As discussed in our article on atmospheric pressure, atmospheric air is a mixture of several kinds of gases. According to Dalton’s Law, the atmospheric pressure is the sum of the vapor pressures of each individual type of gas. The pressure contribution of each kind of gas toward the total atmospheric pressure is called the partial pressure of that gas.

Dry atmospheric air contains 78.1% nitrogen (N2) and 20.95% oxygen (O2) by volume. It follows that 78.1% of the atmospheric pressure can be attributed to the partial pressure of N2 and 20.95% to the partial pressure of O2.

At an atmospheric pressure of 101.3 kPa at sea level (1 atm), oxygen exerts a partial pressure Poxygen of 21.22 kPa, and nitrogen exerts a partial pressure Pnitrogen of 79.12 kPa. The partial pressure of oxygen can be calculated by:

where Xoxygen = 0.2095 (20.95% by volume of O2 molecules in air) and Pair is the total absolute air pressure.

The absolute atmospheric pressure depends on altitude. At sea level the absolute pressure is 101.3 kPa. The pressure decreases approximately 0.2 kPa per every 100 m increase in altitude. This relationship is defined by the barometric-altitude equation:

where Pa is the absolute atmospheric pressure (unit: kPa) at altitude “a,” Ps is the atmospheric pressure at sea level (101.3 kPa), a is the altitude (unit: km) above sea level, and c is a constant equal to 18.4 km. Some absolute pressure values for various altitudes are shown in the table.

## Altitude vs. Atmospheric Pressure

 0 m = 101.3 kPa 1200 m = 87.2 kPa 200 m = 98.8 kPa 1400 m = 85.0 kPa 400 m = 96.4 kPa 1600 m = 82.9 kPa 600 m = 94.0 kPa 1800 m = 80.9 kPa 800 m = 89.4 kPa 2000 m = 78.9 kPa 1000 m = 89.4 kPa

Prior Article - Saturated Vapor Pressure