Pressure of the air is proportional to the rate of collisions between the molecules and the walls of a container.

There are two ways of increasing the pressure:

• increasing the density of the air (by decreasing the volume, or by pumping more air in)
• increasing the temperature of the air (molecules exert higher pressure because they are moving more rapidly).

The relationship between pressure, density, and temperature is called the Equation of State.

Different fluids have different equations of state, depending on their molecular properties.

For dry air the gases in the atmosphere have a simple equation known as the Ideal Gas Law:

P = r R_d T

P = pressure (in Pascals)

r (Rho) = density (in Kg m^–3 )

R_d = Gas Constant for dry air = 287.053 J Kg^–1 K^–1

or  Alternative form of Gas Constant for Dry Air: 

R_d = 0.287053 kPa K^-1 m³ kg^-1

T = Temperature (in Kelvin)

Holding density constant, an increase in temperature means an increase in pressure; and holding temperature constant, an increase in density means an increase in pressure.