Atmospheric pressure may be defined as the force that the atmosphere exerts against a surface of a given area. It is usually a close approximation of the hydrostatic pressure of the air's weight above the measurement point. Atmospheric pressure therefore depends primarily on the altitude of the measurement point, although an accurate calculation must also consider other physical variables. Since atmospheric pressure is generally calculated within the Earth's atmosphere, many of these variables may be considered to be constants.

### Items you will need

- Calculator with scientific functions.

Identify the physical constants of the Earth that we need for a calculation of atmospheric pressure. The universal gas constant R holds throughout the universe and has a value of 8.31432 Newton meters/moles kelvin. The standard gravity go exerted by the Earth at sea level is 9.80665 meters per second squared (m/s^2). The molar mass M of the Earth's air is 0.0289644 kilograms per mole (kg/mol).

Define the terms for height. h will be the height of the measurement point above sea level and hb will be the minimum altitude of the zone that the measurement point lies within. The minimum altitude of these zones are given in thousands of meters as follows: 0, 11, 20, 32, 47, 51 and 71.

Define the standard temperature values. The standard temperatures Tb are based on the altitude zones given in step 2 and are given in kelvins as follows : 288.15, 216.65, 216.65, 228.65, 270.65, 270.65 and 214.65. The standard temperature lapse rate Lb is the rate at which the temperature is decreasing at a given altitude. The standard temperature lapse rates for the altitude zones in step 2 are given in kelvins per meter as follows: -.0065, 0, .001, .0028, 0 -.0028 and -.002.

Provide the standard pressure Pb for the altitude zone in the desired units. For example, the pressure for the seven altitude zones in pascals are as follows: 101,325, 22,632, 5474, 868, 110, 66 and 4.

Calculate the pressure P at height h as follows: P = Pb [Tb/(Tb + Lb (h -hb))]^(goM/RLb).