pressure, internal warmth, and extremely fast speeds.
- This occurs after it reaches land or passes across chilly water.
Extratropical Cyclones
- Mid-latitude depressions, temperate cyclones, frontal depressions, and wave cyclones are all names for extratropical cyclones.
- In both hemispheres, these are active above the mid-latitudinal zone between 35° and 65° latitude.
- The movement is from west to east, and it is most noticeable during the winter months.
- Polar and tropical air masses collide and generate fronts in these latitude zones.
- Formation
- The Polar Front theory best explains the formation and evolution of temperate cyclones.
- According to this idea, a polar front is generated when warm-humid air masses from the tropics collide with dry-cold air masses from the poles.
- Because the cold air mass is denser and heavier, the warm air mass is pushed up.
- The contact of cold and warm air masses causes instability, resulting in a drop in pressure at the intersection, particularly in the centre of the interactions.
- As a result of the reduced pressure, a void is generated.
- The surrounding air rushed in to fill the void, and a cyclone formed as a result of the earth’s rotation.
- Extratropical cyclones contrast with the more violent tropical cyclones or hurricanes, which occur in generally constant temperature zones.
Names Around the World
Naming Process of Cyclone
- An international committee of the World Meteorological Organisation maintains and updates cyclone lists and names (WMO).
- Six different lists are used in a rotating fashion.
- As a result, the 2020 list will be used again in 2026.
- If a country has been devastated by a storm, the names will not be mentioned again for obvious reasons.
- Katrina in the United States (2005), Sandy in the United States (2012), Haiyan in the Philippines (2013), Irma, and Maria in the Caribbean are examples (2017).
Cyclones: Indian Context
- The subcontinent with a long coastline of 8041 kilometres is exposed to nearly 10 percent of the world’s tropical cyclones.
- Of these, the majority of them have their initial genesis over the Bay of Bengal and strike the East coast of India.
- More cyclones occur in the Bay of Bengal than the Arabian Sea and the ratio is approximately 4:1
- Tropical cyclones occur in the months of May-June and October-November.
- Why is the Bay of Bengal the hot-bed for cyclonic storms?
- Cyclones in the Bay of Bengal can be attributed to the vast low pressure created by the warm water of the ocean.
- the bay that gives birth to severe cyclones is concave or shallow where when strong winds push water, it gets concentrated as a storm.
- The Bay of Bengal shaped like a trough that makes it more hospitable for storms to gain force.
- Moreover, the high sea surface temperature makes matters more worse in the Bay triggering the intensity of the storms.
- Bay of Bengal gets more rainfall with sluggish winds and warm air currents around it that keep temperatures relatively high all year
- The constant inflow of fresh warm water from the perineal rivers like Bramhaputra, Ganga makes it further impossible to mix with the cooler water below.
- Lack of landmass between the Pacific Ocean and the Bay of Bengal tend cyclonic winds to move into the coastal areas causing heavy rainfall.
- The absence of air movements from north-western India towards the Bay in the post-monsoon phase is also another reason for the chances of cyclones in the Bay of Bengal.
- What geographical advantage the Arabian sea has?
- Arabian Sea, it is much calm as the stronger winds help dissipate the heat and lack of constant fresh water helps the warm water to mix with the cool water underneath, reducing the surface temperature.
- The Arabian Sea enjoys the locational advantage as the winds from the Pacific Ocean encounter the Western Ghats and the Himalayas cutting down on its intensity and sometimes never reaching the Arabian Sea.
- Impact of Cyclones in India
- Strong winds cause damages to infrastructure, uproots trees, and lead to other catastrophes.
- Torrential rainfall leads to unprecedented floods and damages to houses and buildings.
- Due to storm surge, seawater levels rise, and the coastal areas are exposed to floods.
- The rise in seawater level also erodes beaches and embankments.
- Severe cyclonic storms resulting in floods can damage vegetation and livestock.
- Due to the strong winds and flood conditions, the soil becomes infertile.
- In addition to this, cyclonic storms result in the loss of human, plant and animal lives and affect the country’s economy.
- Cyclone Hazard Zones in India
- As per the Indian Meteorological Department, 13 coastal regions and Union Territories in India are prone to a cyclone.
- This happens because of the long coastline of 8041 Km, which is exposed to nearly 10% of the world’s tropical cyclones.
- The most vulnerable states in India are as follows –
- West Bengal.
- Andhra Pradesh.
- Tamil Nadu.
- Odisha.
- Puducherry (UT)
- Gujarat.