Thermodynamics and Water Vapour - Transport in the Atmosphere-Vegetation-Soil Continuum - page 354

(a) (b)

4.5 0.25

4.0

0.2

3.5

3.0

0.15

2.5

2.0

0.1

1.5

1.0

0.05

over water
over ice

0.5

0.0 0.0

-5 5 -5 5

-10 0 10 15 20 25 30 -10 0 10 15 20 25 30

T ( ° C) T ( ° C)

Figure B.1 Saturation vapour pressure (a) and slope of saturated vapour pressure
curve (b) .

The wet bulb temperature as indicated by a psychrometer is related to the vapour
pressure as:

eeT = () ( − − γ T T )

(B.22)

sat w w

where γ is the psychrometer constant:

c
L p

γ = R
R

p

v

(B.23)

d v

This deinition of the psychrometer constant assumes that the transfer of moisture
from the wet bulb is exactly as eficient as the transfer of heat to the wet bulb; in prac-
tice this is not the case and hence for real calculations regarding psychrometers, the
real psychrometer constant needs to be used, which may depend on ventilation rate
(see Monteith and Unsworth, 2008). Also note that the psychrometer constant is not
constant since it depends on pressure (and to a lesser extent on temperature, through
the temperature dependence of L v ; see below and Table B.3 ).

B.5. Latent Heat of Vaporization
The temperature dependence of the latent heat of vaporization L v of water vapour
is approximately given by 2501000 (

− − T J kg -1 which is a good
approximation over the temperature range 273-313 K (i.e., 0-40 °C) (relative to the
data in Haynes, 2011 , their table 6-5).

1000095 . ( 273 15 . ))