Humidity is the amount of water vapor in the atmosphere.
The main sources of water vapor in the lower atmosphere are evaporation from the Earth's surface and transpiration by plants. In the stratosphere, the breakdown of methane by sunlight is another source. The main sink is precipitation. The total volume of water in the atmosphere is about 1.3 x 1013 m3, the overwhelming majority of which is in the vapor phase. For comparison, the oceans contain about 1.35 x 1018 m3 of water.