Tropical cyclones, also referred to as low-pressure systems, originate from warm tropical waters. These cyclones form when the sea surface temperature exceeds 26.5°C. Typically lasting for days or even weeks, tropical cyclones follow irregular courses. Once a cyclone moves over land or encounters cooler ocean waters, it dissipates.
CYCLONES
- Cyclones are characterized by a low-pressure system with swift inward air circulation.
- In the Northern Hemisphere, the air flows counterclockwise, while in the Southern Hemisphere, it circulates clockwise.
- Cyclones are often linked to storms and adverse weather conditions.
- The term “Cyclone” originates from the Greek word “Cyclos,” meaning snake coils.
- Henry Paddington coined this term, inspired by the resemblance of tropical storms in the Bay of Bengal and the Arabian Sea to coiled sea serpents.
TROPICAL CYCLONE
The term “Tropical Cyclone” is employed by the World Meteorological Organization to denote weather systems with winds exceeding “Gale Force,” with a minimum speed of 63 km per hour.
- These cyclones form in the region between the Tropics of Capricorn and Cancer.
- They are large-scale weather systems originating over tropical or subtropical oceans, characterized by the consolidation of surface wind circulation.
- Tropical cyclones are formidable storms that emerge over tropical oceans and subsequently move inland, causing widespread destruction with their violent winds, heavy rainfall, and storm surges.
- Recognized as one of the world’s most devastating natural disasters, tropical cyclones undergo formation and intensification over warm tropical oceans.
Optimal conditions for the formation of a tropical cyclone include:
Warm Sea Surface Temperature:
- A substantial expanse of water surface with temperatures exceeding 27°C is favorable for tropical cyclone formation.
- The warm ocean provides the necessary heat and moisture to fuel the storm.
Coriolis Force:
- The Coriolis force, a result of the Earth’s rotation, must be sufficiently strong to initiate the development of a cyclonic vortex.
- This force imparts the characteristic rotation to the storm system.
Stable Vertical Wind Speed:
- Minimal variations in vertical wind speed are essential for the development and consolidation of a tropical cyclone.
- Stable atmospheric conditions contribute to the organization of the storm.
Low-Pressure Area or Cyclonic Circulation:
- The presence of a weak low-pressure area or a pre-existing low-level cyclonic circulation is a precursor to tropical cyclone formation.
- Such conditions provide the initial atmospheric disturbance necessary for cyclone development.
Higher Divergence Above Sea Level:
- Above the sea-level system, there should be a higher level of divergence.
- This divergence supports the upward movement of air, contributing to the intensification of the cyclonic system.
These factors collectively create an environment conducive to the initiation and strengthening of tropical cyclones.
DEVELOPMENTAL STAGE OF CYCLONE
Origin of Tropical Cyclones:
- Tropical cyclones originate over tropical oceans during late summers, typically from August to mid-November.
- The thermal conditions during this period provide the necessary warmth for cyclone formation.
- The Coriolis effect, influenced by the Earth’s rotation, imparts a spinning motion to local convectional currents over these tropical regions.
Formation and Initial Development Stage:
- The critical process in the creation and early development of a cyclonic storm involves the transport of water vapor and heat from the warm ocean to the air above.
- This transfer occurs primarily through evaporation from the sea surface.
- Convection sets in, leading to the condensation of rising air above the ocean surface.
- This process stimulates the formation of large vertical cumulus clouds.
Mature Stage:
- As a tropical cyclone intensifies, powerful thunderstorms cause the air to rise, spreading out horizontally at the tropopause level.
- The spreading air creates positive pressure at higher elevations, accelerating the downward movement due to convection.
- This subsiding air warms up through compression, resulting in the formation of a warm “Eye,” which is the low-pressure center of the cyclone.
- In the Indian Ocean, a mature tropical cyclone exhibits a concentric pattern of turbulent cumulus thundercloud bands as a prominent physical feature.
Modification and Decay:
- The weakening of a tropical cyclone begins when its source of warm, moist air diminishes or is abruptly cut off.
- This decline manifests in core low-pressure reduction, loss of internal warmth, and a decrease in speed.
- Factors contributing to the modification and decay include landfall or passage over cooler waters, leading to a gradual dissipation of the cyclonic system.
Understanding the developmental stages of tropical cyclones provides insights into their lifecycle and the conditions that influence their formation, intensification, and eventual weakening.
Structure of a Tropical Cyclone:
1. Eye:
- The “eye” is a roughly circular area at the center of a severe tropical cyclone characterized by mild winds and favorable weather conditions.
