Question

Explain the formation, characteristics, and distribution of tropical cyclones.

Answer 1

Approach:

1. Define tropical cyclones
2. Discuss conditions which favour the formation and intensification of tropical cyclone.
3. Explain characteristics, and global distribution of tropical cyclones with some examples.

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain.

Tropical Cyclones

• Tropical cyclones are regarded as one of the most devastating natural calamities in the world.
• They originate and intensify over warm tropical oceans.
• These are ferocious storms that originate over oceans in tropical areas and move over to the coastal areas causing violent winds, very heavy rainfall, and storm outpourings.

Conditions for the formation of Tropical Cyclone

The conditions which favour the formation and intensification of tropical cyclone storms are:

• Large sea surface with a temperature higher than 27° C
• Presence of the Coriolis force
• Small differences in the vertical wind speed
• A pre-existing weak- low-pressure area or low-level-cyclonic circulation
• Upper divergence above the sea level system

Formation of Cyclone

• The energy that strengthens the storm comes from the condensation process in the towering cumulonimbus clouds, surrounding the centre of the storm.
• With an uninterrupted supply of moisture from the sea, the storm is again strengthened.
• On reaching the terrestrial region the moisture supply is cut off and the storm dissipates.
• The place where a tropical cyclone cuts the coast is called the landfall of the cyclone.
• A landfall is frequently accompanied by sturdy winds, heavy rain and mounting sea waves that could threaten people and cause damage to properties.
• Cyclones which cross 20 degrees North latitude are more destructive.
• They cover a larger area and can originate over the land and sea, whereas the tropical cyclones originate only over the seas and on reaching the land they dissipate.

Characteristics of Cyclone
Tropical Cyclones are climatological phenomena that occur across the globe. They have certain characteristics that distinguish it from other atmospheric disturbances.

These are:

1. The foremost characteristic is that Tropical Cyclones are most violent, most awesome and most disastrous of all the atmospheric disturbances.
2. The average speed is 120 kmph.Although it may vary from 32 kmph to200 kmph or more. At times it reaches400 kmph also.
3. They have closed isobars. The pressure gradient is very sharp. More closely spaced isobars represent greater velocity of the storm and vice versa. The pressure at the center is extremely low. The winds from the surrounding area are drawn towards this low-pressure core called the "eye" of the cyclone.
4. Tropical cyclones develop over oceans and seas only. They are most violent and vigorous over water.On landfall, their velocity decreases due to friction, and as a source of energy is cut off,they dissipate soon. Thus they affect the coastal areas only.
5. The movement of tropical cyclones is affected by the prevailing wind system. Normally they move from east to west under the influence of trade winds.
6. They are seasonal in nature and occur during a specific period of the year only.

Structure of tropical cyclone
Eye of Cyclone

1. A mature tropical cyclone is characterised by the strong spirally circulating wind around the centre which is called the eye.
2. The eye is an area with calm weather descending air.
3. It is characterized by light winds and clear skies.

Eye Wall

1. Around the eye is the eyewall, where there is a strong spiralling rise of air to a greater height reaching the tropopause.
2. The wind reaches maximum velocity in this region and torrential rain occurs here.
3. From the eyewall, rain bands may radiate and trains of cumulus and cumulonimbus clouds may drift into the outer region.

Global Distributions of Tropical Cyclones (TC)

• There are about 94 tropical cyclones worldwide per year. The minimum (about 4 TCs) occurs in December, and the maximum (about 16 TCs) in August.
• The northern hemisphere has more TCs than the southern hemisphere. The western North Pacific (about 27 TCs), South Indian Ocean (16 TCs), eastern North Pacific (15 TCs), North Atlantic (10 TCs), South Pacific (8 TCs) and Australia (7 TCs) are major TC regions.
• There are regional differences of the month of maximum number of TCs. In the Pacific, Atlantic, and North Indian Ocean, the maximum numbers of TCs occur in August, September, and October, respectively. In the South Indian Ocean, it occurs in June. In the South Pacific and Australia regions, the maximums are in February and March, respectively.
• The three regions, the western North Pacific, Bay of Bengal, and South Indian Ocean, may have TCs in all months. The other regions have at least one TC-free month for the year. For example, in the North Atlantic, there is no TC in January.