Cyclone – is a system of winds rotating inwards to an area of low pressure, with an anticlockwise (northern hemisphere) or clockwise (southern hemisphere) circulation. Cyclones are also called depressions. Cyclones bring strong winds, rainfall or snowfall.

Tropical Cyclones
A tropical cyclone is an intense low pressure area or a whirl in the atmosphere over tropical or sub-tropical waters.

From the centre of a cyclonic storm, pressure increases outwards. The rate at which the pressure increases, determines the strength of the cyclone.

As per the criteria adopted by Indian Meteorological Department, the low pressure systems are classified in the following categories:

Low pressure areas with maximum sustained surface winds of speed between 31 Km/h and 61 Km/h are classified as tropical depressions.
1. Wind speed between 62 Km/h and 88 Km/h are called tropical cyclones. Once, winds attain such a speed, tropical cyclone is assigned a name.
2. When wind speed is between 89-118 Km/h, it is called a severe cyclonic storm.
3. When wind speed is between 119-221 Km/h, it is called a very severe cyclonic storm.
4. When wind speed exceeds 221 Km/h, the cyclone is called a super cyclonic storm.

Naming of Tropical Cyclones
Naming of tropical cyclones facilitates ease of communication about tropical cyclone among general public and weather experts. Moreover, if at the same time two or more cyclones affect the same region, naming can help in identification of particular cyclone.

Naming for tropical cyclones was first adopted in WWII in some regions. By mid-1960’s all the regions except North Indian Ocean region began using names for tropical cyclones.

In 2003, north Indian Ocean region also began practice of naming tropical cyclones.

Various regional organizations comprising of member nations prepare a pre- determined list of names. These names are used in sequential order. The list of Northern Indian Ocean tropical cyclone names are as follows:

List of Northern Indian Ocean tropical cyclone names
Contributing nation
Bangladesh India Maldives Myanmar Oman Pakistan Sri Lanka Thailand
Onil Agni Hibaru Pyarr Baaz Fanoos Mala Mukda
Ogni Akash Gonu Yemyin Sidr Nargis Rashmi Khai-Muk
Nisha Bijli Aila Phyan Ward Laila Bandu Phet
Giri Jal Keila Thane Murjan Nilam Viyaru Phailin
Helen Lehar Madi Nanauk Hudhud Nilofar Ashobaa Komen
Chapala Megh Roanu Kyant Nada Vardah Maarutha Mora
Ockhi Sagar Mekunu Daye Luban Titli Gaja Phethai
Fani Vayu Hikaa Kyarr Maha Bulbul Pawan Amphan

Note: The above list is for reference purpose only. A candidate is not required to learn the list for purpose of examination.

Conditions for the Origin of Tropical Cyclones
1. Tropical cyclones emerge over the surface of seas. Development over the sea enables the tropical cyclone to carry large amount of moisture on account of evaporation. The temperature requirement for the development of low pressure and high rate of evaporation is 27oC or more.

2. The winds rotate in anti- clockwise direction in northern hemisphere and in clockwise direction in southern hemisphere on account of coriolis force. Coriolis force increases with latitude and is zero at equator. Beyond 5° latitude, coriolis force is enough to rotate the winds. However, beyond 10° latitude, the temperature is not high enough to create conditions for low pressure required for tropical cyclones. Thus, tropical cyclones emerge between latitudes 5° to 10°.

3. Inter tropical convergence zone (when trade winds meet) provide suitable ground for the creation of tropical cyclones. When the ITCZ is beyond 5° latitude, then there is convergence of winds of different temperature. The warm winds rise, creating vacuum for the cold winds to rotate

4. Vertical winds which form a part of tropical cyclones are uniform in nature. Winds which do not vary with height are called low wind shear.

Characteristics of Tropical Cyclones
1. Size and Shape- Tropical cyclones are elliptical in shape. They have compact size in comparison to temperate cyclones. Their size varies from 80 Kms to 1500 kms. The pressure gradient (i.e. change in pressure) is very high in tropical cyclones as compared to the pressure gradient in temperate cyclones.

2. Wind Velocity – Wind velocity in a tropical cyclone is higher at the margins than that at the centre. Moreover, wind velocity is higher over oceans than that over landmass because landmass acts as a physical barrier to winds.

3.Movement – Tropical cyclones move from east to west direction under the influence of trade winds.

4. Structure of Tropical Cyclones – The centre of tropical cyclone is called an eye of a cyclone. The diameter of an eye of a cyclone varies from 10 – 50 Kms. The wind speed is minimum and the sky is clear in this region. The region surrounding the eye is called eye wall. It is made up of cumulonimbus clouds. A continuous ring of cumulonimbus cloud attains great height. Cumulonimbus clouds are responsible for heaviest precipitation.

Why does east coast of India experiences tropical cyclones whereas west coast of India does not experience tropical cyclones?
East coast of India experiences tropical cyclones whereas west coast of India does not. This is so because cyclones which develop in Bay of Bengal region move under the influence of trade winds towards the east coast of India. On the other hand, cyclones which develop in Arabian Sea move under the influence of trade winds towards Africa.

Why do tropical cyclones emerge in India during the month July – Sept ?
1. Tropical cyclones emerge when the sea surface temperature is 27°C or more, such high temperature arises in summer season over Bay of Bengal.
2. ITCZ passes through Bay of Bengal during this time period. ITCZ shifts in accordance with the maximum sun rays on the earth.

ITCZ is formed where winds coming from sub-tropical regions (both northern and southern hemisphere) meet.  Usually, ITCZ is at equator. However, ITCZ shifts depending upon the maximum intensity of Sun rays. During summers of northern hemisphere, ITCZ shifts northwards. During summers of southern hemisphere, ITCZ shifts southwards.

Depending upon the region, Cyclones are known differently as follows:

S.No. Region Name
1. China Typhoon
2. Indian Ocean Tropical cyclone
3. West Indian Islands Hurricanes
4. West Africa Tornado
5. Southern USA Whirlwind
6. Australia Willy-Willy

Why Tropical Cyclones are rare in Southern hemisphere?
In southern hemisphere, ocean temperatures in summers are lower than that in summers in northern hemisphere. This is because the southern hemisphere has lesser landmass, consequently generating lower warming influence during summers. As a result, the high temperature required for formation of tropical cyclones is never attained.

Moreover, absence of landmass facilitates movements of strong winds which hamper formation of tropical cyclones.