ANTICYCLONES This is the mass of air whose isobars also form an oval or circular 'shape but in which pressure is high at the centre, decreasing towards the outside. Winds in an anticyclone form a clockwise outspiral in the northern hemisphere, whereas they form an anticlockwise outspiral in the southern hemisphere.
TORNADOES Tornadoes are violently rotating storms, characterised by a funnel shaped cloud, in which winds whirl around a small area of extremely low pressure. A
tornado differs from a tropical cyclone in that it forms over the land. The movement of air is, however, similar to that in a depression, moving in spirally from all directions. It is more destructive than a cyclone as the speed of the winds is very high, exceeding 320 km per hour. Tornadoes occur mostly in the Mississippi Valley, and are sometimes known as twisters. They also occur in Australia and occasionally in some mid-latitude places.
MEASURING WIND Description. of wind requires measurement of two qualities: direction and speed. The direction of wind is measured by the wind vane. Wind direction is stated in terms of the direction from which the wind is coming. Wind speed is measured by the anemometer. For wind velocity of higher levels, hydrogen-filled pilot balloons called Rawinsondes are released, carrying a target that reflects radar waves and thus can be followed even when the sky is cloudy. Wind velocity is measured on a Beaufort scale, in miles per hour. But the Beaufort scale of numbers has now been largely replaced by a direct statement of wind velocity in knots.
Tuesday, October 27, 2009
Impact
Impact Severe tropical cyclones cause considerable damage to life, property and agricultural crops. The principal dangers posed are (a) fierce winds, (b) torrential rains and associated flooding, and (c) high storm tides (combined effect of storm surge and tides).
While gales and strong winds as well as torrential rain cause sufficient havoc to property and agriculture, loss of human life and cattle is mainly due to storm surges. If the terrain is shallow and shaped like a funnel, like that of Bangladesh-much of the exposed land is just about at the mean sea level or even less-storm surges get enormously amplified;.Coastal inundation due to a combination of high tide and storm surge can cause the worst disaster.
Tropical Cyclones in the Indian Ocean and Bay of Bengal Regions The frequency, intensity and coastal impact of cyclones vary from region to region. Interestingly, the frequency of tropical cyclones is the least in the north Indian Ocean regions of the Bay of Bengal and the Arabian Sea; they are also of moderate intensities. But the cyclones are deadliest when they cross the coast bordering north Bay of Bengal (coastal areas of Orissa, West Bengal and Bangladesh). This is mainly due to storm surges (tidal waves) that occur in this region inundating the coastal areas.
Tropical cyclones over the Bay of Bengal occur in two distinct seasons, the pre-monsoon months of April-May and the post-monsoon months of October-November. On an average, in fact, almost half a dozen tropical cyclones form in the Bay of Bengal and the Arabian Sea every year, out of which two or three may pe severe. Out of these, the stormiest months are May, June,' October and November. Compared to the pre-monsoon season of May-June, when severe storms are rare, the months of October and November are known for severe cyclones.
'Tropical cyclones in the Bay of Bengal are more frequent than in the Arabian Sea. There could be three reasons fqr this-(i) waters in the Bay of Bengal are comparatively shallow; (ii) the coastline along the Bay of Bengal is more complicated; and (iii) more number of rivers
drain into the Bay of Bengal than into the Arabian Sea.
While gales and strong winds as well as torrential rain cause sufficient havoc to property and agriculture, loss of human life and cattle is mainly due to storm surges. If the terrain is shallow and shaped like a funnel, like that of Bangladesh-much of the exposed land is just about at the mean sea level or even less-storm surges get enormously amplified;.Coastal inundation due to a combination of high tide and storm surge can cause the worst disaster.
Tropical Cyclones in the Indian Ocean and Bay of Bengal Regions The frequency, intensity and coastal impact of cyclones vary from region to region. Interestingly, the frequency of tropical cyclones is the least in the north Indian Ocean regions of the Bay of Bengal and the Arabian Sea; they are also of moderate intensities. But the cyclones are deadliest when they cross the coast bordering north Bay of Bengal (coastal areas of Orissa, West Bengal and Bangladesh). This is mainly due to storm surges (tidal waves) that occur in this region inundating the coastal areas.
Tropical cyclones over the Bay of Bengal occur in two distinct seasons, the pre-monsoon months of April-May and the post-monsoon months of October-November. On an average, in fact, almost half a dozen tropical cyclones form in the Bay of Bengal and the Arabian Sea every year, out of which two or three may pe severe. Out of these, the stormiest months are May, June,' October and November. Compared to the pre-monsoon season of May-June, when severe storms are rare, the months of October and November are known for severe cyclones.
