What is a Tropical Cyclone?
Tropical cyclones originating over the oceans at the tropical latitudes are characterised as intense circular storms accompanied by strong winds and torrential rain. There is a depression at the centre of a tropical cyclone, which is marked by very low atmospheric pressure. Tropical cyclones are also termed hurricanes or typhoons and generally run at a speed of about 119 kilometres per hour. They maintain a steady pace as long as they draw energy from the warm oceans.
Characteristics of a Tropical Cyclone
The tropical cyclones are majorly hazardous for the coastal regions, as the sea level can rise by 20 feet above normal when the cyclones hit the coasts. These cyclones commonly occur in summer, when the temperature at the tropical oceans is quite high, and a low-pressure area develops over it. The depression creates a great pull at the centre of the cyclone, and it takes the shape of a circular hurricane. Tropical cyclones are likely to occur in the months of June, July, August, and September in the Northern hemisphere and in the months of January, February, and March in the Southern hemisphere. At times, tropical cyclones surpass the speed of 240 kilometres per hour or even 320 kilometres per hour, devastating the coastal areas.
The typical characteristics of a tropical cyclone are listed below.
Tropical cyclones develop over the warm regions of oceans, close to the equator. They are so named because they are storms rotating rapidly above the tropical oceans.
The surface winds of a tropical cyclone usually exceed 100 knots.
The centre of the cyclone is termed the eye of the cyclone. There are no clouds at the eye of the cyclone. The sea-level pressure at the centre of a tropical cyclone is 900 Mb or even lower.
A severe low pressure develops at the centre of the tropical cyclone. In general, the central part of a tropical cyclone extends up to about 200 kilometres to 500 kilometres. However, at times, it may extend up to 1000 kilometres.
In the southern hemisphere, the tropical cyclones are characterized by winds blowing clockwise and by winds blowing anti-clockwise in the northern hemisphere.
Tropical cyclones may be highly destructive and are accompanied by torrential rains, violent storms, high waves, and coastal floods.
These cyclones are often given names to generate a high alert among the public when the wind strength exceeds the safe limits.
How is a Tropical Cyclone Formed?
Tropical cyclones develop over ocean waters that have a temperature above 26 or 27 degrees Celsius. The air just above the warm water heats up and rises, creating a low-pressure point over the water. The air layer undergoes adiabatic expansion and remains warmer in comparison with the surrounding undisturbed air column. The disturbances that may potentially lead to a cyclone are detected within 5 degrees from the equator. However, the disturbances that may potentially turn into hurricanes or typhoons are detected beyond 5 degrees from the equator.
The formation of tropical cyclones is explained by the convective and the counter-current or frontal theories. The convective theory states that a large column of air over the warm water becomes unstable and moist and rises up. The cold air from the surroundings tends to rush towards the low-pressure area created due to the upward shift of the warm air. Hence a cyclonic circulation develops in that area. However, the centrifugal force and the rotation of the earth retard the air movement towards the low-pressure central area. This leads to a further fall in pressure at the centre, and it continues till the vigorous cyclonic system is formed. An outward flow of air from the central area of the cyclonic system also lowers the pressure further.
On the other hand, according to the frontal theory, tropical cyclones are developed along the fronts between the equatorial air in the doldrums and the trade winds. The two air masses merge along the front leading to an upward motion of the air. The low-pressure area thus formed is even aggravated due to the centrifugal force, the deflective effect of the earth’s rotation, and the divergence of air at the upper levels. Thereafter, a spiral circulation with a low-pressure area at the centre is formed along the front.
The convective and frontal theories account for the formation of low-pressure central areas for tropical cyclones. These cyclonic winds develop over ocean waters that are at least 26 degrees Celsius hot, and the heat of condensation further facilitates the process and provides sufficient energy for a cyclone or hurricane to be developed. Tropical cyclones are also developed in disturbances along the intertropical zone of convergence.
