Tropical Cyclones

Cyclones are the centres of low pressure surrounded by closed isobars having increasing pressure outward and closed air circulation from outside towards central low pressure. The winds move anti-clockwise in northern hemisphere and clockwise in southern hemisphere. Based on location, cyclones are classified in two major types:

  • Tropical cyclones
  • Extra Tropical cyclones


A tropical cyclone is a type of low-pressure weather system that forms over tropical or subtropical waters. They are characterized by strong winds, heavy rainfall, and a low-pressure centre. Tropical cyclones are known by different names depending on their location, such as hurricanes in the Atlantic and north-eastern Pacific, Willy-willy in North West Australia, typhoons in the northwest Pacific, and cyclones in the South Pacific and Indian Ocean.

Factors which facilitate their formation:

Warm water: Tropical cyclones form over warm tropical or subtropical waters with a surface temperature of at least 26.5°C. Warmth of water provides energy needed to fuel the storm.

Moist air: Tropical cyclones require moist air to form. Moisture provides fuel for thunderstorms that make up the storm.

Low wind shear: Wind shear is the change in wind speed or direction with height. Low wind shear is essential for development of a tropical cyclone because it allows the storm to maintain its organization and strength. (High wind shear removes the heat and moisture they need from the area near their center. Shear also distorts the shape of a hurricane by shearing it (blowing the top away from the lower portion), so that the vortex is tilted. A tilted vortex is usually a less efficient heat engine–the delicate balance of inflowing low-level winds and outflowing upper-level winds that ventilate the storm gets disrupted.)

A pre-existing weather disturbance: Tropical cyclones typically form from pre-existing weather disturbances, such as a tropical wave or an area of low pressure. These disturbances provide initial rotation and organization needed for a tropical cyclone to form.

Converging winds: Tropical cyclones form in areas where winds are converging and rising, this allows for the development of thunderstorms and the low-pressure area that is the tropical cyclone.

Coriolis force: It helps the wind to rotate. This is the reason that cyclones are not formed at Equator.

It’s worth noting that all these conditions have to be met and be in the right balance, otherwise it would lead to the tropical cyclone dissipating, or fail to form. Also, the storm needs to be in an environment where it can maintain its strength and not be disrupted by other weather systems or wind shear.


IMD uses a color-coded warning system to classify severity of tropical cyclones. The system uses four colors: Green, Yellow, Orange, and Red.

  • Green: This color is used for cyclones that are not expected to cause significant damage. The IMD issues a “Green” warning for cyclones that are likely to cause light to moderate rainfall and winds of up to 40 km/h.
  • Yellow: This color is used for cyclones that are expected to cause moderate damage. The IMD issues a “Yellow” warning for cyclones that are likely to cause heavy rainfall and winds of 40-60 km/h.
  • Orange: This color is used for cyclones that are expected to cause substantial damage. The IMD issues an “Orange” warning for cyclones that are likely to cause very heavy to extremely heavy rainfall and winds of 60-100 km/h.
  • Red: This color is used for cyclones that are expected to cause severe damage. The IMD issues a “Red” warning for cyclones that are likely to cause extremely heavy rainfall and winds of more than 100 km/h.
  • The warning system is designed to help people prepare for a cyclone, and to take appropriate action to protect themselves, their families, and their property. The color code is based on the forecasted wind speed, rainfall, and surge height. The IMD also issues forecasts and updates on the progress of the storm and provides advice on what actions to take in response to the storm.
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