Tropical Cyclones | Famous Books for UPSC Exam (Summary & Tests) PDF Download

Tropical Cyclones

• Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large-scale destruction due to violent winds (squalls), very heavy rainfall (torrential rainfall), and storm surge.
• They are irregular wind movements involving the closed circulation of air around a low-pressure center. This closed air circulation (whirling motion) is a result of rapid upward movement of the hot air which is subjected to Coriolis force. The low pressure at the center is responsible for the wind speeds.

Tropical Cyclone

Coriolis Force (f)

• The Coriolis force is zero at the equator (no cyclones at the equator because of zero Coriolis Force) but it increases with latitude. Coriolis force at 5° latitude is significant enough to create a storm [cyclonic vortex].
• About 65 percent of cyclonic activity occurs between 10° and 20° latitude.

Origin and Development of Tropical Cyclones

• The tropical cyclones have a thermal origin, and they develop over tropical seas during late summers (August to mid-November).
• At these locations, the strong local convectional currents acquire a whirling motion because of the Coriolis force.
• After developing, these cyclones advance till they find a weak spot in the trade wind belt.

Origin

• Under favorable conditions, multiple thunderstorms originate over the oceans. These thunderstorms merge and create an intense low pressure system (wind is warm and lighter).

Early-stage

• In the thunderstorm, the air is uplifted as it is warm and light. At a certain height, due to lapse rate and adiabatic lapse rate, the temperature of the air falls and moisture in the air undergoes condensation.
• Condensation releases latent heat of condensation making the air warmer. It becomes much lighter and is further uplifted.
• Space is filled with fresh moisture-laden air. Condensation occurs in this air and the cycle is repeated as long as the moisture is supplied.
• Due to excess moisture over oceans, the thunderstorm intensifies and sucks in the air at a much faster rate. The air from surroundings rushes in and undergoes deflection due to Coriolis force creating a cyclonic vortex (spiraling air column. Similar to a tornado).

Due to centripetal acceleration (centripetal force pulling towards the center is countered by an opposing force called the centrifugal force), the air in the vortex is forced to form a region of calmness called an eye at the center of the cyclone. The inner surface of the vortex forms the eyewall, the most violent region of the cyclone.

• All the wind that is carried upwards loses its moisture and becomes cold and dense. It descends to the surface through the cylindrical eye region and at the edges of the cyclone.
• Continuous supply of moisture from the sea is the major driving force behind every cyclone. On reaching the land the moisture supply is cut off and the storm dissipates.
• If the ocean can supply more moisture, the storm will reach a mature stage.

Question for Tropical Cyclones

Try yourself:What is the major driving force behind the development and intensification of tropical cyclones?

• A.

  Centripetal force
• B.

  Coriolis force
• C.

  Trade wind belt
• D.

  Continuous supply of moisture from the sea

Explanation

D) Continuous supply of moisture from the sea

• The major driving force behind the development and intensification of tropical cyclones is the continuous supply of moisture from the sea. Tropical cyclones have a thermal origin and develop over warm tropical seas during late summers.
• Under favorable conditions, multiple thunderstorms originate over the oceans, which merge and create an intense low-pressure system.
• The warm and moisture-laden air is uplifted in these thunderstorms, and due to condensation, it releases latent heat, becoming lighter and further uplifted.
• The excess moisture over oceans causes the thunderstorm to intensify and create a cyclonic vortex with spiraling air columns.
• The air from the surroundings rushes in and undergoes deflection due to the Coriolis force, forming the cyclonic vortex.
• The continuous supply of moisture from the sea is essential for the cyclone to maintain its strength and intensity.
• Once the cyclone moves over land, the moisture supply is cut off, leading to the storm's weakening and eventual dissipation. If the ocean can supply more moisture, the cyclone may reach a mature stage and potentially intensify further.

