Definition -
Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above that surface in the atmosphere of Earth (or that of another planet).
In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air
above the measurement point. On a given plane, low-pressure areas have
less atmospheric mass above their location, whereas high-pressure areas
have more atmospheric mass above their location. Likewise, as elevation
increases, there is less overlying atmospheric mass, so that
atmospheric pressure decreases with increasing elevation. On average, a
column of air one square centimeter in cross-section, measured from sea level to the top of the atmosphere, has a mass of about 1.03 kg and weight of about 10.1 N (2.28 lbf) (A column one square inch in cross-section would have a weight of about 14.7 lbs .
SI UNIT OF PRESSURE -
The Pascal unit is derived from Newton per meter-squared. However, the Newton is derived from kilogram meter per second-squared. Hence, by the use of base SI units only, the value of standard atmospheric pressure equals 101325 kg/( m s2), "kilogram per meter per second-squared".
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