A litre of air weighs about 1.3g.At the sea level the air pressure is around 1033.6 g cm . This is referred to as one atmosphere.
The pressure exerted by the atmosphere is expressed in
millibars.
The average pressure over the earth's surface at sea-level is 1013.25 mb.
Most of the atmospheric mass is concentrated in the lower layers:
- Below 5.6 km 50% of atmospheric mass
- Below 16 km 90% of atmospheric mass
- Below 32 km 99% of atmospheric mass