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From Wikipedia, the free encyclopedia. Vapor density is the density of a vapour in relation to that of hydrogen. It may be defined as mass of a certain volume of a substance divided by mass of same volume of hydrogen. vapor density = mass of n molecules of gas / mass of n molecules of hydrogen. Therefore: vapour density = molar mass of gas / molar mass of H 2.
The term humidity generally refers to the amount of this water vapor in the atmosphere. Each atmospheric gas has its own vapor pressure, a measure of the number of molecules present at a given temperature. The vapor pressure of water thus measures the amount of water vapor in the air. The saturation vapor pressure is the vapor pressure when liquid water begins to condense. Relative humidity is determined by using the actual vapor pressure divided by the saturation vapor pressure (see below).