Atmospheric pressure is the weight of the column of air at any given place and time. It is measured by means of an instrument called a Barometer. It is measured as a force per unit area. The units used by metrologists for this purpose are called millibars (mb). One millibar is equal to the force of one gram on a square centimetre.
Distribution of Atmospheric Pressure:
Pressure in mb
In general, the atmospheric pressure decreases on an average at the rate of about 34 millibars per every 300 metres of height. It does not always decrease at the same rate. Table gives the average pressure and temperature at selected levels of elevation for a standard atmosphere.
Its main feature is its zonal character known as pressure belts. On the earth’s surface there are in all seven pressure belts. They are-
Factors controlling the direction and speed of wind:-
Pressure gradient force: The differences in atmospheric pressure produce a force. The rate of change of pressure with respect to distance is the pressure gradient. The pressure gradient 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 and its influence generally extends upto an elevation of 1 - 3 km. Over the sea surface the friction is minimal.
The Coriolis Effect: The rotation of the earth about its axis affects the direction of the wind. This force is called the Coriolis force after the French physicist who described it in 1844. It deflects the wind to the right direction in the northern hemisphere and to the left in the southern hemisphere. The deflection is more when the wind velocity is high. The Coriolis force is directly proportional to the angle of latitude. It is maximum at the poles and is absent at the equator.
The Coriolis force acts perpendicular to the pressure gradient force. The pressure gradient force is perpendicular to an isobar. The higher the pressure gradient force, the more is the velocity of the wind and the larger is the deflection in the direction of wind. As a result of these two forces operating perpendicular to each other, in the low-pressure areas the wind blows around it. 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. That is the reason why tropical cyclones are not formed near the equator.
Centripetal acceleration and friction: The velocity and direction of the wind are the net result of the wind generating forces. The winds in the upper atmosphere, 2 - 3 km above the surface, are free from frictional effect of the surface and are controlled by the pressure gradient and the Coriolis force. 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. The wind circulation around a low is called cyclonic circulation. Around a high it is called anti cyclonic circulation. The direction of winds around such systems changes according to their location in different hemispheres.
The wind circulation at the earth’s surface around low and high on many occasions is closely related to the wind circulation at higher level. Generally, over low pressure area the air will converge and rise. Over high pressure area the air will subside from above and diverge at the surface. Apart from convergence, some eddies, convection currents, orographic uplift and uplift along fronts cause the rising of air, which is essential for the formation of clouds and precipitation.
General circulation of the atmosphere
The pattern of planetary winds largely depends on: (i) latitudinal variation of atmospheric heating; (ii) emergence of pressure belts; (iii) the migration of belts following apparent path of the sun; (iv)the distribution of continents and oceans; (v) the rotation of earth. The pattern of the movement of the planetary winds is called the general circulation of the atmosphere. The general circulation of the atmosphere also sets in motion the ocean water circulation which influences the earth’s climate
The air at the Inter Tropical Convergence Zone (ITCZ) rises because of convection caused by high insolation and a low pressure is created. The winds from the tropics converge at this low pressure zone. 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 30o N and S. Part of the accumulated air sinks to the ground and forms a subtropical high. Another reason for sinking is the cooling of air when it reaches 30o N and S latitudes. Down below near the land surface the air flows towards the equator as the easterlies. The easterlies from either side of the equator converge in the Inter Tropical Convergence Zone (ITCZ). Such circulations from the surface upwards and vice-versa are called cells. Such a cell in the tropics is called Hadley Cell
In the middle latitudes the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high. At the surface these winds are called Westerlies and the cell is known as the Ferrel cell. At polar latitudes the cold dense air subsides near the poles and blows towards middle latitudes as the polar easterlies. This cell is called the polar cell. These three cells set the pattern for the general circulation of the atmosphere. The transfer of heat energy from lower latitudes to higher latitudes maintains the general circulati
The general circulation of the atmosphere also affects the oceans. The large-scale winds of the atmosphere initiate large and slow moving currents of the ocean. Oceans in turn provide input of energy and water vapour into the air. These interactions take place rather slowly over a large part of the ocean.
TYPES OF WINDS
1. Planetary winds:- the winds which blow from the subtropical high pressure towards the equatorial region of low pressure regularly throughout the year in many areas especially the oceans and the hot deserts from north –east in the northern hemisphere.
