Atmospheric Pressure
Atmospheric pressure is the weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere.

Therefore, at a place, if the air is dense for instance near the Earth’s surface (due to Gravity), the atmospheric pressure will be more.

It is expressed in atm (Atmosphere), mb (millibar) and Pa (Pascal).

It is measured with the help of a mercury barometer or the aneroid barometer.

At sea level, the average atmospheric pressure is 1 atm or 1,013.2 mb or 1,013.2 h Pa (kilo Pascal).
VARIATIONS OF ATMOSPHERIC PRESSURE
 As the Pressure depends on the number of air molecules present at any place, it varies both vertically as well as horizontally.
 This variation of the Atmospheric Pressure has been playing a very important role in Weather and Climate.
 Its variation is the main cause of air motion/ wind.
VERTICAL VARIATION
 The pressure decreases with height because air gets thinner.
 The average decrease is about 1 mb per each 10m increase in elevation, subject to other factors such as Temperature, local topography, closeness to the sea, etc.
 Therefore, if the surface Atmospheric Pressure at any place is 1,000 mb, then the Pressure at 1 km above the surface will be (1000 – 100) mb i.e. 900mb.
 Despite high vertical pressure gradient, there is weak upward wind because the pressure gradient gets weakened by the Gravitational force.
HORIZONTAL VARIATION

The horizontal variation of the Pressure depends on the differential heating (insolation) of the surface which causes the differential air volumes.

These variations are highly significant in terms of wind direction and speed. Though the direction and speed depend also on Frictional force and Coriolis force.

Horizontal distribution of pressure is studied by drawing isobars (lines connecting places having equal pressure) at constant levels.