Humidity Equations and Conversion Chart B-1
January 2006
Introduction The following symbols appear in the equations below:
e = Vapor Pressure, millibars
e
i
= Vapor Pressure with respect to ice, millibars
e
w
= Vapor Pressure with respect to water, millibars
e
is
= Saturation vapor pressure, ice, millibars
e
ws
= Saturation vapor pressure, water, millibars
P = Total Pressure, millibars
T = Temperature, °C
T
a
= Ambient temperature, °C
T
d
= Dew point temperature, °C
T
f
= Frost point temperature, °C
Vapor Pressure Saturation vapor pressure with respect to water is a function of
temperature only and is given by the following:
(B-1)
Saturation vapor pressure with respect to ice requires a minor
adjustment of the constants as given by the following:
(B-2)
In addition to yielding saturation vapor pressure as a function of
ambient temperature, the above equations also yield ambient vapor
pressure as a function of dew/frost point.
The total pressure of a gas mixture is equal to the sum of the partial
pressure each gas would exert, were it to occupy the same total
volume, according to Dalton’s law.
E
WS
6.1121EXP
17.502T
240.97 T+
------------------------
=
E
IS
6.1115EXP
22.452T
272.55 T+
------------------------
=
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