2. Westerlies: The winds which blow from the Horse latitudes towards the polar region throughout the year with varying intensity and cause rain near the Polar Regions. Westerlies are stronger in Southern Hemisphere because of the vast expanse of ocean water. They are also described as Roaring Forties, furious Fifties and Shrieking Sixties.
3. Periodic Winds:-It refers to the wind system that has pronounced seasonal reversal of direction. The monsoon winds blow over India, Pakistan, Bangladesh, Myanmar, Sri Lanka, Arabian Sea, Bay of Bengal, South Eastern Asia, Northern Australia, China and Japan.
4. Local Winds-
Loo: A very hot and dry wind in N.W India and Pakistan which blow from the west in afternoon.
Mistral: The cold wind which originates over the show covered mountain of Alps and blown towards the Mediterranean Sea.
Chinook and Foehn: Warm and dry local winds blowing on the leeward sides of the mountains are called Chinook in the USA and foehn in Switzerland.
Harmattan: The warm and dry winds blowing from north-east and east to west in the eastern part of Sahara desert. Similar winds are called Brickfielder in Australia, Blackioller in USA, Shamal in Iraq and Persian Gulf and Northwester in New Zealand.
Sirrocco: It is a warm, dry and dusty wind which blows in northernly direction from Sahara desert and reaches Italy, Spam etc. similar winds are called Khamsim in Egypt, Gibli in Libya, Chilli in Ternisia and Simoon in Arabian desert.
Bora: It is an extremely cold and dry north easterly wind blowing in the Adriatic Sea.
Blizzard: It is a violent, stormy, cold and powdery polar wind laden with dry snow in Siberia, Canada and USA.
Pinga: It is a snow laden cold wind in Rusian Tundra.
Bise: It is a cold wind in France.
Zonda: It is a warm wind in Central Europe.
Land and Sea Breezes
The land and sea absorb and transfer heat differently. 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. 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
In mountainous regions, during the day the slopes get heated up and air moves upslope and to fill the resulting gap the air from the valley blows up the valley. This wind is known as the valley breeze. During the night the slopes get cooled and the dense air descends into the valley as the mountain wind. The cool air, of the high plateaus and ice fields draining into the valley is called katabatic wind. Another type of warm wind occurs on the leeward side of the mountain ranges. The moisture in these winds, while crossing the mountain ranges condenses and precipitates. When it descends down the leeward side of the slope the dry air gets warmed up by adiabatic process. This dry air may melt the snow in a short time.
UPPER AIR CIRCULATIONS
In the mid- latitude, high speed winds known as jet streams blow from west to east in the upper troposphere near the Tropopause. The jet streams are narrow meandering bands of swift winds which are embedded in the prevailing Westerlies and encircle the globe. They may range from 40-160kms.in width and 2-3kms.in depth Average wind speed is very high with a lower limit of about 120 km/hr. in winter and 50 km/hr. in summer.
There are two main jet streams. They are:–
The sub-tropical jet stream is located over the low latitude margins of the Westerlies. It persists through most of the year. It is produced by the rotation of the earth.
The mid-latitude or polar front jet is produced by a temperature difference. It has a more variable position than the sub-tropical jet. In summer, its position shifts towards the poles and in winter towards the equator.
An air mass is a large body of air whose physical properties especially temperature and moisture content are relatively uniform horizontally. Normally, an air mass extends over hundreds of kilometers and consists of several layers, each having homogenous conditions. Regions where homogenous air masses tend to be created are known as source regions. A large land mass or water body which has evenly distribution insolation provides a suitable location for the development of an air mass. Some of the well known source regions are sub- tropical and tropical oceans, i.e., low- latitude deserts like the Sahara in the summer and the continental interiors especially those of North America and Eurasia in the winter. Another prerequisite for the development of an air mass is large scale subsidence over the source region. Submitting air over a homogenous source region gradually acquires the characteristics of the region and retains them even when it moves away. The heat and moisture properties pf the air mass may, however, change when it moves over other surface conditions. An air mass is said to be cold when it is colder than the surface over which it rests or is moving. An air mass is said to be warm when it is warmer than the surface over which it rests or is moving.
