# Distribution of Pressure

Distribution of atmospheric pressure on the surface of the earth is not uniform. It varies both vertically and horizontally.

• Air is a mixture of various gases.
• It is highly compressible. As it compresses, its density increases.
• The higher the density of air, the greater is the air pressure and vice versa.
• The mass of air above in the column of air compresses the air under it hence its lower layers are more dense than the upper layers. As a result, the lower layers of the atmosphere have higher density, hence, exert more pressure.
• Conversely, the higher layers are less compressed and, hence, they have low density and low pressure.
• The columnar distribution of atmospheric pressure is known as vertical distribution of pressure.
• Air pressure decreases with increase in altitude but it does not always decrease at the same rate.
• Dense components of atmosphere are found in its lowest parts near the mean sea level.
• Temperature of the air, amount of water vapour present in the air and gravitational pull of the earth determine the air pressure of a given place and at a given time.
• Since these factors are variable with change in height, there is a variation in the rate of decrease in air pressure with increase in altitude.
• The normal rate of decrease in air pressure is 34 millibars per every 300 metres increase in altitude.
• The effects of low pressure are more clearly experienced by the people living in the hilly areas as compared to those who live in plains.
• In high mountainous areas rice takes more time to cook because low pressure reduces the boiling point of water.
• Breathing problem such as faintness and nose bleedings are also faced by many trekkers from outside in such areas because of low pressure conditions in which the air is thin and it has low amount of oxygen content.
• The distribution of atmospheric pressure over the globe is known as horizontal distribution of pressure.
• It is shown on maps with the help of
• An isobar is a line connecting points that have equal values of pressure.
• Isobars are analogous to the contour lines on a relief map.
• The spacing of isobars expresses the rate and direction of change in air pressure.
• This change in air pressure is referred to pressure gradient.
• Pressure gradient is the ratio between pressure difference and the actual horizontal distance between two points.
• Close spacing of isobars expresses steep pressure gradient while wide spacing indicates gentle pressure gradient.
• The horizontal distribution of atmospheric pressure is not uniform in the world. It varies from time to time at a given place; it varies from place to place over short distances.
• The factors responsible for variation in the horizontal distribution of pressure are as follows:
•  (i) Air temperature
• (ii) The earth’s rotation
• (iii) Presence of water vapour

(i) Air Temperature:

• The earth is not heated uniformly because of unequal distribution of insolation, differential heating and cooling of land and water surfaces.
• Generally there is an inverse relationship between air temperature and air pressure.
• The higher the air temperature, the lower is the air pressure.
• The fundamental rule about gases is that when they are heated, they become less dense and expand in volume and rise. Hence, air pressure is low in equatorial regions and it is higher in polar regions.
• Along the equator lies a belt of low pressure known as the “equatorial low or doldrums”.
• Low air pressure in equatorial regions is due to the fact that hot air ascends there with gradual decrease in temperature causing thinness of air on the surface.
• In polar region, cold air is very dense hence it descends and pressure increases. From this one might expect, a gradual increase in average temperature and a gradual decrease in pressure towards the equator.
• However, actual readings taken on the earth’s surface at different places indicate that pressure does not increase latitudinally in a regular fashion from equator to the poles.
• Instead, there are regions of high pressure in subtropics and regions of low pressure in the subpolar areas.

(ii) The Earth’s Rotation:

• The earth’s rotation generates centrifugal force.
• This results in the deflection of air from its original place, causing decrease of pressure.
• It is believed that the low pressure belts of the sub polar regions and the high pressure belts of the sub-tropical regions are created as a result of the earth’s rotation.
• The earth’s rotation also causes convergence and divergence of moving air.
• Areas of convergence experience low pressure while those of divergence have high pressure.

(iii) Pressure of Water Vapour :

• Air with higher quantity of water vapour has lower pressure and that with lower quantity of water vapour has higher pressure.
• In winter the continents are relatively cool and tend to develop high pressure centres; in summer they stay warmer than the oceans and tend to be dominated by low pressure, conversely, the oceans are associated with low pressure in winter and high pressure in summer.
• The variation of pressure from place to place and from season to season over the earth plays an important role in affecting the weather and climate.
• Therefore pressure distribution is studied through isobar maps.
• While drawing isobar maps, the pressures of all places are reduced to sea level to avoid the effect of altitude on air pressure.

(i) January Conditions

• In January, with the south-ward apparent movement of the Sun, the equatorial low pressure belt shifts a little south of the mean equatorial position (see fig.).
• Areas of lowest pressure occurs in South America, Southern Africa and Australia.
• This is because the land tends to get hotter rapidly than water.
• Sub-tropical high pressure cells are centered over the ocean in the southern hemisphere.
• The belt of high pressure is interrupted by the continental land masses where the temperature is much higher.
• They are well developed in eastern part of the ocean where cold ocean currents dominate.

• The Aleutian and Icelandic lows disappear from the oceans while the landmasses, which developed high pressure during winter months, have extensive low pressure cells now.
• In Asia, a low pressure develops. The subtropical highs of the northern hemisphere are more developed over the oceans – Pacific and Atlantic.
• In the southern hemisphere, the sub-tropical high pressure belt is continuous.
• Sub-polar low forms a continuous belt in the southern hemisphere while in northern hemisphere, there is only a faint oceanic low