Campbell Scientific VisualWeather Software - Manual (Page 69)

Appendix C. Dew Point
C.1 Dew Point
Dew point is the temperature at which air saturates, upon cooling, without
change in its water content.
C.1.1 Method
1. The value of saturation vapor pressure, e
s
(kPa), is calculated using the
polynomial in Ta, the air temperature 1
e
s
=(a0 + Ta*(a1+Ta*(a2+Ta*(a3+Ta*(a4+Ta*(a5+a6*Ta))))))*0.1; (1)
where Ta is the mean hourly temperature and the polynomial coefficients
are:
a0=6.107799961; a1=4.436518521E-1; a2=1.428945805E-2;
a3=2.650648471E-4; a4=3.031240396E-6; a5=2.034080948E-8;
a6=6.136820929E-11;
The quantity e
s
is in mbars. Since 1000 mbars = 100 kPA, 1 mbar = 0.1
kPa. Thus, multiplying the polynomial result by 0.1 converts e
s
from
mbar into kPa.
2. The vapor pressure, e
a
(kPa), is calculated from e
a
= RH * e
s
/100, where
RH is a measured value of relative humidity (%).
3. The dewpoint, Td (Celsius), is calculated using the equation 2
T
d
= C
3
*ln(e
a
/ C
1
) / (C
2
-ln(e
a
/ C
1
)) (2)
where C
1
= 0.61078, C
2
= 17.558, and C
3
= 241.88
C.2 References
1. 'An Approximating Polynomial for the Computation of Saturation Vapor
Pressure', Lowe, P.R., J. Appl. Meteor., Volume 16, Issue 1: 100-103,
January 1977.
2. 'Uber einige meteorologische Begriffe', Tetens,O. , Z. GeoPhys., 6:297-
309, 1930.
C-1