When two different air masses meet, the boundary zone between them is called a front. The process of formation of the fronts is known as frontogenesis. There are four types of fronts: (a) Cold; (b) Warm; (c) Stationary; (d) occluded.
When the front remains stationary, it is called a stationary front. When the cold air moves towards the warm air mass, its contact zone is called the cold front, whereas if the warm air mass moves towards the cold air mass, the contact zone is a warm front. If an air mass is fully lifted above the land surface, it is called the occluded front. The fronts occur in middle latitudes and are characterised by steep gradient in temperature and
pressure. They bring abrupt changes in temperature and cause the air to rise to form clouds and cause precipitation.
Extra Tropical Cyclones
The systems developing in the mid and high latitude, beyond the tropics are called the middle latitude or extra tropical cyclones. The passage of front causes abrupt changes in the weather conditions over the area in the middle and high latitudes. Extra tropical cyclones form along the polar front. Initially, the front is stationary. 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. The cyclonic circulation leads to a well developed extra tropical cyclone, with a warm front and a cold front.
There are pockets of warm air or warm sector wedged between the forward and the rear cold air or cold sector. 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. The cold front approaches the warm air from behind and pushes the warm air up. As a result, cumulus clouds develop along the cold front. The cold front moves faster than the warm front ultimately overtaking the warm front. The warm air is completely lifted up and the front is occluded and the cyclone dissipates. The processes of wind circulation both at the surface and aloft are closely interlinked. The extra tropical cyclone differs from the tropical cyclone in number of ways. The extra tropical cyclones have a clear frontal system which is not present in the tropical cyclones. They cover a larger area and can originate over the land and sea. Whereas the tropical cyclones originate only over the seas and on reaching the land they dissipate. The extra tropical cyclone affects a much larger area as compared to the tropical cyclone. The wind velocity in a tropical cyclone is much higher and it is more destructive. The extra tropical cyclones move from west to east but tropical cyclones, move from east to west.
Tropical cyclones 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. This is one of the most devastating natural calamities. 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.
Tropical cyclones originate and intensify over warm tropical oceans. The conditions favourable for the formation and intensification of tropical storms are:
The energy that intensifies the storm comes from the condensation process in the towering cumulonimbus clouds, surrounding the centre of the storm. With continuous supply of moisture from the sea, the storm is further strengthened. On reaching the land the moisture supply is cut off and the storm dissipates. The place where a tropical cyclone crosses the coast is called the landfall of the cyclone. The cyclones, which cross 20o N latitude generally, recurve and they are more destructive.
A mature tropical cyclone is characterised by the strong spirally circulating wind around the centre, called the eye. The diameter of the circulating system can vary between 150 and 250 km. The eye is a region of calm with subsiding air. Around the eye is the eye wall, where there is a strong spiralling ascent of air to greater height reaching the Tropopause. The wind reaches maximum velocity in this region, reaching as high as 250 km per hour.
Torrential rain occurs here. From the eye wall rain bands may radiate and trains of cumulus and cumulonimbus clouds may drift into the outer region. The diameter of the storm over the Bay of Bengal, Arabian Sea and Indian Ocean is between 600 -1200 km. The system moves slowly about 300 - 500 km per day. The cyclone creates storm surges and they inundate the coastal low lands. The storm peters out on the land.Other severe local storms are thunderstorms and tornadoes. They are of short duration, occurring over a small area but are violent. Thunderstorms are caused by intense convection on moist hot days. A thunderstorm is a well-grown cumulonimbus cloud producing thunder and lightening. When the clouds extend to heights where sub-zero temperature prevails, hails are formed and they come down as hailstorm. If there is insufficient moisture, a thunderstorm can generate duststorms.
A thunderstorm is characterised by intense updraft of rising warm air, which causes the clouds to grow bigger and rise to greater height. This causes precipitation. Later, downdraft brings down to earth the cool air and the rain. From severe thunderstorms sometimes spiralling wind descends like a trunk of an elephant with great force, with very low pressure at the centre, causing massive destruction on its way. Such a phenomenon is called a tornado. Tornadoes generally occur in middle latitudes. The tornado over the sea is called water sprouts.
These violent storms are the manifestation of the atmosphere’s adjustments to varying energy distribution. The potential and heat energies are converted into kinetic energy in these storms and the restless atmosphere again returns to its stable state.