- In the eye, there is minimal to no precipitation, and occasional sightings of blue sky or stars are possible.
- It has the lowest surface pressure and the warmest temperatures aloft. The eye’s temperature can be 10°C or more warmer than the surrounding environment at a height of 12 km, with only 0-2°C warmer temperatures near the surface.
- Eyes vary in size, ranging from 8 kilometers to over 200 kilometers, with the majority falling between 30 and 60 kilometers in diameter.
2. Eyewall:
- The “eyewall” is a circular ring of deep convection that surrounds the eye and harbors the strongest surface winds in the tropical cyclone.
- It consists of numerous moderate to powerful updrafts and downdrafts, contributing to the heated temperatures of the eye.
- The eyewall region has the greatest sustained winds, representing the fastest winds in the cyclone.
- Sinking air is prevalent in the eye, while the eyewall experiences a net upward flow.
- Soundings within the eye typically reveal a low-level moist layer with an inversion above.
- Sinking in the eye may not reach the ocean surface but extends to a depth of 1-3 kilometers.
3. Spiral Bands:
- Eyewall convection plays a role in the formation and maintenance of the eye, and spiral bands are associated features of tropical cyclones.
- Spiral bands are long, thin rain bands that face the same direction as the horizontal wind and appear to spiral into the storm’s core.
- These bands contribute to low-level convergence and upper-level divergence, forming a direct circulation pattern.
- Convergence at the surface, upward motion through the bands, upper-level divergence, and descending air on both sides create a dynamic circulation pattern.
- Adiabatic warming occurs as the air cools, leading to a significant pressure drop across the bands and strengthening tangential winds around the cyclone.
4. Vertical Structure:
- The vertical structure of a tropical cyclone is divided into three layers:
- Inflow Layer (Up to 3 km): Crucial for storm generation.
- Cyclonic Storm Layer (3 km to 7 km): Main region for cyclonic activity.
- Outflow Layer (Above 7 km): Features anticyclonic air movement, with the highest outflow observed at 12 kilometers and above.
Favorite Breeding Grounds for Tropical Cyclones:
- 1. South-east Caribbean Region:
- Commonly referred to as hurricanes in this region.
- 2. Philippines Islands, Eastern China:
- Known as typhoons when they form in this area.
- 3. Bay of Bengal and Arabian Sea:
- Referred to as cyclones in the Bay of Bengal and Arabian Sea.
- 4. South-east African Coast and Madagascar-Mauritius Islands:
- Regions around the south-east African coast and Madagascar-Mauritius islands are conducive to cyclone formation.
- 5. North-west Australia:
- Favorable for the development of tropical cyclones.
Local Names of Tropical Cyclones:
- 1. Hurricanes:
- Used to describe tropical cyclones in the Atlantic and Eastern Pacific regions.
- 2. Typhoons:
- Refers to tropical cyclones in Southeast Asia.
- 3. Cyclones:
- Commonly used term for tropical cyclones in the Indian Ocean and Western Pacific around Australia.
FAQs – Tropical Cyclones
1. What is a tropical cyclone?
A tropical cyclone is a powerful storm characterized by a low-pressure system with rapid inward air circulation. It originates over warm tropical waters and is known by different names in various regions, such as hurricanes, typhoons, or cyclones.
2. How do tropical cyclones form?
Tropical cyclones form when the sea surface temperature exceeds 26.5°C. They develop over warm tropical or subtropical oceans, typically between the Tropics of Capricorn and Cancer. The cyclone follows irregular courses, lasting for days or weeks, and dissipates when moving over land or cooler ocean waters.
3. What are the optimal conditions for tropical cyclone formation?
Several factors contribute to the formation of tropical cyclones, including warm sea surface temperatures (above 27°C), a strong Coriolis force, stable vertical wind speed, the presence of a low-pressure area or cyclonic circulation, and higher divergence above sea level.
4. How are tropical cyclones classified?
Tropical cyclones are classified by the World Meteorological Organization based on their wind speeds. The categories include Tropical Depression, Tropical Storm, Category 1 (Gale Force), Category 2 (Moderate), Category 3 (High), Category 4 (Very High), and Category 5 (Devastating).
5. What is the lifecycle of a tropical cyclone?
The lifecycle of a tropical cyclone involves three stages: Formation and Initial Development, Mature Stage, and Modification and Decay. The cyclone originates over warm tropical oceans, intensifies through convective processes, reaches maturity with a well-defined eye and eyewall, and weakens when its heat and moisture source diminishes.
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