Specific Humidity:
Specific humidity (or moisture content) is the ratio of water vapor,
, to unit mass of dry air
in any given volume of the mixture, and is sometimes referred to as the humidity ratio.
Specific humidity is approximately equal to the "mixing ratio", which
is defined as the ratio of the mass of water vapor in an air parcel to
the mass of dry air for the same parcel
.
Specific Humidity is defined as:
Specific humidity can be expressed in other ways including:
or:
Using this definition of specific humidity, the relative humidity can be expressed as
However, specific humidity is also defined as the ratio of water
vapor to the total mass of the system (dry air plus water vapor).
For example, the ASHRAE 2009 Handbook, Ch1,1.2, (9a) defines specific
humidity as "the ratio of the mass of water vapor to total mass of the
moist air sample".
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