# Horizontal Distribution

• 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.