'Tropical cyclones in the Bay of Bengal are more frequent than in the Arabian Sea. There could be three reasons fqr this-(i) waters in the Bay of Bengal are comparatively shallow; (ii) the coastline along the Bay of Bengal is more complicated; and (iii) more number of rivers
drain into the Bay of Bengal than into the Arabian Sea.
Major Breeding Grounds
Major Breeding Grounds The major areas affected by
cyclones are-
(i) South-east Carribean region where they are called
hurricanes.
(ii) Philippines islands, eastern China and Japan where
they are called typhoons.
(iii) Bay of Bengal and Arabian Sea where they are
called cyclones.
(iv) Around south-east African coast and Madagascar
Mauritius islands.
(v) North-west Australia.
Characteristics 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 1,500 km.
Wind Velocity all~ 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 1,200 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 30° latitude. They then lose energy and subside. Tropical cyclones follow a parabolic path, their axis being parallel to the isobars.
Structure The centre is characterised by a patch of clear sky, called eye of cyclone. Here, because of the dry, descending air, quiet conditions prevail. Outwards of the centre, there are cirrus clouds and still outwards, dark nimbus clouds, which cause occasional torrential downpour and thunderstorms. The right hand back comer of the cyclone gets heavy squalls and rainfall, but the left hand back comer gets clear weather as the trough passes. These cyclones are associated with destructive weather conditions, especially at the front of the cyclone. Also, these cyclones are highly unpredictable as the thermal effect over ocean and coastal areas changes very rapidly.
Associated Weather The arrival of a tropical cyclone at a particular place is marked by a sudden increase in air temperature and wind velocity, decrease in air pressure, appearance of cirrus or cirrostratus clouds in the sky, and emergence of high waves in the oceans. These clouds thicken and become cumulonimbus, and torrential rains begin. Giant waves spring up and the spray blows in continuous sheets which reduces visibility almost to zero.
This terrible storm continues for few hours and is abruptly followed by total calm and clear skies. This happens when the 'eye' of the cyclone arrives. Temperature rises, but the pressure is at its lowest. The eye is merely a hollow vortex produced by the rapid spiralling of air in the storm. The period of calm lasts for half an hour, and the weather suddenly changes with the arrival of the rear or the tail of the cyclone. The sky becomes overcast again; there is heavy downpour accompanied by lightning and thunder. Winds of high velocity again set in but this time in reverse direction to those of the first half of the storm. This situation continues for few hours, then gradually the winds abate, the clouds break and fair weather returns.
cyclones are-
(i) South-east Carribean region where they are called
hurricanes.
(ii) Philippines islands, eastern China and Japan where
they are called typhoons.
(iii) Bay of Bengal and Arabian Sea where they are
called cyclones.
(iv) Around south-east African coast and Madagascar
Mauritius islands.
(v) North-west Australia.
Characteristics 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 1,500 km.
Wind Velocity all~ 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 1,200 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 30° latitude. They then lose energy and subside. Tropical cyclones follow a parabolic path, their axis being parallel to the isobars.
Structure The centre is characterised by a patch of clear sky, called eye of cyclone. Here, because of the dry, descending air, quiet conditions prevail. Outwards of the centre, there are cirrus clouds and still outwards, dark nimbus clouds, which cause occasional torrential downpour and thunderstorms. The right hand back comer of the cyclone gets heavy squalls and rainfall, but the left hand back comer gets clear weather as the trough passes. These cyclones are associated with destructive weather conditions, especially at the front of the cyclone. Also, these cyclones are highly unpredictable as the thermal effect over ocean and coastal areas changes very rapidly.
Associated Weather The arrival of a tropical cyclone at a particular place is marked by a sudden increase in air temperature and wind velocity, decrease in air pressure, appearance of cirrus or cirrostratus clouds in the sky, and emergence of high waves in the oceans. These clouds thicken and become cumulonimbus, and torrential rains begin. Giant waves spring up and the spray blows in continuous sheets which reduces visibility almost to zero.
This terrible storm continues for few hours and is abruptly followed by total calm and clear skies. This happens when the 'eye' of the cyclone arrives. Temperature rises, but the pressure is at its lowest. The eye is merely a hollow vortex produced by the rapid spiralling of air in the storm. The period of calm lasts for half an hour, and the weather suddenly changes with the arrival of the rear or the tail of the cyclone. The sky becomes overcast again; there is heavy downpour accompanied by lightning and thunder. Winds of high velocity again set in but this time in reverse direction to those of the first half of the storm. This situation continues for few hours, then gradually the winds abate, the clouds break and fair weather returns.
