- 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.
- The wind redistributes the heat and moisture across the planet, thereby, maintaining a constant temperature for the planet as a whole. The vertical rising of moist air cools it down to form the clouds and bring precipitation.
- 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 milibar. At sea level the average atmospheric pressure is 1,013.2 milibar. Due to gravity the air at the surface is denser and hence has higher pressure.
- Air pressure is measured with the help of a mercury barometer or the aneroid barometer
- 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. It does not always decrease at the same rate..
- The vertical pressure gradient force is much larger than that of the horizontal pressure gradient. But, it is generally balanced by a nearly equal but opposite gravitational force. Hence, we do not experience strong upward winds.
Horizontal Distribution of Pressure
- Small differences in pressure are highly significant in terms of the wind direction and velocity.
- Horizontal distribution of pressure is studied by drawing isobars at constant levels.
Isobars are lines connecting places having equal pressure. In order to eliminate the effect of altitude on pressure, it is measured at any station after being reduced to sea level for purposes of comparison.
- Low- pressure system is enclosed by one or more isobars with the lowest pressure in the centre.
- High-pressure system is also enclosed by one or more isobars with the highest pressure in the centre.
World Distribution of Sea Level Pressure
- Near the equator the sea level pressure is low and the area is known as equatorial low.
- Along 30° N and 30o S are found the high-pressure areas known as the subtropical highs.
- Further pole wards along 60o N and 60o S, the low-pressure belts are termed as the sub polar lows.
- Near the poles the pressure is high and it is known as the polar high.
- 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.
Forces Affecting the Velocity and Direction of Wind
- The air in motion is called wind. The wind blows from high pressure to low pressure. The wind at the surface experiences friction.
- In addition, rotation of the earth also affects the wind movement. The force exerted by the rotation of the earth is known as the Coriolis force.
- Thus, the horizontal winds near the earth surface respond to the combined effect of three forces – the pressure gradient force, the frictional force and the Coriolis force. In addition, the gravitational force acts downward.
Pressure Gradient Force
- The differences in atmospheric pressure produces 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.
- 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 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.(important need more point )
Pressure and Wind
- The winds in the upper atmosphere, 2 - 3 km above the surface, are free from frictional effect of the surface and are controlled mainly 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 (Figure 10.4).
- 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 (Figure10.5).
- 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.
Pattern of Wind Direction
at the Centre
- 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.
- 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 circulation.
- 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.
- General Atmospheric Circulation and its Effects on Oceans
- Warming and cooling of the Pacific Ocean is most important in terms of general atmospheric circulation. The warm water of the central Pacific Ocean slowly drifts towards South American coast and replaces the cool Peruvian current.
- Such appearance of warm water off the coast of Peru is known as the El Nino.
- The El Nino event is closely associated with the pressure changes in the Central Pacific and Australia. This change in pressure condition over Pacific is known as the southern oscillation.
- The combined phenomenon of southern oscillation and El Nino is known as ENSO.
- In the years when the ENSO is 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. This phenomenon is closely monitored and is used for long range forecasting in major parts of the world.