What is atmospheric pressure?

Atmospheric pressure is the force per unit area exerted by the weight of the Earth's atmosphere on a surface. It is caused by the gravitational pull of the Earth on the air molecules in the atmosphere. Atmospheric pressure decreases with increasing altitude, as there are fewer air molecules above a given point. It is commonly measured in units of millibars (mb) or hectopascals (hPa). The average atmospheric pressure at sea level is about 1013.25 millibars or 1013.25 hectopascals.