## 5.1. MEAN SEA LEVEL PRESSURE

Instead of checking the station pressure, the reduced pressure has been checked.

For the data of the GTS-stations the reduced pressure is already given in the SYNOPs.
For the data of the MAP-stations the reduced pressure has to be calculated:

This is done with the following equations ...

1. Gravity acceleration as a function of station high and latitude:

2. Geopotential height:

3. Saturation pressure:
, tD [°C]

4. specific moisture

5. virtual temperature

... so at the end we get the reduced sea surface pressure:

Due to reduction effects the station can have some deviation.

• Positive deviation means: pressure of the station is higher than the surrounding stations.
• Negative deviation means: pressure of the station is lower than the surrounding stations.

## 5.2. POTENTIAL TEMPERATURE

The temperature is influenced by a lot of physical processes with a diurnal cycle (e.g. difference between valley and foreland). Heat island effects of big cities, etc. are represented in temperature measurements, too.

• Positive deviation means: temperature of the station is higher than the surrounding stations.
• Negative deviation means: temperature of the station is lower than the surrounding stations.

The potential temperature is calculated with the following equation:

with:

• q: potential temperature
• T: station temperature
• p: station pressure
• po = 1000 hPa
• R = 287 JK-1kg-1
• cp = 1004 J kg-1 k -1

Up to now only low level stations (0 m £ h £750 m) have been checked. T (emperatue) und p(ressure) are the measured values.

## 5.3. 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

## 5.4. HORIZONTAL WIND

As we get information about the direction ("direc") and the velocity ("velo") of the horizontal wind
we can calculate the u - und v - components of the horizontal wind as followed:

u=velo*cos(direc(i)+(pi/2.))
v=velo*cos(direc(i)+pi)