Explanation: Since air has mass, it also has weight. The pressure of air at a given place is defined as a force exerted in all directions by virtue of the weight of all the air above it.
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 various units.
Measurement of Air Pressure
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 barometer.
The units used by meteorologists for this purpose are called millibars (mb).
One millibar is equal to the force of one gram on a square centimeter. A pressure of 1000 millibars is equal to the weight of 1.053 kilograms per square centimeter.
In other words, it will be equal to the weight of a column of mercury 75 cm high.
The normal pressure at sea level is taken to be about 76 centimeters (1013.25 millibars).
Vertical Variation of Pressure
In the lower atmosphere the pressure decreases rapidly with height.
