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Cyclone


1. Low-Pressure System

A low-pressure system is an area where the atmospheric pressure is lower than its surroundings. These systems are associated with rising warm air, which leads to cloud formation and precipitation. They are the primary drivers of weather disturbances like cyclones and storms.

  • Concept: Warm air rises, creating a region of lower pressure at the surface. As air converges to fill this void, it starts to rotate due to the Coriolis effect.
  • Example: A monsoon low-pressure system forming over the Bay of Bengal, leading to heavy rains in eastern India.

2. Depression

A depression is a more developed form of a low-pressure system with a well-defined circulation. It brings moderate to heavy rainfall and gusty winds.

  • Concept: When a low-pressure system intensifies with wind speeds between 31-49 km/h, it is classified as a depression.
  • Example: The depression over the Arabian Sea that causes heavy rainfall in Mumbai during the monsoon season.

3. Deep Depression

A deep depression is a further intensification of a depression with stronger winds and heavier rainfall.

  • Concept: A depression becomes a deep depression when wind speeds increase to 50-61 km/h. This stage is a precursor to a tropical storm or cyclone.
  • Example: A deep depression forming over the Bay of Bengal, later developing into Cyclone Yaas in 2021.

4. Cyclone

A cyclone is a large-scale air mass that rotates around a strong center of low atmospheric pressure. It is classified into different categories based on wind speed.

  • Concept: A cyclone forms when a deep depression further intensifies, with wind speeds exceeding 62 km/h. Warm ocean waters provide energy, causing rapid intensification.
  • Types of Cyclones:
    • Tropical Cyclones: Form over warm ocean waters (e.g., Cyclone Amphan in 2020).
    • Extratropical Cyclones: Occur outside tropical regions and are associated with frontal systems (e.g., Nor'easters in the U.S.).

5. Storm

A storm is a general term for a disturbed state of the atmosphere that can bring strong winds, heavy rain, thunder, lightning, and sometimes snow.

  • Concept: When wind speeds reach 62-88 km/h, the system is called a "Cyclonic Storm." If it intensifies further, it may become a Severe Cyclonic Storm (89-117 km/h) or even a Super Cyclone (>222 km/h).
  • Example: Cyclone Fani (2019) was classified as an Extremely Severe Cyclonic Storm with wind speeds exceeding 250 km/h, causing significant destruction in Odisha, India.

Wind Speed Classifications

System TypeWind Speed (km/h)
Low Pressure< 31
Depression31-49
Deep Depression50-61
Cyclonic Storm62-88
Severe Cyclonic Storm89-117
Very Severe Cyclonic Storm118-165
Extremely Severe Cyclonic Storm166-221
Super Cyclone> 222

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