"Martin Rowley" wrote in message
...
Can anyone tell me how the field which I interpret to be boundary layer
cloud fraction (Grenzschichtbewolkung)
differs from the low cloud fraction he-

Boundary layer:-
http://www2.wetter3.de/GFS_00_UTC/gfs_6-12_37.gif

Low cloud:-

http://www2.wetter3.de/GFS_00_UTC/gfs_6-12_14.gif

What I'm trying to work out is what vertical 'cut-off' is used - is it an
arbitrary altitude (say 900m) or is it based upon something else? My very
limited German is not up to the job I'm afraid.

Martin, looking at the "expert" maps on the WeatherOnline site,
they have RH charts for 0-300m, and for 925 hPa, which seem
to be similar to the ones you quote.

Philip Eden