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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

(kPa), is calculated using the

polynomial in Ta, the air temperature

=(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

is in mbars. Since 1000 mbars = 100 kPA, 1 mbar = 0.1

kPa. Thus, multiplying the polynomial result by 0.1 converts e

from

mbar into kPa.

2. The vapor pressure, e

(kPa), is calculated from e

= RH * e

/100, where

RH is a measured value of relative humidity (%).

3. The dewpoint, Td (Celsius), is calculated using the equation

= C

*ln(e

/ C

) / (C

-ln(e

/ C

)) (2)

where C

= 0.61078, C

= 17.558, and C

= 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

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