Density of Air

The density of air, ρ, is the mass per unit volume of Earth's atmosphere. There are several factors which can affect air density, these being temperature, humidity and altitude.

Temperature

As with most other gases, solids and liquids an increase in temperature causes particles to begin moving more and thus usually maintain a greater average separation. This is what causes the expansion in the air and the variance in the density of air.

Temperature in °C	Density of air ρ in kg·m−3
35	1.1455
30	1.1644
25	1.1839
20	1.2041
15	1.225
10	1.2466
+5	1.269
±0	1.2922
-5	1.3163
-10	1.3413
-15	1.3673
-20	1.3943
-25	1.4224

Graph shows relationship between Air Density and Temperature (°C) at a constant pressure.

Humidity

The addition of water vapor to air reduces the density of air, this is due to molecular mass differences between water and air. As Avogadro's Law states at a given temperature and pressure, the number of molecules present is constant for a particular volume. This means that as water molecules begin to mix with the dry air the air molecules must also decrease by the same number.