How Tropical cyclones are formed?

News: Cyclone Burevi has weakened into a deep depression as its interaction with a landmass in Tamil Nadu has slowed its movement and intensity.

Cyclonic disturbances are rapid weather systems, which include weather depressions, deep depressions, and tropical cyclones (of severe, very severe, extremely severe, and super cyclonic storms depending on their intensity).

What is Tropical cyclone Burevi ?

Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large-scale destruction caused by violent winds, very heavy rainfall, and storm surges.

Cyclones create a whirl in the atmosphere with very strong winds circulating around it in an anti-clockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere.

The pressure gradient falls towards the center from all directions and therefore winds try to converge towards the center from all directions. The air blows inwards in an anticlockwise direction in the northern hemisphere and clockwise in the southern hemisphere.

Conditions for formation of Cyclones

The conditions favorable for the formation and intensification of tropical storms are:

  1. A consistent source of heat as tropical cyclones are thermally induced low-pressure systems.
  2. Large sea surface with a temperature higher than 27° C which is possible only during the late summers i.e. September, October, and November
  3. Presence of the Coriolis force.
  4. Small variations in the vertical wind speed.
  5. A pre-existing weak low-pressure area or low-level-cyclonic circulation;
  6. Upper divergence above the sea level system.

Structure of Tropical Cyclone

The structure of a Tropical Cyclone is typically a massive cumulonimbus cloud with rapidly rising air spiraling upwards at the margins of the eye. An eye is a region of calm with subsiding air. Around the eye is the Eyewall, where strong spiraling winds ascends and the height can reach up to the tropopause.

Tropical cyclones are formed by the following parts:

Eye:  

The eye is the center of cyclones which is characterized by a calm area, sinking, and light wind. The eye is the calmest part of the Cyclone structure.

Conservation of angular momentum and centrifugal force are the reasons behind its formation.

Eyewall:

A band around the eye of the greatest wind speed, where clouds reach the highest and precipitation is the heaviest. The heaviest wind damage occurs where a hurricane’s eyewall passes over land.

Rain bands:

Curved bands of clouds and thunderstorms that trail away from the eyewall in a spiral fashion. These bands are capable of producing heavy bursts of rain and wind. Sometimes gaps are found between spiral rain bands, where no impact (wind or rain) of cyclones are found.

Formation of Tropical Cyclones

Cyclone Structure

  • As mentioned in the conditions, air temperature higher than 27° C with an abundant and turbulent transfer of water vapor to the overlying atmosphere (air) by evaporation is required for the formation of Cyclones, which is facilitated by direct insolation.
  • As the low-pressure area is created over sea pockets and high pressure on the surrounding areas, air starts to flow from the high-pressure area to low-pressure pockets.
  • The Coriolis force causes the wind to spiral around a low-pressure area. As the presence of Coriolis force is negligible in the equatorial belt between 5 degrees north and 5 degrees south latitudes, hence cyclonic systems do not develop in this region.
  • Heated sea surface starts to heat the air over that leading to air moving up and away from the ocean surface due to convection, it leaves less air near the surface. Cool Air from the surrounding areas rushes towards the empty area to fill it, which after reaching there gets heated and picks moisture and starts rising upwards. It creates the cycle of air moving in and up.
  • As the moist air rises up, it starts cooling with the altitude (temperature falls with the height), and the process of condensation starts. It results in the release of the latent heat of condensation. The latent heat of condensation is what drives the storm and leads to the formation of clouds.
  • The energy that intensifies the storm, comes from the condensation process in the towering cumulonimbus clouds, surrounding the center of the storm (Eye).
  • With the increasing altitude, the air cools down to an extent at tropopause where it fails to rise any further and starts to diverge outside horizontally.
  • The whole system of clouds and wind spins and grows, fed by the ocean’s heat and water evaporating from the ocean surface. As the storm system rotates faster and faster, an eye forms in the center.
  • A mature tropical cyclone is characterized by the strong spirally circulating wind around the center, called the eye. The diameter of the circulating system can vary between 150 and 250 km.
  • Rain bands regions with cumulonimbus clouds are created, facilitating intense rainfall in that region. Cloud formation is dense at the center and density decrease towards the outside.

Conditions that slow or end Cyclones

  • With a continuous supply of moisture from the sea, the storm is further strengthened. On reaching the land the moisture supply is cut off and the storm dissipates. The place where a tropical cyclone crosses the coast is called the landfall of the cyclone.
  • Another condition that slowdown or ends a Cyclone is when dry, cool air is suddenly present in the system, which reduces the possibility of convection to keep the storm going.

Why More Cyclones are formed in Bay of Bengal?

There are other coastlines around the world that are vulnerable to surging storms – the Gulf Coast of Louisiana, for example – but the “north coast of the Bay of Bengal is more prone to catastrophic surges than anywhere on Earth”.

