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Air expands when heated and gets compressed when cooled. This results in variations in the atmospheric pressure. The result is that it causes the movement of air from high pressure to low pressure, setting the air in motion. Air in horizontal motion is wind. Atmospheric pressure also determines when the air will rise or sink.
Atmospheric Pressure
i). The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure. The atmospheric pressure is expressed in units of millibar.
ii). Due to gravity the air at the surface is denser and hence has higher pressure.
iii). Air pressure is measured with the help of a mercury barometer or the aneroid barometer.
iv). The pressure decreases with height. At any elevation it varies from place to place and its variation is the primary cause of air motion, i.e. wind which moves from high pressure areas to low pressure areas.
Vertical Variation of Pressure
In the lower atmosphere the pressure decreases rapidly with height. The decrease amounts to about 1 mb for each 10 m increase in elevation. The vertical pressure gradient force is much larger than that of the horizontal pressure gradient.
Horizontal Distribution of Pressure
Horizontal distribution of pressure is studied by drawing isobars at constant levels. Isobars are lines connecting places having equal pressure.
World Distribution of Sea Level Pressure
i). Near the equator the sea level pressure is low and the area is known as equatorial low.
ii). Along 30° N and 30° S are found the high-pressure areas known as the subtropical highs.
iii). Further pole wards along 60° N and 60° S, the low-pressure belts are termed as the sub polar lows.
iv). Near the poles the pressure is high and it is known as the polar high.
v). These pressure belts are not permanent in nature. They oscillate with the apparent movement of the sun. In the northern hemisphere in winter they move southwards and in the summer northwards.
Factors Affecting The Velocity And Direction Of Wind
i) The air in motion is called wind. The wind blows from high pressure to low pressure, and rotation of the earth also affects the wind movement. The force exerted by the rotation of the earth is known as the Coriolis force.
ii) The horizontal winds near the earth surface respond to the combined effect of three forces. They are:
(a) The pressure gradient force,
(b) The frictional force and
(c) The Coriolis force.
Pressure Gradient Force
The rate of change of pressure with respect to distance is the pressure gradient. It is strong where the isobars are close to each other and is weak where the isobars are apart.
Frictional Force
It affects the speed of the wind. It is greatest at the surface (extends upto an elevation of 1 - 3 km), whereas over the sea surface the friction is minimal.
Coriolis Force
i) The rotation of the earth about its axis affects the direction of the wind. This force is called the Coriolis force.
ii) It deflects the wind to the right direction in the Northern Hemisphere and to the left in the Southern Hemisphere.
iii) The Coriolis force is directly proportional to the angle of latitude. It is maximum at the poles and is absent at the equator. At the equator, the Coriolis force is zero and the wind blows perpendicular to the isobars. The low pressure gets filled instead of getting intensified, so tropical cyclones are not formed near the equator.
Pressure and Wind
i). Velocity and direction of wind are the net result of wind generating force.
ii). Wind circulation around a low is called cyclonic circulation & around a high it is called anticyclonic circulation.
iii). When isobars are straight and when there is no friction, the pressure gradient force is balanced by the Coriolis force and the resultant wind blows parallel to the isobar. This wind is known as the geostrophic wind.
iv). The wind circulation at the earth’s surface is closely related to the wind circulation at higher level. Over high pressure area the air will subside from above and diverge at the surface.
General Circulation Of The Atmosphere
i). The pattern of planetary winds largely depends on:
(a) Latitudinal variation of atmospheric heating;
(b) Emergence of pressure belts;
(c) The migration of belts following apparent path of the sun;
(d) The distribution of continents and oceans;
(e) The rotation of earth.
ii). The air at the Inter Tropical Convergence Zone (ITCZ) rises because of convection caused by high insolation and a low pressure is created.
iii). The converged air rises along with the convective cell. It reaches the top of the troposphere up to an altitude of 14 km and moves towards the poles. This causes accumulation of air at about 30 N and S.
iv). Down below near the land surface the air flows towards the equator as the easterlies.
v). Easterlies converge in at ITCZ – circulations are known as cells.
vi). In tropics it is called Hadley Cell.
vii). Mid latitude is called Ferrel Cells – westerlies (warm air that blows from subtropical highs).
viii). At poles – cold dense air subsides near the poles and blows as polar easterlies, called Polar cells.
ix). The transfer of heat energy from lower latitudes to higher latitudes maintains the general circulation.
x). General atmospheric circulation and its effects on oceans: Oceans in turn provide input of energy and water vapour into the air. Warm water of the central Pacific Ocean slowly drifts towards South American coast and replaces cool Peruvian current. Such appearance of warm water off the coast of Peru is known as the El Nino. The El Nino events are associated with the pressure changes in the Central Pacific and Australia. This change in pressure condition over Pacific is known as the Southern Oscillation. Southern Oscillation + El Nino = ENSO (strong, large-scale variations in weather occur over the world. The arid west coast of South America receives heavy rainfall, drought occurs in Australia and sometimes in India and floods in China).
Pressure systems | Pressure condition at the centre | Patterns of wind direction
Cyclone | Low | Anticlockwise in the N.H and clockwise in the S.H.
