The word ‘Cyclone’ has been derived from the Greek word ‘Cyclos’ meaning the coils of a snake. This is because the tropical storms appear like coiled serpents of the sea. Cyclones are low-pressure areas that are distinguished by swift and often destructive air circulation. They cause atmospheric disturbances such as violent storms and bad weather. In cyclones, the general circulation of air is in an anticlockwise direction in the Northern hemisphere while it is clockwise in the Southern hemisphere.
Classification of Cyclones
Cyclones have been classified into two main types. These are:
A. Extratropical Cyclones
Extratropical Cyclones or Temperate Cyclones originate in temperate zones and high-latitude regions. These cyclones are formed in the regions extending between 35°- 65° latitudes in both hemispheres. They are formed due to the convergence of two contrasting air masses having different characteristics. The process of formation of temperate cyclones is called ‘Frontogenesis’. The temperate cyclones move in an easterly direction after formation under the influence of westerly winds. These cyclones control the weather conditions in the middle latitudes.
Regions of origin
Temperate Cyclones develop mainly in the mid and high latitudes. These Cyclones are also called wave cyclones. Favorable breeding areas of temperate cyclones are:
Gulf of Alaska
North-West Atlantic Ocean Near North America
Islandic Low
Colorado Low (Near USA)
Mediterranean Sea
Gulf of Mexico
Shape and Size
The isobars in the case of temperate cyclones are not close but they take the shape of an inverted V. Hence, these cyclones are also known as ‘V-shaped’ depressions. The diameter of temperate cyclones varies between 160 Km to 3200 km. The temperate cyclones cover very large areas of about 1.6 million square kilometers.
Winds
There are two contrasting winds in the temperate cyclone. The cold dry and denser winds come from the northern direction while the warm and moist wind comes from the Southern side. At the center of the temperate cyclone, these winds converge. There is a sudden change in the wind direction which is known as the Shift Line. Different temperatures are noted in different parts of temperate cyclones.
Weather Conditions in the Temperate Cyclones
The observation point of the eastward-moving temperate cyclones experiences a wide range of weather conditions. These are under the various phases of the cyclone:
Arrival of the Cyclone: Wind velocity decreases with the arrival of the temperate cyclone. The Cirrus clouds are prevalent and the Sun and the Moon are encircled by the Halo due to the Cirro-Stratus Cloud. Temperature decreases because of the arrival of the cold sector of the cyclone while the weather remains clear and dry.
Warm Front Precipitation: Cumulonimbus Clouds are found due to the rise of the warm air which rises and cools adiabatically. It gives the warm front precipitation in the form of molecular snowfall or rainfall.
Warm Sector: Temperature increases suddenly with the advancement of this sector and there is a sudden change in the wind direction is noticed. The winds start to flow southerly and South-Westerly. The weather becomes clear and pleasant.
Cold Front Precipitation: The cold front quickly arrives with a reduction in visibility. The sky is covered by a nimbostratus cloud giving heavy downpours along with lightning and thunder.
Cold Sector: With the advancement of the cold sector of the cyclone, the weather again becomes clear and winds become westerly in nature. The sky becomes cloudless and clear.
B. Tropical Cyclones
A tropical disturbance that develops between 8°-35° N and S latitudes. These latitudes are characterized by extreme low-pressure conditions, closed isobars and counterclockwise wind circulation in the northern hemisphere over the oceanic surfaces are known as Tropical cyclones.
Regions of Development
These cyclones develop in the Inter-tropical Convergence zone (ITCZ) regions due to the convergence of warm tropical air masses. Tropical cyclones do not occur between 8° N- 8° S latitudes due to the absence of Coriolis Force.
The general direction of tropical cyclones is east to west while that of temperate cyclones is west to east.
Conditions for the development of tropical cyclones
Continuous supply of warm and moist air: They originate in the regions of the warm oceans with a surface temperature of more than 27°C upto a depth of 60 m which transfers a huge amount of water vapour to the surrounding air mass. They are very strong in the regions of warm ocean currents because such regions promote or cause shifting of the ITCZ.
