In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. As elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. Pressure measures force per unit area, with SI units of Pascals (1 pascal = 1 newton per square metre, 1 N/m2). On average, a column of air with a cross-sectional area of 1 square centimetre (cm2), measured from mean (average) sea level to the top of Earth's atmosphere, has a mass of about 1. 03 kilogram and exerts a force or "weight" of about 10. 1 newtons or 2. 37 lbf, resulting in a pressure at sea level of about 10. 1 N/cm2 or 101 kN/m2 (101 kilopascals, kPa). A column of air with a cross-sectional area of 1 in2 (6. 45 cm2) would have a mass of about 6. 65 kg and a weight of about 65. 4 N or 14. 7 lbf, resulting in a pressure of 10. 1 N/cm2 or 14. 7 lbf/in2.
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