At this zone, the cyclonic storms are formed under the superimposed upper disturbances or on the transverse waves. The deepening of the surface pressures must be favoured at the intertropical convergence zone to facilitate the formation of a cyclone. The divergence at the upper levels must exceed the convergence at the lower levels to favour the deepening of surface pressures. Hence, the formation of tropical cyclones is favoured by an optimum combination of divergence, circulation, and convergence, maintained on a standard scale over a considerable amount of time.
Ranking and Naming a Cyclone
Based on the speed and intensity of the wind, a tropical cyclone can be ranked as moderate, severe, intense, and very intense.
Tropical cyclonic storms are popular as Typhoons in the northwestern Pacific.
The storms developing in the Arabian Sea and Bay of Bengal area are called cyclones. The storms developing in the southwestern region of the Indian Ocean are termed tropical cyclones.
They are popularly known as hurricanes in the central and eastern regions of the North Pacific ocean, in the Gulf of Mexico, Caribbean Sea, and North Atlantic Ocean.
They are called severe tropical cyclones in the western parts of the South Pacific Ocean and the southeastern parts of the Indian Ocean.
Tropical cyclones are classified on the basis of the maximum wind speed sustained. The latent heat of condensation is the source of energy for these cyclones. Hence, these cyclones are formed over warm oceans and fail to sustain over the land. Tropical depressions, storms, hurricanes, cyclonic storms, and tropical cyclones are classified depending upon the speed of the wind. The following section will help you understand how to name a cyclone based on sustained wind speed.
Tropical Depression: When the maximum sustained speed of the wind measures up to 61 kilometres per hour or 33 knots, it is termed a tropical depression.
Tropical Storm: When the maximum sustained speed of the wind is between 62 kilometres per hour and 88 kilometres per hour, or between 34 and 47 knots, it is termed a tropical storm.
Severe Tropical Storm: When the maximum sustained wind speed measures between 89 kilometres per hour and 117 kilometres per hour, or between 48 and 63 knots, it is classified as a severe tropical storm.
Typhoon: When the maximum wind sustained speed measures between 118 kilometres per hour and 220 kilometres per hour or between 64 and 120 knots, it is classified as a typhoon.
Super Typhoon: When the maximum wind sustained speed measures beyond 220 kilometres per hour or beyond 120 knots, it is classified as a super typhoon.
Effects of Tropical Cyclones
Tropical cyclones are most likely to have devastating effects on the lives and property in coastal areas. The storms and heavy rains cause losses in property and lives. The effects of tropical cyclones are discussed below.
Strong Winds: The strong winds accompanying tropical cyclones increase by about 1.2 to 1.6 times or more in magnitude and are maintained for several minutes or even for more than an hour. The wind speed increases gradually or suddenly and results in unprecedented damage. The spatial expanse of these strong winds ranges between 2 kilometres and 10 kilometres.
Storm Surge: Storm surge occurs in the oceans due to tropical cyclones and is characterised by an abnormal rise in the water level. The meteorological driving forces lead to these surges. Storm surges, occurring along the coasts having low-lying terrain, lead to inland inundation or surge across inland water bodies like estuaries, bays, or lakes. Typically, a cyclonic storm comes with a surge that can impact about 160 kilometres of the coastline for several hours.
Tornadoes: Tornadoes are formed along with strong convection. These storms have a greater concentration in the right-front quadrant, wherein the air undergoes a shorter trajectory over the land.
Rain and Flood: Tropical cyclones always bring along heavy rainfall that can be harmful or beneficial. The cyclonic rains are beneficial for the water-deficit areas, whereas they can cause flooding in the low-lying areas and can be highly destructive.
Conclusion
Tropical cyclones are among the most destructive natural calamities. Their wind speeds, location of origin, and direction determine how to name a cyclone. In order to generate a safety alert beforehand, the meteorological departments across the globe adopt various scientific methods and techniques to track the evolution of any cyclone, its intensity, and course. Before every tropical cyclone, official warnings are circulated among the people living along the coastlines, and especially among the fishermen.
FAQs on Tropical Cyclone
1. What is a tropical cyclone?
A tropical cyclone is a powerful, rapidly rotating storm system with a low-pressure centre, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. These storms form exclusively over warm tropical or subtropical oceans, as they derive their energy from the heat and moisture of the sea surface.