Higher Value of Coriolis Force: The development of the gradient wind is assisted by the presence of Coriolis force. The Coriolis force is completely absent between 5° N and 5° S latitudes due to which the development of the tropical cyclones does not occur.
Presence of ITCZ: Recent understandings of Tropical Cyclones show that when the warm air mass converges along the ITCZ and if there is a sufficient Coriolis force to cause deflection then there are depressions and cyclones are formed. Hence, the regions of cold ocean currents in the Inter-Tropical Convergence Zone do not shift. Therefore, cyclones are absent on the western coast of the continents.
Upper-Air Divergence/Outflow: Anti-cyclonic conditions must exist between 9000-15000 m above the surface disturbance or the surface low-pressure existing near the ground. This allows pumping out or throwing out of the ascending air currents from the eye.
Weak Vertical Wind Shear: Weak vertical wind shear is a requisite condition for the development of a tropical cyclone. Therefore, Hurricanes and cyclones are formed in the equatorward latitudes of the subtropical jet streams.
Existence of mild tropical disturbance: It will help in intensifying the low-pressure depression into a violent tropical cyclone.
What is Wind shear? Wind shear is defined as a sudden change in wind velocity and/or direction over a short distance. This phenomenon can occur in all directions. But for ease of understanding, it is considered along the vertical and horizontal axis. These wind shears are termed Vertical and Horizontal Wind Shears. Vertical Wind Shear Vertical wind shear is defined as the variations of wind speed and direction along the vertical axis. Horizontal Wind Shear Horizontal wind shear consists of variations in the wind’s direction and speed along the horizontal axis.
Development and Life Cycle of a Tropical Cyclone
Initially, the advocates of the frontal theory argue that all cyclones originate due to frontogenesis. According to it, even tropical cyclones originate due to the formation of fronts formed due to the meeting of land and sea winds.
With the advent of the positivist models in geography, the frontal concept has been rejected.
The new concept says that they are formed due to the development of the low-pressure centers of the thermal origin. These low-pressure centers have emerged where the North-East and South-East trade winds meet.
The development of tropical cyclones can be divided into three stages. These are:
Stage I: Initial Stage
The warm sea surface temperature causes a huge transfer of water vapor through the process of evaporation. This causes atmospheric instability resulting in the formation of vertical cumulus clouds associated with convection. On the intensification of the low-pressure center on the addition of latent heat of condensation in the form of moist heat, air vertically rises upwards up to the tropopause intensifying the convection. This intensification of the low pressure causes the winds from the surrounding regions to rise upwards in a spiraling manner further intensifying the cyclones.
Stage II: Maturity of the Tropical Cyclone
A huge wall of cumulonimbus clouds develops in a different spiraling band. Due to an increase in the pressure at the higher level, some air descends in the central part giving rise to a rainless, cloudless center which is known as the ‘Eye’.
The temperature of the eye is highest due to compression caused by the subsidence. The fully developed tropical cyclone has numerous bands of cumulonimbus clouds which cause heavy rainfall along with lightning and thunder.
Stage 3: Decay of the Tropical Cyclone
When the eye of a tropical cyclone makes a landfall (eye touching the land), the source of moisture is cut off and the cyclone begins to ebb. This can also occur when the cyclones touch over the cold water. The most dangerous part of the cyclone during this stage is high sea waves which hit the landmass with strong velocity known as the “Storm Surge.”
Anticyclones
These are high-pressure centres with decreasing pressure outwards. They are characterized by descending air, resulting in cloudless and clear skies with dry weather conditions promoting aridity. Due to this reason, the anticyclones are termed a weatherless phenomenon. They are the regions of subsidence with horizontal divergence near the surface. As the Pressure gradient is very low the isobars are widely spaced and wind velocity is low.
Size: The size of the anticyclones is very large having a diameter between 10000 km to 1 lakh kilometres.
Regions of Development: These are formed in the areas of subsidence of the air. These are sub-tropical high-pressure belts and Polar high-pressure belts. Generally, anticyclones often exist between two cyclones which influences their characteristics and tracks. Temporary anti-cyclones are formed in the regions of the cold weather due to snowfall. For example, In Himalayan regions such high-pressure zones develop.
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