Climatology

What are Tropical Cyclones and their characteristics?

Tropical cyclones , one of the most deadly storm on the planet.

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What are Tropical Cyclones?

Tropical cyclones represent a circulatory motion of air towards a low-pressure centre.
These low-pressure centres are known by various names based upon their characteristics, intensity and, distribution.
The origin and development of these cyclones require the presence of some favourable conditions.
In these cyclones, the wind blows counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere.
They develop in tropics and move from east to west.
They mainly originate on the western side of oceans within the tropics.
Most of the tropical cyclones develop in the doldrums region.
In Northern Hemisphere, they are most frequent in August-October and Southern Hemisphere during March-April.
Tropical low-pressure centres are one of the most powerful and deadly atmosphere storms on the planet.
They are associated with violent winds and heavy rainfall and represent destructive weather phenomena.
Tropical cyclones cause a great loss of life and property.

Video source- https://youtu.be/W2UDbDXXYGE

Typical Isobars in a Tropical Cyclone Development

Image Source: http://3.bp.blogspot.com/_jeetJTfzUoU/TMovh81Dh0I/AAAAAAAAJ4I/E7kLf6-8LFU/s1600/ExtratropicalCyclone-min-sfc-pressure-27Oct2010.jpg
Diagram-1. Isobars in a low-pressure centre

Characteristics of tropical cyclone

Size of tropical cyclone- On average they are smaller than a temperate cyclone. Their diameter ranges between 500– 600 km. Sometimes their diameter is restricted to 50 km or even less.

Pressure- Due to a steep pressure gradient, they are more intense than temperate cyclones. The central pressure varies between 50-60 Mb. Tropical cyclones are associated with the extreme velocity of the wind.

Intensity- These low-pressure centres are vigorous over the oceans but become weak over land. They die out after reaching the interior of the continents. These cyclones are energised by the release of latent heat of condensation.

Mobility- Tropical cyclones are not always mobile. Sometimes they become stationary over a place for several days and cause heavy precipitation. They move in well-frequented tracks.

Direction- They move westward under the influence of prevailing trade winds.

Areas- Tropical cyclones are found in well-defined areas in certain seasons. They develop mostly in the tropical oceans between 5⁰ to 20⁰ latitudes.

Identification- A great variety of these cyclones can be identified based on their shape, size and associated weather.

Circular centre of a cyclone– The almost circular centre is the most fascinating feature of a tropical cyclone which is called its eye. The diameter of an eye varies between 8 to 50km.

Classification of Tropical Cyclones-

Based upon the intensity, size and weather conditions, the tropical cyclones are classified into the following four types.

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Tropical disturbances-

These are wave-like cyclones, move east to west due to easterly trade winds. They are also known as easterly waves.
About 80 per cent of these disturbances develop between 5⁰ and 20⁰ N latitudes on the western side of the oceans.
These disturbances originate near the boundary of trade winds with doldrums.
Western disturbances are associated with moderate to heavy rain due to cumulus and cumulonimbus clouds.
Good weather before the disturbance is the general characteristic and it is followed by bad weather. Initially, the weather is fine and scattered cumulus clouds. It is followed by occasional rainfall, and finally by moderate to heavy rainfall from heavy cumulus and cumulonimbus clouds occurs.

Tropical Depressions-

Tropical depressions are low-pressure centres encircled by more than one closed isobar.
On average, their velocity varies between 40-60km/hr.
These depressions mainly occur in the Intertropical convergence zone (ITCZ).
In summer season these depressions influence the weather conditions of India and Australia.
They normally fail to attain the size of a storm and die out as weak disturbances.

Tropical Storms-

Tropical storms are low-pressure centres encircled by precisely placed isobars. The wind velocities in Tropical storms range between 63 to 118 km per hour.
These storms are common in the Bay of Bengal, Arabian Sea, Caribbean Sea and in the vicinity of Philippines, during the summers.
They are associated with heavy rainfall and storm surges in coastal areas.

Hurricanes or typhoons-

Tropical cyclones are warm vortex circulatory wind systems with closed circular isobars.
They accompanied with torrential rains, sustained with maximum wind velocity (119kmph).
These cyclones are known by different names in the different parts of the world.
Hurricanes represent the most powerful and destructive tropical cyclones.
The term Hurricane is used for tropical cyclones of the Caribbean Sea and Gulf of Mexico region.
The pressure gradient is very steep at the centre and having a pressure of 950mb.
Hurricanes have very high speed, having 119kmph.
These cyclones are also called typhoons in the western North Pacific Ocean.
They are known as Willy Willies in Australia, cyclones in the Indian ocean, Baguio in Philippines and Taifu in Japan.
If a cyclone attains a speed of 200kmph or more, it is classified as Super Cyclone.

Image Source: http://faculty.uml.edu/nelson_eby/87.202/IMAGES/Tropical%20cyclone.jpg
Diagram-2. Hurricanes

Major oceanic currents pdf

The Origin of Tropical cyclones-

A tropical cyclone is like a heat engine that is energized by the latent heat of condensation. The exact mechanism that brings about the formation of a tropical cyclone is not well understood as yet. Certain basic requirements must be fulfilled for the development of a tropical cyclone.

Continuous Supply of a Large Amount of Warm and Moist Air– A well-defined tropical cyclones originate only over large tropical ocean surface where the temperature is 27⁰C or above. Generally, tropical low-pressure centres originate on the western side of tropical oceans. As this condition is not fulfilled in the South Atlantic Ocean they fail to develop. The power of a tropical cyclone depends on the latent heat of condensation. These cyclones are most frequent over the warmer tropical oceans especially during the warmer parts of the year.

Strong Coriolis force– Due to the absence of Coriolis force, the tropical cyclones are not developed over the equator. It is only at 5⁰ latitudes that the minimum required level of Coriolis force prevails. Therefore, these cyclones are mostly concentrated in the belt of 5⁰ to 30⁰ latitude.

Upper level air divergence- – At a height of 9000 to 12000 m above the surface disturbance, there must be an anti-cyclonic circulation.

Minimal vertical Wind shear- There must be minimal vertical wind shear between lower and upper troposphere. In case, different wind directions and speeds operate vertically over an area, cyclones will fail to develop.

Existence of Mild Tropical Disturbances- Sometimes the weak tropical disturbances occasionally develop into a large tropical cyclone. This happens when abundant warm and moist air results in the formation of an intense column of latent heat.

Structure of Tropical Cyclone

The following are the parts of a tropical cyclone-

Eye- At the centre of a fully developed tropical cyclone lies the ‘eye’ which is more or less circular. The diameter of an eye varies between 20 to 40km. This is a calm zone, having light and variable wind. The cloud is either absent or scattered, so the sky appears to blue having intermittent sunlight. Here the pressure is low, the temperature is high and relative humidity is very high. As a result, the weather is hot and sultry. The low pressure in the eye induces the air from above to sink into the eye. The sinking air is heated due to adiabatic compression, which is the reason for abnormal high temperature is the eye and absence of cloud cover.

Eyewall– The eye is surrounded by a wall of cumulonimbus clouds known as an eyewall. This is a more or less circular region of about 10-20 km wide. It is here, the strongest winds are found having vertical motion. As a result, most intense rainfall occurs in this region.

Spiral bands– They surround the eyewall and give the cyclone the appearance of a galaxy from the space. They are also called rain bands because they are associated with cumulonimbus clouds and thunderstorms.

Annular zone–This belt surrounds spiral bands and is characterized by low humidity, suppressed cloudiness and high temperature. This is formed due to subsidence of air from aloft at the outer limit of the cyclone.

Outer convective zone- This belt surrounds the annular belt and is characterized by intense convective activity.

Diagram-3 Structure of Tropical cyclone

Distribution of Tropical Cyclones- Tropical Cyclones are confined to tropical areas. The six major source regions of tropical cyclones are as following.

Tropical North Atlantic– Gulf of Mexico, West Indies and the Caribbean Sea.
The western part of the tropical north pacific- the Philippines, the china sea and areas around Japan.
Eastern part of the tropical north pacific-the western coastal areas of Mexico and Central America.
The Bay of Bengal and Arabian sea.
The south Indian ocean – coastal regions of Madagascar.
Western South Pacific Ocean – in the regions of Samoa, Fiji Island and the east and north coasts of Australia. The largest number of tropical cyclones develops in the southern segment of the North Pacific Ocean.

Diagram-4. Distribution of Tropical Cyclone.

Saffir-Simpson Hurricane Scale.

Category	Central Pressure (mb)	Wind Speed (km/hr)	Wind Speed (miles per hour)	Storm surge (metre)	Damage
1.	>979	119-153	74-95	1.2-2.5	Minimal
2.	965-979	154-177	96-110	1.6-2.4	Moderate
3.	945-964	178-209	111-130	2.5-3.6	Extensive
4.	920-944	210-250	131-155	3.7-5.4	Extreme
5.	<920	>250	>155	>5.4	Catastrophic