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:

, t

4. specific moisture

5. virtual temperature

... so at the end we get the

Due to reduction effects the station can have some deviation.

The potential temperature

__with:__

- q: potential temperature
- T: station temperature
- p: station pressure
- p
_{o }= 1000 hPa - R = 287 JK
^{-1}kg^{-1} - c
_{p}= 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.

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.

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

q = T (p

q

exp(e) ~ 1 + e for e <<1

=> q

q / T»1

- q: potential temperature
- q
_{e}: equivalent potential temperature - T: station temperature
- p: station pressure
- q
_{s}: saturation mixing ratio at condensation level [g kg^{-1}] - p
_{o }= 1000 hPa - R = 287 JK
^{-1}kg^{-1} - L
_{c}= 2.5 x 10^{6 }J kg^{-1} - c
_{p}= 1004 J kg^{-1 }k^{ -1 }

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)