Anticyclone | High | Clockwise in N.H and Anticlockwise in S.H.
Seasonal Winds
The pattern of wind circulation is modified in different seasons due to the shifting of regions of maximum heating, pressure and wind belts. Such shifts can be seen in the monsoon season - over South East Asia.
Local Winds
Differences in the heating and cooling of earth surfaces and the cycles those develop daily or annually can create several common, local or regional winds.
Land And Sea Breezes
i) The land and sea absorb and transfer heat differently.
ii) During the day the land heats up faster and becomes warmer than the sea. Therefore, over the land the air rises giving rise to a low pressure area, whereas the sea is relatively cool and the pressure over sea is relatively high. Thus, pressure gradient from sea to land is created and the wind blows from the sea to the land as the sea breeze.
iii) In the night the reversal of condition takes place. The land loses heat faster and is cooler than the sea. The pressure gradient is from the land to the sea and hence land breeze results.
Mountain And Valley Winds
During day - Valley breeze slopes are heated and air moves upslope to fill the gap air from valley blows up.
During night - Slopes are cool and air descends in the valley as the mountain wind. The cool air, of the high plateaus and ice fields draining into the valley is called katabatic wind.
Air Masses
i). When the air remains over a homogenous(ocean surface or plains) area for a sufficiently longer time, it acquires the characteristics of the area.
ii). Airmass is defined as a large body of air having little horizontal variation in temperature and moisture. The homogenous surfaces, over which air masses form, are called the source regions.
iii). The air masses are classified according to the source regions. There are five major source regions. They are:
(a) Warm tropical and subtropical oceans;
(b) The subtropical hot deserts;
(c) The relatively cold high latitude oceans;
(d) The very cold snow covered continents in high latitudes;
(e) Permanently ice covered continents in the Arctic and Antarctica.
iv). These types of air masses are recognised:
(a) Maritime tropical (mT)- Warm
(b) Continental tropical (cT)- Warm
(c) Maritime polar (mP)-Cold
(d) Continental polar (cP)-Cold
(e) Continental arctic (cA)-Cold.
Fronts
When two different air masses meet ( i.e, warm, cold, stationary and occluded) then the boundary zone between them is called a front. The process of formation of the fronts is known as frontogenesis.
i). Stationary Front – front remains stationary
ii). Cold front – cold air moves over warm air mass
iii). Warm front – warm air moves over cold air mass
iv). Occluded front – Air mass is fully lied above the ground.
Extra Tropical Cyclones
i). The system developing in the mid and high latitude, beyond the tropics are called the middle latitude or extra tropical cyclones.
ii). In the Northern Hemisphere, warm air blows from the south and cold air from the north of the front. When the pressure drops along the front, the warm air moves northwards and the cold air move towards, south setting in motion an anticlockwise cyclonic circulation.
iii). Cyclonic circulation leads to extra tropical cyclone with warm and cold front.
iv). The warm air glides over the cold air and a sequence of clouds appear over the sky ahead of the warm front and cause precipitation.
v). Cold front pushes the warm front up (cumulus cloud develop along cold front) – cold front moves faster and overtakes warm front.
vi). Warm air is completely lied up and the front is occluded and the cyclone dissipates.
Extra tropical cyclones | Tropical cyclones
Clear frontal system | Frontal systems aren't present
Large area | Smaller area
At both land and sea | Only at seas
Wind speed is less | Wind at high speed and destructive
Move from west to east | Move from east to west
Tropical Cyclones
i). These are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges. These are one of the most devastating natural calamities. They originate and intensify over warm tropical oceans.
ii). They are known as Cyclones in the Indian Ocean, Hurricanes in the Atlantic, Typhoons in the Western Pacific and South China Sea, and Willy-willies in the Western Australia.
iii). Favourable Conditions are:
(a). Large sea surface with temperature higher than 27° C.
(b). Presence of the Coriolis force.
(c). Small variations in the vertical wind speed.
(d). A pre-existing weak low- pressure area or low-level-cyclonic circulation.
(e) Upper divergence above the sea level system.
iii). Towering cumulonimbus clouds, surrounding the centre of the storm. With storm form the sea, moisture is strengthened on reaching land moisture is cut off and storm dissipates. Place where a tropical cyclone crosses the coast is called the landfall of the cyclone.
iv). The cyclones, which cross 20° N latitude generally, recurve and they are more destructive.
v). Strong spirally circulating wind around the centre, called the eye - its diameter can vary between 150 and 250 km. The eye is a region of calm with subsiding air.
vi). Around the eye is the eye wall, the wind reaches maximum velocity in this region, reaching as high as 250 km per hour. Torrential rain occurs here.
vii). From the eye wall rain bands may radiate and trains of cumulus and cumulonimbus clouds may drift into the outer region.
Thunderstorms And Tornadoes
i). Other severe local storms are thunderstorms and tornadoes. They are of short duration occurring over a small area but are violent.
ii). Thunderstorms are caused by intense convection on moist hot days. Such a phenomenon is called a tornado.
iii). Tornadoes generally occur in middle latitudes. The tornado over the sea is called water sprouts.
iv). These violent storms are the manifestation of the atmosphere’s adjustments to varying energy distribution.
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