Saturation vapor density (SVD) is a concept closely tied with saturation vapor pressure. It is useful for getting an exact quantity of water vapor

Water vapor

Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 in the air from a relative humidity

Relative humidity

Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...

 (RH). Given an RH percentage, the density of water in the air is given by RH * SVD = Actual Vapor Density. Alternatively, RH can be found by RH = Actual Vapor Density / SVD. As Relative Humidity is a percent, vapor density can be in whatever units are necessary, usually grams or kilograms per cubic meter.

For low temperatures (below 400 kelvin

Kelvin

The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

s or so), SVD can be approximated from the SVP by the ideal gas law

Ideal gas law

The ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behavior of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of Boyle's law and Charles's law...

: PV = nRT where n is the number of moles

Mole (unit)

The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

, which is related by density by n = M/m, where M is the mass of water present and m is the molar mass of water (18.01528 grams/mole). Thus, setting V to 1 cubic meter, we get P*m/(R*T) = M/V = density.

R is the gas constant

Gas constant

The gas constant is a physical constant which is featured in many fundamental equations in the physical sciences, such as the ideal gas law and the Nernst equation. It is equivalent to the Boltzmann constant, but expressed in units of energy The gas constant (also known as the molar, universal,...

, and T is the temperature in kelvin.