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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 CycloneTropical 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  latitude is significant enough to create a storm [cyclonic vortex].
  • About 65 percent of cyclonic activity occurs between 10° and 20° latitude.

Tropical Cyclones | Famous Books for UPSC Exam (Summary & Tests)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
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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 EyeThe Eye

Question for Tropical Cyclones
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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.

Question for Tropical Cyclones
Try yourself:Which factor primarily influences the path of a tropical cyclone?
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The document Tropical Cyclones | Famous Books for UPSC Exam (Summary & Tests) is a part of the UPSC Course Famous Books for UPSC Exam (Summary & Tests).
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FAQs on Tropical Cyclones - Famous Books for UPSC Exam (Summary & Tests)

1. What is a tropical cyclone?
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, strong winds, and thunderstorms. It develops over warm tropical or subtropical waters and can cause significant damage due to high winds, heavy rainfall, storm surges, and tornadoes.
2. How are tropical cyclones formed?
Tropical cyclones are formed when warm, moist air over the ocean rises and begins to rotate due to the Earth's rotation (Coriolis effect). As the warm air rises, it cools and condenses, forming clouds and releasing heat energy. This process creates a low-pressure area, and if the conditions are favorable, the system can strengthen and develop into a tropical cyclone.
3. What are the different categories of tropical cyclones?
Tropical cyclones are categorized based on their maximum sustained wind speeds using the Saffir-Simpson Hurricane Wind Scale. The categories are as follows: - Category 1: 74-95 mph (119-153 km/h) - Category 2: 96-110 mph (154-177 km/h) - Category 3: 111-129 mph (178-208 km/h) - Category 4: 130-156 mph (209-251 km/h) - Category 5: 157 mph or higher (252 km/h or higher)
4. How long do tropical cyclones typically last?
The duration of a tropical cyclone can vary depending on various factors such as environmental conditions and the size of the storm. On average, tropical cyclones can last for about a week, but some can dissipate within a couple of days, while others may persist for several weeks.
5. How can we prepare for a tropical cyclone?
To prepare for a tropical cyclone, it is important to: - Stay informed: Monitor weather forecasts and warnings issued by meteorological agencies. - Create an emergency kit: Include essential supplies such as food, water, medications, flashlights, batteries, and a first aid kit. - Secure your property: Trim trees, secure loose objects, and reinforce windows and doors to minimize damage. - Evacuation plan: Know the evacuation routes and have a plan in place in case evacuation is necessary. - Follow official instructions: Follow the guidance of local authorities and evacuate if advised to do so for your safety.
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