2. What are the primary conditions necessary for the formation of a tropical cyclone?
Several atmospheric and oceanic conditions must be met for a tropical cyclone to form. The key requirements include:
- A large and continuous supply of warm, moist air from a sea surface with a temperature of at least 27°C.
- The presence of the Coriolis force to initiate the cyclonic rotation, which is why these storms do not form near the equator (0°-5° latitude).
- A pre-existing area of weak low pressure or atmospheric disturbance.
- Low vertical wind shear, meaning the wind speed and direction do not change significantly with height, allowing the storm's structure to remain intact and grow vertically.
- Upper-level atmospheric divergence, which helps pump air away from the storm's centre at high altitudes, further intensifying the surface low pressure.
3. What are the main parts of a mature tropical cyclone's structure?
A fully developed tropical cyclone has three main parts:
- The Eye: The calm, low-pressure centre of the storm. It is often cloud-free with light winds.
- The Eyewall: A dense ring of powerful thunderstorms surrounding the eye. This is where the strongest winds and heaviest rainfall are found.
- Rainbands: Spiral bands of clouds, rain, and thunderstorms that extend outwards from the eyewall. These bands can be hundreds of kilometres long and contain heavy bursts of rain and wind.
4. How are tropical cyclones named and categorised based on their intensity?
Tropical cyclones are named from a pre-determined list maintained by the World Meteorological Organization (WMO) and regional meteorological bodies. Names are recycled and retired if a storm is particularly destructive. They are categorised based on their maximum sustained wind speed. For example, in the North Indian Ocean region, the India Meteorological Department (IMD) classifies them as Cyclonic Storm, Severe Cyclonic Storm, Very Severe Cyclonic Storm, and so on, with increasing wind speeds.
5. Why do tropical cyclones spin in different directions in the Northern and Southern Hemispheres?
The direction of a cyclone's spin is determined by the Coriolis effect, an apparent force created by the Earth's rotation. In the Northern Hemisphere, this effect deflects moving air to the right, causing cyclones to rotate in a counter-clockwise direction. Conversely, in the Southern Hemisphere, the air is deflected to the left, resulting in a clockwise rotation.
6. What is the 'eye' of a cyclone, and why is it so calm?
The 'eye' is the calm, low-pressure centre of a mature tropical cyclone. The calmness is a result of sinking air. As the powerful winds in the eyewall rotate, some of this air is forced into the centre, where it sinks. This sinking motion compresses and warms the air, which prevents cloud formation and leads to light winds and often clear skies, creating a stark contrast to the violent conditions in the surrounding eyewall.
7. How does a tropical cyclone weaken and eventually dissipate?
A tropical cyclone begins to weaken and dissipate when its primary energy source is cut off or its structure is disrupted. This typically happens when:
- The cyclone moves over land, depriving it of the warm, moist air from the ocean that fuels it.
- The cyclone moves over colder ocean waters that cannot provide enough heat and moisture to sustain its intensity.
- The cyclone encounters strong vertical wind shear, which tilts the storm's vertical structure and disrupts the circulation, causing it to fall apart.
8. What is the difference between a tropical cyclone and an extratropical (temperate) cyclone?
While both are low-pressure systems, they differ significantly. A tropical cyclone forms over warm tropical oceans, gets its energy from the latent heat of condensation, has a warm core, and is symmetrical in shape. In contrast, an extratropical cyclone (or temperate cyclone) forms along frontal boundaries in mid-latitudes, derives its energy from the temperature difference between warm and cold air masses, has a cold core, and is asymmetrical.
9. Which regions in India are most vulnerable to tropical cyclones and why?
India's East Coast, particularly the states of Odisha, Andhra Pradesh, West Bengal, and Tamil Nadu, is more vulnerable to tropical cyclones than the West Coast. The primary reason is that the Bay of Bengal is a more active cyclonic basin. Its sea surface temperatures are generally higher, and its enclosed shape can intensify storms. Furthermore, it often receives the remnants of typhoons from the Northwest Pacific, which can re-intensify into cyclones.
