The cyclones are irregular wind movements involving closed air circulation around a low pressure centre. This closed air circulation is caused by atmospheric disturbances over and above the earth’s surface, coupled with the earth’s ‘rotation which imparts to these disturbances a whirling motion.
Cyclones are associated with destructive and violent disturbances, such as heavy squalls and torrential rainfall.
Significance of Cyclones:
The cyclones level down inequalities of pressure and wind movement over the globe. They play an important role in the complex process of heat exchange between various latitudinal zones. Cyclones have a bearing over the phenomenon of precipitation, especially in mid-latitude regions, by lifting up the moist air from oceans and taking it into the surrounding landmasses.
The cyclonic wind movements are anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere. The cyclones are often characterised by existence of an anticyclone between two cyclones. Depending on their area of origin and principal tracks followed, the cyclones can be either tropical or temperate/extra-tropical.
1. Tropical Cyclones:
Origin and Development:
The tropical cyclones have a thermal origin, and they develop over tropical seas during certain seasons. At these locations, the local convectional currents acquire a whirling motion because of the Coriolis force generated by the earth’s rotation. After developing, these cyclones advance till they find a weak spot in the trade wind belt.
The ideal conditions for the development of tropical cyclones are:
(i) Quiet air
(ii) High temperature
(iii) Highly saturated atmospheric conditions.
Such conditions exist over the equatorial doldrums, especially in western margins of oceans, which have great moisture carrying capacity because the trade winds continuously replace the saturated air. Also, the whirling motion is enhanced when the doldrums are farthest from the equator. This happens during the autumnal equinox (August-September). At this time, there are two advantages—the air is overheated and the sun is exactly over the equator. The source of energy for the development of tropical cyclones is latent heat of condensation.
Conditions Favourable for Tropical Cyclones:
Various conditions are favourable for the formation of tropical cyclones. These are described below.
Source of Latent Heat:
Tropical cyclones are formed over warm waters having temperatures of 26°C or more. Sufficient evaporation leads to the accumulation of moisture above the ocean surface. This, in turn, provides the necessary latent heat to supply the energy for the storm and is later released in the process of cloud and rain formation.
As D.S. Lai points out, tropical cyclones originate in the western part of the oceans where temperatures are relatively higher than their eastern parts. The cold currents lower the surface temperatures of the eastern parts of the tropical oceans making them unfit for the breeding of such storms.
The depth of warm water (26-27°C) should extend for 60-70 m from surface of the ocean/sea, so that deep convection currents within the water do not churn and mix the cooler water below with the warmer water near the surface.
Coriolis Force (f):
The Coriolis force is zero, at the equator but it increases with latitude. A Coriolis force exceeding 10-5 /second in magnitude occurs above 5° latitude. This magnitude is strong enough to help the development of a cyclonic vortex. So we find that about 65 per cent of cyclonic activity occurs between 10° and 20° latitude. Of course, the Coriolis force continues to increase with latitude, but at the higher latitudes the other factors necessary for cyclone formation cease to exist.
Low-level disturbance in the form of easterly wave disturbances in the Inter-Tropical Convergence Zone (ITCZ) should preexist. Small local differences in the temperature of water and of air produce various low pressure centres of small size. A weak cyclonic circulation develops around these areas. Then, because of the warm humid air and the latent instability of the air column, a true hurricane vortex may develop very rapidly. However, it may be pointed out that only a few of these disturbances develop into hurricanes.
Trade winds from both the hemispheres meet along a line called-the inter-tropical front. Temperature contrasts between these air masses must exist when the ITCZ is farthest, from the equator. Thus, the convergence of these winds of different temperatures and the resulting instability are the prerequisites for the origin and growth of violent tropical storms.
Upper Air Disturbance:
The remains of an upper tropospheric cyclone from the Westerlies move deep into the tropical latitude regions. As divergence prevails on the eastern side of the old abandoned troughs, a rising motion occurs; this leads to the development of thunderstorms. Further, these old abandoned troughs usually have cold cores, suggesting that the environmental lapse rate is steeper and unstable below these troughs. Such instability encourages thunderstorms.
The vertical wind shear between the upper and lower layers of the troposphere should remain at the minimum level. Tropical cyclones develop when the wind is uniform. Because of weak vertical wind shear, hurricane formation processes are limited to latitude equator ward of the subtropical jet stream.
Upper Tropospheric Divergence:
A well- developed divergence in the upper layers of the atmosphere is necessary so that the rising air currents within the cyclone continue to be pumped out and a low pressure maintained at the centre.
High humidity (around 50 to 60 per cent) is required in the mid-troposphere, since the presence of moist air leads to the formation of cumulonimbus cloud.
Favourite Breeding Grounds:
Major areas of origin and tracks of tropical cyclones are shown in Fig. 2.23.
These breeding grounds are:
1. South-east Carribean region where they are called hurricanes.
2. Philippines islands, eastern China and Japan where they are called typhoons.
3. Bay of Bengal and Arabian Sea where they are called cyclones.
4. Around south-east African coast and Madagascar-Mauritius islands.
5. North-west Australia.
The main features of tropical cyclones are as follows:
Size and Shape:
Tropical cyclones have symmetrical elliptical shapes (2:3 ratio of length and breadth) with steep pressure gradients. They have a compact size—80 km near centre, which may develop upto 300 km to 1500 km.
Wind Velocity and Strength:
Wind velocity, in a tropical cyclone, is more in poleward margins than at centre and is more over oceans than over landmasses, which are scattered with physical barriers. The wind velocity may range from nil to 1200 km per hour.
Orientation and Movement:
These cyclones start with a westward movement, but turn northwards around 20° latitude. They turn further north-eastwards around 25° latitude, and then eastwards around 80° latitude. They then lose energy and subside. Tropical cyclones follow a parabolic path, their axis being parallel to the isobars.
