Variation in Air Pressure

a) Vertical Variation in Air Pressure:

• Air is highly compressible.
• Atmosphere is held to the earth by the force of gravity and air pressure is the weight of all air above the level of which it is measured.
• So obviously as the elevation increases, air molecules are more dispersed or their intermolecular spaces becomes greater, the weight of the air above the point of measurement decreases, and finally the atmospheric pressure also decreases.
• In other word, air near the ground is naturally denser than that found at a certain elevation

b) Horizontal Distribution of Air Pressure

• The horizontal variations of pressure are much less than the vertical variations; nevertheless, these relatively small variations are of fundamental significance for determining wind direction and speed.
• Factors which control the distribution of temperature on the earth surface equally govern the distribution of pressure as well.
• Horizontal distribution of air pressure on the earth surface is shown by mean of isobars

c) Pressure Gradient

• The difference of pressure between two points along a line perpendicular to the isobars divided by the distance between the points is called pressure gradient.
• In other worlds, pressure gradient is the rate of the change of pressure per unit horizontal distance.
• Mathematically, pressure gradient ;
• The rate and direction of the change in air pressure is also known as barometric slope. It is expressed in millibar per hundred kilometers or per degree of latitude.

d) Isobar

• It is the Imaginary Line joining the places of equal pressure.
• Isobars drawn closely on a weather map represent a steep(strong) pressure gradient and high wind velocity; while isobar drawn farther indicates a weak pressure gradient and light wind