What is atmospheric humidity and how is it measured?
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. Atmospheric water vapor accounts for only about 1/10,000th of the total amount of water in the global hydrological cycle. 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. (The ratio of global atmospheric and oceanic water volumes is approximately the same as the ratio of the volume of water that can be held in a thimble to that in a bath tub.) Nevertheless, atmospheric water vapor is one of the most important factors in determining Earth’s weather and climate, because of its role as a greenhouse gas and because of the large amounts of energy involved as water
