Cyclones

A cyclone is a localized low-pressure system of winds rotating around a low-pressure centre. The pressure gradient is falling towards the centre from all directions and the winds converge at the centre from all directions. While converging towards the centre, the wind moves along the curved spiralling path in the anticlockwise direction in the Northern Hemisphere and clockwise direction in the Southern Hemisphere.

An anticyclone is the opposite of a cyclone in which the winds rotate clockwise in the Northern Hemisphere. There is a high-pressure centre where the air from above sinks to the ground.

Types of Cyclones

1) Tropical Cyclones:

• Tropical cyclones are among the most destructive natural disasters. These intense storms form over tropical oceans and move towards coastal areas, causing widespread damage due to powerful winds, heavy rainfall, and storm surges.
• Depending on the region, tropical cyclones are referred to by different names: Cyclones in the Indian Ocean, Hurricanes in the Atlantic, Willy-willies in Western Australia, and Typhoons in the Western Pacific and South China Sea.
• Tropical cyclones form and intensify over warm tropical waters.
• The favorable conditions for their formation and intensification include:
  • • 1. A large sea surface with temperatures above 27°C
      2. The presence of the Coriolis force;
      3. Minimal variation in vertical wind speeds;
      4. A pre-existing weak low-pressure area or low-level cyclonic circulation;
      5. Upper-level divergence above the sea-level system.
• The energy driving the storm comes from the condensation process in cumulonimbus clouds around the storm’s center.
• Continuous moisture supply from the sea strengthens the storm.
• Once the storm reaches land, the moisture supply is cut off, and it dissipates. The point where it crosses the coast is called the cyclone’s landfall.
• Cyclones crossing 20°N latitude often recurve and become more destructive.
• The eye is a calm region with subsiding air, surrounded by strong spiraling winds.
• The system’s diameter ranges from 150 to 250 km, with maximum wind speeds up to 250 km/h in the eyewall.
• Storms in the Bay of Bengal, Arabian Sea, and Indian Ocean are 600-1200 km wide and move 300-500 km per day.
• Cyclones cause storm surges, inundating coastal areas, and fade upon reaching land.

2) Extra-Tropical Cyclones:

• Extra-tropical cyclones are systems that develop in the mid and high latitudes, outside of the tropics. They cause sudden weather changes when a front moves through an area in these latitudes.
• These cyclones form along the polar front.
• At first, the front remains stationary, with warm air moving from the south and cold air from the north in the Northern Hemisphere.
• As pressure decreases along the front, warm air moves northward while cold air moves southward, initiating an anticlockwise cyclonic circulation.

• The cyclonic circulation leads to the formation of a fully developed extra-tropical cyclone, characterized by both a warm front and a cold front.
• A pocket of warm air, known as the warm sector, is wedged between the forward and rear cold air, or cold sector. The warm air rises above the cold air, creating a sequence of clouds ahead of the warm front, resulting in precipitation.
• The cold front follows the warm air from behind, forcing it upward, causing cumulus clouds to form along the cold front. The cold front moves more quickly than the warm front and eventually overtakes it.
• This lifts the warm air completely, resulting in an occluded front, which leads to the dissipation of the cyclone. The wind circulation at the surface and higher altitudes is intricately connected throughout the process.