Specifications

Section 8. Processing and Math Instructions
8-10
Dew point temperature is calculated as follows:
1. The saturation vapor pressure (S
vp
; units kPa) is calculated using Lowe’s
equation (see SatVP).
2. The vapor pressure (V
p
; units kPa) is calculated from V
p
= RH ∗ S
vp
/
100).
3. The dew point (T
d
; units °C) is calculated from the inverse of a version of
Tetens’ equation, optimized for dewpoints in the range -35 to 50°C:
T
d
= (C
3
∗ ln(V
p
/ C
1
)) / (C
2
. ln(V
p
/ C
1
))
where:
C
1
= 0.61078
C
2
= 17.558
C
3
= 241.88
Error in the Estimation of Dew Point
Tetens’ equation is an approximation of the true variation of saturated vapor
pressure as a function of temperature. However, the errors in using the
inverted form of the equation result in dew point errors much less than 0.1°C.
The largest component of error, in reality, comes from errors in the absolute
calibration of the temperature and RH sensor.
Figure 8-1 shows how dew point varies as a function of temperature and
humidity. It can be seen that the response is non-linear with respect to both
variables. Errors in the measurement of RH and temperature thus form a
complex function in relation to the resultant error in estimated dew point. In
practice, the effect of errors in the calibration of air temperature can be taken to
translate to an equivalent error in dew point, e.g. if the air temperature sensor
is 0.2°C high, then the estimated dew point is approximately 0.2°C high.
Figure 8-2 shows the errors in dew point as a function of a ‘worst case’ 5%
error in the calibration of the RH sensor.
For sensors installed in the field there are additional errors associated with
exposure of the sensor, e.g. sensors in unaspirated shields get slightly warmer
than true air temperature in conditions of low wind speeds and high solar
radiation. However, if the RH and air temperature sensors are installed in the
same shield and are thus exposed identically, the estimate of dew point is not
subject to the same error as the measurement of air temperature would be.
This is because the temperature sensor will measure the actual temperature of
the RH sensor, which is what is required for the derivation of air vapor
pressure and thereby dew point.