Humidity's the amount of water vapour in the air. In daily language the term "humidity"'s normally taken to mean relative humidity. Relative humidity's defined as the ratio of the partial pressure of water vapour in a parcel of air to the saturated vapour pressure of water vapour at a prescribed temperature. Humidity may also be expressed as absolute humidity and specific humidity. Relative humidity's an important metric used in forecasting weather. Humidity indicates the likelihood of precipita… (More on Humidity)