The molecular forces which lead to more or less cohesion of solids and liquids, are so small in the case of gases that the gas particles are practically free to move. Gases spread over every available space and mix with gases already present there. The gas volume consists mainly of free volume and the net volume of the gas molecules, the latter being considerably smaller than their free volume. As a result, all gases and, of course, also air, can be compressed. At atmospheric pressure and a temperature of 0°C, 1m3 of air contains approximately 27 x 1018 molecules which constantly enter into collisions, whereupon direction of motion and speed are altered in each case. In a closed vessel with 27 x 1018 molecules, each square millimetre of the inner surface of the vessel is exposed to about 3 x 1021 molecular impacts per second. The total effect of these 3 x 1021 impacts onto the walls is described as air pressure.
The energy of motion of a gas particle is therefore solely dependent on the temperature. At -273°C every molecule of gas is totally devoid of motion. Air pressure is measured by means of a barometer and is the current weight of the air column above the surface of the earth related to sea level. The pressure upon the surface of the earth caused by the weight of air amounts to 1.033 kg/cm2. If the air were of even density this would equate to a height of the atmosphere of 
where 1.293 represents the density of air in kg/m3 at 0°C. In reality, however, pressure decreases with height.
Meteorology uses the terms Torr for indicating pressure. 1 Torr corresponds here to 1 mm of mercury column. This denomination was introduced in honour of the Italian Professor Torricelli.