Tropical Cyclones
Tropical Cyclones Tropical cyclones, the most destructive of nature's phenomena, are known to form over all tropical oceans, except South Atlantic and the South Pacific, during certain seasons. These cyclones have a thermal origin. They are believed to form in the InterTropical Convergence Zone. (ITCZ), a narrow belt at the equator, where the trade winds of the northern and southern hemispheres meet.
ITCZ is a 'legion of high radiation energy which supplies the necessary heat Jor the vaporisation of sea water into the air. This moist unstable air rises, generates convective clouds and leads to an atmospheric disturbance with a fall in surface atmospheric pressure. This causes a convergence of surrounding air towards this region of low pressure.
The converging mass of air gains a rotary motion because of what is known as the Coriolis force caused by the rotation of the earth. However, under favourable circumstances, such as high sea-surface temperatures, this low pressure area can get accentuated. The convective instability builds up into an organised system with high speed winds circulating around the low pressure interior. The net result is a well-formed cyclone.
The ideal conditions for the development of tropical cyclones are-(i) quiet air, (ii) high temperature, (iii) highly saturated atmospheric conditions.
Tropical cyclones never originate over land, although they often penetrate far into the margins of continents. But they soon lose their strength after crossing the coast and penetrating inland.
ITCZ is a 'legion of high radiation energy which supplies the necessary heat Jor the vaporisation of sea water into the air. This moist unstable air rises, generates convective clouds and leads to an atmospheric disturbance with a fall in surface atmospheric pressure. This causes a convergence of surrounding air towards this region of low pressure.
The converging mass of air gains a rotary motion because of what is known as the Coriolis force caused by the rotation of the earth. However, under favourable circumstances, such as high sea-surface temperatures, this low pressure area can get accentuated. The convective instability builds up into an organised system with high speed winds circulating around the low pressure interior. The net result is a well-formed cyclone.
The ideal conditions for the development of tropical cyclones are-(i) quiet air, (ii) high temperature, (iii) highly saturated atmospheric conditions.
Tropical cyclones never originate over land, although they often penetrate far into the margins of continents. But they soon lose their strength after crossing the coast and penetrating inland.
Characteristics Temperate cyclones
Characteristics Temperate cyclones have the following, characteristics.
Size and Shape The temperate cyclQnes are asymmetri cal and shaped like an inverted 'V'. They stretch from 50 km to 600 km. They may go upto 2,500 km over Norf America. They have a height of 8 to 11 km.
Wind Velocity and Strength These aspects of a temperat cyclone vary with season, location and from cyclone t cyclone, and the wind is directed a little to the right c the centre, rather than into it. The wind strength is mOl in eastern and southern portions, more over North Americ compared to Europe. The wind velocity increases with th approach but decreases after the cyclone has passed.
Orientation and Movement Since these cyclones mov with the westerlies, they are oriented east-west. If the storr front is east-west, the centre moves swiftly eastwards. : the storm front is directed northwards, the centre mov€ towards the north, but after two or three days, the pressuI difference declines and the cyclone dissipates. In case th storm front is directed southwards, the centre moves qui! deep southwards---even upto the Mediterranean regio (sometimes causing the. Mediterranean cyclones).
Structure The north-western sector is the cold sector and the north-eastern sector is the warm sector. As 011 moves eastwards in the northern sector, dark nimbI; clouds and altrostratus are foUowed by cirrostratus highl up with cirrus clouds finally at the storm front. In t~ eastern sector, the extent of cloudiness and rainfall limited. This sector is generally dominated by cumulonin bus which cause heavy downpour, thunderstorm, lightnil1 and hail-storm.
Associated Weather The approach of a temperate cyclor 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.
Size and Shape The temperate cyclQnes are asymmetri cal and shaped like an inverted 'V'. They stretch from 50 km to 600 km. They may go upto 2,500 km over Norf America. They have a height of 8 to 11 km.
Wind Velocity and Strength These aspects of a temperat cyclone vary with season, location and from cyclone t cyclone, and the wind is directed a little to the right c the centre, rather than into it. The wind strength is mOl in eastern and southern portions, more over North Americ compared to Europe. The wind velocity increases with th approach but decreases after the cyclone has passed.
Orientation and Movement Since these cyclones mov with the westerlies, they are oriented east-west. If the storr front is east-west, the centre moves swiftly eastwards. : the storm front is directed northwards, the centre mov€ towards the north, but after two or three days, the pressuI difference declines and the cyclone dissipates. In case th storm front is directed southwards, the centre moves qui! deep southwards---even upto the Mediterranean regio (sometimes causing the. Mediterranean cyclones).
