Atmospheric Pressure

"Atmospheric pressure is the pressure at any point in the Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point.".There is less atmospheric mas above low pressure areas, and high have more atmospheric mass above them. When elevation increases there is less atmospheric mass, so pressure decreases with the increase of elevation. "A column of air 1 square inch in cross section, measured from sea level to the top of the atmosphere, would weigh approximately 14.7 lbf. A 1 m² (11 sq ft) column of air would weigh about 100 kilonewtons (equivalent to a mass of 10.2 tonnes at the surface)." A unit of pressureis defined s being the equal to 101.325 kPa."The standard atmosphere (symbol: atm)"

resource:http://en.wikipedia.org/wiki/Atmospheric_pressure