Air is a non-homogenous mixture of gases, solid particles, and liquids;
It consistes of gases and particles with small settling velocity that exhibit
stability in a gravitational field.
A somewhat more anthropocentric definition would be:
Air is the part of the Earth's atmosphere that humans breathe.
It is made up of 78% nitrogen, 21% oxygen and 1% other substances (including water vapor).
An atmosphere is a layer of gases that may surround a material body of sufficient mass (Earth would qualify)
held in place by the gravity of the body. An atmosphere may be retained for a longer duration,
if the gravity is high and the atmosphere's temperature is low. Some planets consist mainly of
various gases, but only their outer layer is their atmosphere (see gas giants).
Earth's Atmosphere: vertical structure, layers
Troposphere 0-17 km Stratosphere 17-50 km Mesosphere 50-90 km Ionosphere 90-100 km
95% of the air by weight is contained in the Troposphere, which also where the weather "happens"..
Nominal Composition of Air
| Major Components (99%) in % by w or fraction of total molecules*
|Component ||Percentage by Weight ||Fraction of Total Molecules |
|N2 ||75.51 ||0.7808 |
|O2 ||23.14 ||0.2095 |
|40Ar ||1.28 ||0.0093 |
|Water Vapor || ||0.0004 |
* Fractions of total molecules
** 325 ppm: 325 of each 1 million particles is CO2
| Minor Components
|Component ||ppm (parts per million**) |
|Carbon dioxide CO2 ||325.0 |
|Ne ||18.0 |
|He ||5.0 |
|CH4 ||2.0 |
|Kr ||1.0 |
|H2 ||0.5 |
|N2O ||0.5 |
|Xe ||0.1 |