Humidity is a measure of the quantity of water vapor in the air. There are different methods for determining this quantity and those methods are reflected in a variety of humidity indexes and readings.

The humidity reading in general use by most meteorologists is relative humidity. The relative humidity of air describes the saturation of air with water vapor. Given in terms of percent humidity (e.g., 50% relative humidity), the measurement allows a comparison of the amount of water vapor in the air with the maximum amount water vapor that—at a given temperature—represents saturation. Saturation exists when the phase state changes of evaporation and condensation are in equilibrium.

Approximately one percent of Earth's total water content is suspended in the atmosphere as water vapor, precipitation, or clouds. Humidity is a measure only of the vapor content.

Because water vapor exerts a pressure, the presence of water...