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Mature stage

• At this stage, the spiraling winds create multiple convective cells with successive calm and violent regions.
• The regions with cumulonimbus cloud (rising limbs of the convective cell) formation are called rain bands below which intense rainfall occurs.
• The ascending air will lose moisture at some point and descends (subsides) back to the surface through the calm regions (descending limbs of convection cell – subsiding air) that exist between two rain bands.
• Cloud formation is dense at the center. The cloud size decreases from center to periphery.
• Rain bands are mostly made up of cumulonimbus clouds. The ones at the periphery are made up of nimbostratus and cumulus clouds.
• The dense overcast at the upper levels of the troposphere is due to cirrus clouds which are mostly made up of hexagonal ice crystals.
• The dry air flowing along the central dense overcast descends at the periphery and the eye region.

Structure of a tropical cyclone

Eye

• A mature tropical cyclone is characterized by the strong spirally circulating wind around the center which is called the eye.
• The “eye” is a roughly circular area of comparatively light winds, clear skies. and fair weather found at the center of a severe tropical cyclone.
• There is little or no precipitation and sometimes blue sky or stars can be seen.
• The eye is the region of lowest surface pressure and warmest temperatures aloft (in the upper levels) – the eye temperature may be 10°C warmer or more at an altitude of 12 km than the surrounding environment, but only 0-2°C warmer at the surface in the tropical cyclone.
• Eyes range in size from 8 km to over 200 km across, but most are approximately 30-60 km in diameter.

The Eye

Favorite Breeding Grounds for Tropical Cyclones

• South-east Caribbean region where they are called hurricanes.
• Philippines islands, eastern China, and Japan where they are called typhoons.
• The Bay of Bengal and the Arabian Sea where they are called cyclones.
• Around the south-east African coast and Madagascar-Mauritius islands.
• North-west Australia.

Characteristics of Tropical Cyclones

The main features of tropical cyclones are as follows.

Size and Shape

• Tropical cyclones have symmetrical elliptical shapes (2:3 ratio of length and breadth) with steep pressure gradients. They have a compact size—80 km near the center, which may develop up to 300 km to 1500 km.

Wind Velocity and Strength

• Wind velocity, in a tropical cyclone, is more in poleward margins than at center and is moreover oceans than over landmasses, which are scattered with physical barriers. The wind velocity may range from nil to 1200 km per hour.

Path of Tropical Cyclones

• These cyclones start with a westward movement but turn northwards around 20° latitude. They turn further north-eastwards around 25° latitude, and then eastwards around 30° latitude. They then lose energy and subside.
• Tropical cyclones follow a parabolic path, their axis being parallel to the isobars.
• Coriolis force or earth’s rotation, easterly and westerly winds influence the path of a tropical cyclone.
• Tropical cyclones die at 30° latitude because of cool ocean waters and increasing wind shear due to westerlies.

Warning of Tropical Cyclones

• Detection of any unusual phenomena in the weather leading to cyclones has three main parameters: fall in pressure, increase in wind velocity, and the direction and movement (track) of the storm.
• There is a network of weather stations monitoring pressure fall and wind velocities in all countries of the world, including the Arctic and Antarctic regions.
• The islands attain special significance in this as they facilitate monitoring of these developments.
• In India, there are detection radars along both coasts.
• Monitoring is also done by aircraft which carry a number of instruments including weather radar.
• Cyclone monitoring by satellites is done through very high-resolution radiometers, working in the visual and infra-red regions (for night view) of the spectrum to obtain an image of the cloud cover and its structure.
• Remote sensing by radars, aircraft, and satellites helps predict where exactly the cyclone is going to strike. It helps in taking advanced steps in the following areas:
  • closing of ports and harbors,
  • suspension of fishing activities,
  • evacuation of the population,
  • stocking of food and drinking water, and
  • provision of shelter with sanitation facilities (safety homes).
• Today, it is possible to detect a cyclone right from its genesis in the high seas and follow its course, giving a warning at least 48 hours prior to a cyclone strike.
• However, the predictions of a storm course made only 12 hours in advance do not have a very high rate of precision.