Both the Bay of Bengal and the Arabian Sea are prone to Cyclonic storms, but Cyclonic activities are more intense and frequent in Bay of Bengal Compared to the Arabian Sea

  • High sea surface temperatures along with high humidity due to higher rainfall in the Bay of Bengal, triggers extremely strong cyclones.
  • Sluggish winds, along with warm air currents in the Bay of Bengal keep temperatures relatively high.
  • The supply of constant inflow of fresh water from the Ganga and Brahmaputra rivers makes it impossible for the warm water to mix with the cooler water below.
  • Cyclonic winds easily move into the Bay of Bengal due to the presence of moisture source from rivers and the absence of any large landmass unlike the Arabian Sea, where Cyclones usually weaken due to the presence of Western Ghats.
  • Whereas Arabian Sea receives stronger winds that help dissipate the heat, and the lack of constant fresh water supply helps the warm water mix with the cool water, reducing the temperature.

Classification of storms

Indian Meteorological Department (IMD), which classifies the low pressure systems in the Bay of Bengal and the Arabian Sea on the basis of capacity to damage, which is adopted by the WMO.

tropical cyclone classification

Benefits of Tropical Cyclones:

Although Tropical cyclones are known for the destruction they cause, when they strike, they also bestow certain benefits to the climatic conditions of that area such as

  • Relieve drought conditions: By bringing rain to the coastal areas, cyclones relieve the drought-like conditions in the surrounding areas.
  • Maintain equilibrium in the Earth’s troposphere: They Carry heat and energy away from the tropics towards temperate latitudes, thus helps in maintaining an equilibrium of the troposphere.
  • Cyclones help in maintaining a relatively stable and warm temperature worldwide.

Causes of destruction caused by Cyclones:

There are three elements associated with cyclones that cause destruction during its occurrence. These are:

Strong Gusts/Squall:
    • These are very strong winds that accompany a cyclonic storm damages installation, dwellings, communications systems, trees, resulting in loss of life and property.
  • Gusts: These are short but rapid bursts in wind speed. These are the main cause of damage. Gusts are generally short-lived.
  • Squalls: A squall is a strong rise in wind speed which generally lasts for some time. Squalls generally associated with the bands of thunderstorms.
Torrential rains and inland flooding:
  • Torrential rainfall (more than 30 cm/hour) associated with cyclones is another major cause of damages.
    • Unabated rain gives rise to unprecedented floods.
    • Rain is a serious problem for the people which become shelterless due to cyclone.
    • Heavy rainfall from a cyclone is usually spread over a wide area and causes large-scale soil erosion and weakening of embankments.
Storm Surge:
  • It can be defined as an abnormal rise of sea level near the coast caused by a severe tropical cyclone;
    • Seawater inundates low-lying areas of coastal regions drowning human beings and life stock, causes eroding beaches and embankments, destroys vegetation, and leads to a reduction of soil fertility.

Apart from these Cyclones also create destructions such as:

    • Sudden Change in Regional climate: The ability of cyclone to bring in warmer air is high. So, the elderly and children in those areas have a high vulnerability to develop heat-related problems such as heat strokes.
    • Loss of Livelihood: The majority of the coastal people generally depend on fishing which is completely halted by cyclones.
    • Loss of economy: The economic loss is in multifront from infrastructure loss, relief packages to people, etc.

Government Initiatives:

  • Government is carrying out a National Cyclone Risk Mitigation Project (NCRMP) with the help of the World Bank to upgrade cyclone forecasting, tracking, and warning systems in India
  • Government is also implementing the Integrated Coastal Zone Management Project (ICZMP) to improve national capacity for the implementation of comprehensive coastal management in India.
  • Government also separated Structural(includes construction) and non-structural measures for effective disaster management of cyclones
  • Solutions:
  • The government should consider the NDMA Guidelines for the management of cyclones:
  • Ensemble Warning System(EWS): Establish EWS involving observations, predictions, warnings, and customized local level advice for decision-makers (national, state, district level) to manage the impact of the cyclone (Read more about EWS)
  • Commissioning of Aircraft Probing of Cyclone (APC): Guidelines calls for the combination of manned and Unmanned Aerial Vehicles (UAV) for critical observational data gaps.
  • Cyclone Disaster Management Information System (CDMIS): Establishing a comprehensive department for coverage of all management information and provide online services to the departments of Disaster management.
  • Specifying the roles and responsibilities in institutionalizing Cyclone risk mitigation with Developmental planning.
  • Community-Based Disaster Management (CBDM): Guidelines asked to launch such activities in all villages of the 84 districts vulnerable to cyclones.

Way forward:

  • With the adverse Climate Change risks posted by IPCC reports the only option for India is to better preparedness for the disaster with better urban planning, community awareness, etc.
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