Definitions

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Dew Point (DP)

Dew point is that unique temperature to which the air (or any gas) must be cooled in order that it shall be saturated with respect to water.

Frost Point (FP)

Frost point is that unique temperature to which the air (or any gas) must be cooled in order that it shall be saturated with respect to ice.

The dew point or frost point DEFINES the partial pressure of the water vapor in the gas, from the Smithsonian Meteorological Tables.

Relative Humidity (RH)

Relative Humidity (RH) is the ratio of the actual vapor pressure in the mixture to the saturation vapor pressure, with respect to water, at the prevailing dry bulb temperature.

NOTE: RH is arbitrarily defined with respect to water even though it seems that it should be with respect to ice at -40&186;C (-40&186;F).

Dalton's Law (Pm)

John Dalton was the first to surmise that the total pressure (PM) exerted by a mixture of gases or vapors is the sum of the pressures of each gas if it were to occupy the same volume individually. The pressure which each gas component of a multiple constituent gas (such as air) exerts is called its partial pressure. If Px, Py, and Pz represent the respective partial pressure of gases X, Y, and Z in a mixture, Dalton's Law states:

PM = Px + Py + Pz + ...

Elementary as it may seem, the concept of Dalton's Law is often overlooked when considering problems in humidity, because one forgets that the "water vapor" in a gas is actually a gas itself and must therefore be treated in accordance with the gas laws. Air must be considered a mixture of gases: oxygen, nitrogen, and water vapor (neglecting the minor constituents). All discussions of humidity can then be reduced to discussions of water vapor pressure, and all definitions encountered in humidity can be expressed in terms of vapor pressure.

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