Structure The north-western sector is the cold sector and the north-eastern sector is the warm sector. As 011 moves eastwards in the northern sector, dark nimbI; clouds and altrostratus are foUowed by cirrostratus highl up with cirrus clouds finally at the storm front. In t~ eastern sector, the extent of cloudiness and rainfall limited. This sector is generally dominated by cumulonin bus which cause heavy downpour, thunderstorm, lightnil1 and hail-storm.
Associated Weather The approach of a temperate cyclor 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.
DEPRESSIONS
DEPRESSIONS Depression is a mass of air whose isobars form an oval or circular shape, with low pressure at the centre. The air converges at the centre and rises to be disposed off. In a depression, the winds rotate anticlockwise in the northern hemisphere. While in the southern hemisphere, the circular movement of winds is in a clockwise direction. Depressions are rarely stationary and tend to follow definite tracks. They are most influential over the ocean spreads and they weaken over land areas. They are of two types.
1. Depression or Temperate Cyclones Also known as extra-tropical or wave cyclones, temperate cyclones are active over mid-latitudinal region between 35° and 65° latitudes in both hemispheres.
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 rushes in to occupy this void and, coupled with the earth's rotation, a cyclone is formed which advances with the westerlies.
(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.
Distribution The favourite breeding grounds of temper
ate cyclones 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 cyclone shift northwards and there is no temperate cyclone ove sub-tropics and the warm temperate zone, although a hig] concentration of storms occur over Bering Strait, USA ani Russian Arctic and sub-Arctic zone.
1. Depression or Temperate Cyclones Also known as extra-tropical or wave cyclones, temperate cyclones are active over mid-latitudinal region between 35° and 65° latitudes in both hemispheres.
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 rushes in to occupy this void and, coupled with the earth's rotation, a cyclone is formed which advances with the westerlies.
(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.
Distribution The favourite breeding grounds of temper
ate cyclones 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 cyclone shift northwards and there is no temperate cyclone ove sub-tropics and the warm temperate zone, although a hig] concentration of storms occur over Bering Strait, USA ani Russian Arctic and sub-Arctic zone.
Mountain and Valley Winds
Mountain and Valley Winds These are local winds responding to local pressure gradients set up by heating or cooling of the lower air. During the day when the slopes are intensely heated by the sun, the air in contact with the slopes is also heated by conduction from the ground.
This warmed-up air rises and is replaced by the air moving up from the valley, thus creating an upslope wind during the day. It is particularly well developed when one side of thtt valley is heated much more than the other, for example<; the valleys which have an east-west trend in mid and high latitudes. When the same slopes have been cooled at night by radiation of heat from ground to air, the wind moves valleywards. The wind is at its strongest just before sunrise when radiational cooling is greatest. Valley winds are a kind of anabatic winds.
Katabatic Winds A cold downslope wind caused by the gravitational movement of cold dense air near the earth's surface is a katabatic or drainage wind. Such cold dense air may accumulate in winter over a high plateau or high interior valley. Favourable conditions cause some of this cold air to spill over low divides and flow down as a strong cold wind.
The strongest katabatic winds are those that blow from an ice cap off the Greenland or Antarctic ice caps. In other parts of the world, these are known by various local names such as the bora in the northern Adriatic coast and the mistral in southern France. The Santa-Ana of southern California is of desert origin, carrying much dust and silt suspension.
Foehn and Chinook These result when strong regional winds passing over a mountain range are forced to descend on the ice side with the result that the air is heated and dried.
This warmed-up air rises and is replaced by the air moving up from the valley, thus creating an upslope wind during the day. It is particularly well developed when one side of thtt valley is heated much more than the other, for example<; the valleys which have an east-west trend in mid and high latitudes. When the same slopes have been cooled at night by radiation of heat from ground to air, the wind moves valleywards. The wind is at its strongest just before sunrise when radiational cooling is greatest. Valley winds are a kind of anabatic winds.
Katabatic Winds A cold downslope wind caused by the gravitational movement of cold dense air near the earth's surface is a katabatic or drainage wind. Such cold dense air may accumulate in winter over a high plateau or high interior valley. Favourable conditions cause some of this cold air to spill over low divides and flow down as a strong cold wind.
The strongest katabatic winds are those that blow from an ice cap off the Greenland or Antarctic ice caps. In other parts of the world, these are known by various local names such as the bora in the northern Adriatic coast and the mistral in southern France. The Santa-Ana of southern California is of desert origin, carrying much dust and silt suspension.
Foehn and Chinook These result when strong regional winds passing over a mountain range are forced to descend on the ice side with the result that the air is heated and dried.
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