What is Relative Humidity?

Relative Humidity:

Relative humidity is the ratio of the partial pressure of water vapor in the air–water mixture to the saturated vapor pressure of water at those conditions. The relative humidity of air is a function of both its water content and temperature.
Relative humidity is normally expressed as a percentage and is calculated by using the following formula. It is defined as the ratio of the partial pressure of water vapor (H₂O) in the mixture to the saturated vapor pressure of water at a prescribed temperature.

Relative humidity is an important metric used in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, a rise in relative humidity increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin. For example, according to the Heat Index, a relative humidity of 75% at 80.0°F (26.7°C) would feel like 83.6°F ±1.3 °F (28.7°C ±0.7 °C) at ~44% relative humidity.