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. Definitions of atmospheric humidity Meteorologists have defined several different measures of humidity.
Air is saturated when it holds the maximum possible amount of water vapor.
At that point, the rate at which water molecules enter the air by evaporation exactly balances the rate at which they leave by condensation. Definitions of water vapor concentration Specific humidity is the concentration, by mass, of water vapor in a sample of moist air. It is, therefore, the ratio of the mass of water vapor in a sample to the total mass of the sample moist air, including both the dry air and the water vapor. The mixing ratio is the ratio of the mass of water vapor to the mass of dry air in the sample. As ratios of masses, both specific humidity and mixing ratio are dimensionless numbers.