Structure of Tropical Cyclone:
The eye lies at the centre of the cyclone. The diameter of this core varies from 10 to 50 km. The wind speed is minimum and the sky remains generally clear in this region. Temperature is high due to the descending air currents which heat up by compression.
The eye wall, surrounding the eye, is made of cumulonimbus clouds, and it is characterised by winds of maximum velocities. A continuous ring of cumulonimbus clouds moves vertically. Therefore, the eye wall region witnesses the heaviest precipitation.
Two spiral bands are located outside the eye wall. A few km wide, they are also called rainbands or feeder bands. They rotate at speeds ranging from 20 km to 55 km. These regions are marked by high winds and precipitation.
The annular zone is characterised by suppressed cloudiness, fairly high temperature and low humidity conditions. The outer connective band, found at the edge of the
cloudmass, has external fringe of deep convective clouds. These are produced due to the instability of air consequent upon converging air movement. A belt of limited cloud cover is found, away from the main cloudmass.
There are three divisions in the vertical structure of tropical cyclones.
The lowest layer, extending up to 3 km and known as the inflow layer, is responsible for driving the storm. The middle layer, extending from 3 km to 7 km, is where the main cyclonic storm takes place. The outflow layer lies above 7 km. The maximum outflow is found at 12 km and above. The movement of air is anticyclonic in nature.
The approach of a tropical cyclone is marked by cirrus clouds of the eye of the cyclone, and then dark nimbus clouds cover the sky and heavy downpour begins. This downpour is evenly distributed around the centre and continues till the arrival of the tail with sleet and clear weather.
But just before the tail comes, there is lightning and thunder accompanied by calm, oppressive weather. The wind forms an upward moving spiral, so there are no marked wind shifts. The aftereffect is felt in the form of a fall in temperature and rise in pressure.
2. Temperate Cyclones:
Temperate cyclones are active over mid-latitudinal region between 35° latitude and 65° latitude in both hemispheres. These cyclones are also called extra-tropical or wave cyclones.
Origin and Development:
There are two theories of origin of temperate cyclones:
(i) Polar Front Theory by Bjerkenes:
According to this theory, the warm-humid air masses from the tropics meet the dry-cold air masses from the poles and thus a polar front is formed as a surface of discontinuity. Such conditions occur over sub-tropical highs, sub-polar lows and along the tropopause. The cold air pushes the warm air upwards from underneath. Thus a void is created because of lessening of pressure. The surrounding air rushed in to occupy this void and coupled with the earth’s rotation, a cyclone is formed which advances with the westerlies. (Fig. 2.25)
(ii) Thermodynamic Theory by Lampert and Shaw:
According to this theory, in sub-tropical areas, an overcrowding of vertical currents releases the surplus energy upwards which, after meeting the upper cool air, converts into an eddy. This eddy tends to settle down as an inverted ‘V’ shaped cyclone.
The temperate cyclones occur mostly in winter, late autumn and spring. They are generally associated with rainstorms and cloudy weather.
The favourite breeding grounds of temperate cyclones are shown in Fig. 2.26 and are listed below.
1. Over USA and Canada, extending over Sierra Nevada, Colorado, Eastern Canadian Rockies and the Great Lakes region.
2. Mexican Gulf
3. The belt extending from Iceland to Barents Sea and continuing over Russia and Siberia.
4. Winter storms over Baltic Sea.
5. Mediterranean basin extending upto Russia and even upto India in winters (called western disturbances).
6. The Antarctic frontal zone.
During summer, all the paths of temperate cyclones shift northwards and there is no temperate cyclone over sub-tropics and the warm temperate zone, although a high concentration of storms occurs over Bering Strait, USA and Russian Arctic and sub-Arctic zone.
Temperate cyclones have the following characteristics.
Size and Shape:
The temperate cyclones are asymmetrical and shaped like an inverted ‘V’. They stretch from 500 km to 600 km. They may go upto 2500 km over North America. They have a height of 8 to 11 km.
Wind Velocity and Strength:
These aspects of a temperate cyclone vary with season, location and from cyclone to cyclone, and the wind is directed a little to the right of the centre, rather than into it. The wind strength is more in eastern and southern portions, more over North America compared to Europe. The wind velocity increases with the approach but decreases after the cyclone has passed.
Orientation and Movement:
Since these cyclones move with the westerlies, they are oriented east-west. If the storm front is east-west, the centre moves swiftly eastwards. If the storm front is directed northwards, the centre moves towards the north, but after two or three days, the pressure difference declines and the cyclone dissipate. In case the storm front is directed southwards, the centre moves quite deep southwards—even upto the Mediterranean region (sometimes causing the Mediterranean cyclones).
The north-western sector is the cold sector and the north-eastern sector is the warm sector. As one moves eastwards in the northern sector, dark nimbus clouds and altrostratus are followed by cirrostratus higher up with cirrus clouds finally at the storm front. In the eastern sector, the extent of cloudiness and rainfall is limited. This sector is generally dominated by cumulonimbuses which cause heavy downpour, thunderstorm, lightning and hailstorm.
The approach of a temperate cyclone is marked by fall in temperature, fall in the mercury level, wind shifts and a halo around the sun and the moon, and a thin veil of cirrus clouds. A light drizzle follows which turns into a heavy downpour. These conditions change with the arrival of the warm front which halts the fall in mercury level and the rising temperature. Rainfall stops and clear weather prevails until the cold front of an anticyclonic character arrives which causes a fall in temperature, brings cloudiness and rainfall with thunder. After this, once again clear weather is established.
The temperate cyclones experience more rainfall when there is slower movement and a marked difference in rainfall and temperature between the front and rear of the cyclone. These cyclones are generally accompanied by anticyclones.