HUMIDITY

As a measure for humidity, the difference between potential and equivalent-potential temperature has been chosen.
The results for humidity are more difficult to interpret than those for pressure and temperature. We think that most moist and dry deviations may also be explained by location or exposition of the station without being erroneous.

Positive deviation means: the station is moister than the surrounding stations.
Negative deviation means: the station is drier than the surrounding stations.

The influence of temperature has to be considered, too. The water content depends strongly on temperature. Therefore the variability of the deviation estimates is higher during summer than during winter due to saturation.

The humidity is calculated with the following equation:

q = T (po/p) R/cp
q e= q exp((Lcqs)× (cpT)-1)

with approximation:

exp(e) ~ 1 + e       for e <<1

=> q e-q = q (1 + (Lcqs)× (cpT)-1 - 1)

with approximation:

q / T»1

=>q e  -   q ~ 2.5 qs

with:
• q: potential temperature
• qe: equivalent potential temperature
• T: station temperature
• p: station pressure
• qs : saturation mixing ratio at condensation level [g kg-1]
• po = 1000 hPa
• R = 287 JK-1kg-1
• Lc = 2.5 x 106 J kg-1
• cp = 1004 J kg-1 k -1