Vaisala DM500 - Manual (Page 131)

Appendix A __________________________________________________________ Humidity Theory
VAISALA ______________________________________________________________________ 129
Partial Pressure of Water Vapor (P
w
)
Partial pressure of water vapor refers to the part of the overall pressure
exerted by the water vapor component of a gas. Note that the partial
pressure of water vapor can be calculated when the dewpoint
temperature of a gas is known.
)) /( ( 10
n d d
T T T m A Pw
+ ⋅ ⋅ = (2)
Likewise, dewpoint temperature can be calculated for a given partial
pressure of water vapor.
1) - ) / ( log / /(
10
A P m T T
w n d
= (3)
where T
d
is a dewpoint temperature T
n
, m and A are constants and
their value depend on the temperature range as follows:
over water A m Tn
0...60 ºC 6.1078 7.5 237.3
-20...50 ºC 6.1162 7.5892 240.71
-70...0 °C (over ice) 6.1134 9.7911 273.47
0.0001
0.001
0.01
0.1
1
10
100
1000
-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60
T d ,T f / °C
Pw / hPa
over water
over ice
Figure 25 Partial Pressure of Water Vapor Over Water and
Ice
Relative Humidity (RH)
Relative humidity (RH) is the ratio of the amount of water vapor
actually in the air compared to the maximum amount the can be mixed