Explanation: 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 units of milibar. At sea level the average atmospheric pressure is 1,013.2 milibar.
• Due to gravity the air at the surface is denser and hence has higher pressure.
• Air pressure is measured with the help of a mercury barometer or the aneroid barometer. The pressure decreases with height.
• At any elevation it varies from place to place and its variation is the primary cause of air motion, i.e. wind which moves from high pressure areas to low pressure areas.
• The vertical pressure gradient force is much larger than that of the horizontal pressure gradient.
• But, it is generally balanced by a nearly equal but opposite gravitational force. Hence, we do not experience strong upward winds

Source: NCERT – Fundamental of Physical Geography