Like many other facets of meteorology, mathematics comes in handy when determining absolute and relative humidities. The absolute humidity is the mass of water vapor divided by the mass of dry air in a specific volume of air at a specific temperature. In this instance, the warmer the air, the more water vapor it contains.

On the other hand, relative humidity (RH) is the ratio of the absolute humidity to the highest possible absolute humidity, which in turn depends on the current air temperature. Mathematically, RH is often defined as the ratio of the water vapor density (mass per unit volume) to the saturation water vapor density, usually expressed as a percent. The equation for relative humidity is: RH = actual water vapor density / vapor saturation density × 100 percent.

More commonly, RH is thought of as the amount of water vapor in the air at a given temperature in comparison to the amount that the air could contain at the same temperature. For example, if an area is experiencing 100 percent relative humidity, that usually means the air is saturated with (can’t hold anymore) water vapor.