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Mass per unit volume of the Earth's atmosphere
The density of air or atmospheric density, denoted ρ, is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variations in atmospheric pressure, temperature and humidity. At 101.325 kPa (abs) and 20 °C (68 °F), air has a density of approximately , according to the International Standard Atmosphere (ISA). At 101.325kPa (abs) and , air has a density of approximately , which is about that of water, according to the International Standard Atmosphere (ISA). Pure liquid water is
Air density is a property used in many branches of science, engineering, and industry, including aeronautics; gravimetric analysis; the air conditioning industry; atmospheric research and meteorology; agricultural engineering (modeling and tracking of Soil Vegetation Atmosphere Transfer (SVAT) models); and the engineering community that deals with compressed air. Depending on the measuring instruments used, different sets of equations for the calculation of the density of air can be applied. Air is a mixture of gases and the calculations always simplify, to a greater or lesser extent, the properties of the mixture.

Other things being equal, hotter air is less dense than cooler air and will thus rise through cooler air. This can be seen by using the ideal gas law as an approximation.

Further note that since g, Earth's gravitational acceleration, is approximately constant with altitude in the atmosphere, the pressure at height h is proportional to the integral of the density in the column above h, and therefore to the mass in the atmosphere above height h. Therefore the mass fraction of the troposphere out of all the atmosphere is given using the approximated formula for p:

For nitrogen, it is 75%, while for oxygen this is 79%, and for carbon dioxide, 88%.

Higher than the troposphere, at the tropopause, the temperature is approximately constant with altitude (up to ~20km) and is 220K. This means that at this layer and , so that the exponential drop is faster, with for air (6.5 for nitrogen, 5.7 for oxygen and 4.2 for carbon dioxide). Both the pressure and density obey this law, so, denoting the height of the border between the troposphere and the tropopause as U: