[Jos]

Steven,

From the excellent book Schlachtflieger! by Rick Duiven & Dan-San Abbott:

The weather on the 21 st March was both favorable an unfavorable to the attacking German troops. The Somme River valley in the middle of the attacking 2 Armee was completely enveloped with fog, which coupled with the mist and low clouds had a great effect on the batlle. The weather was favorable for the attacking German infantry in that they suddenly appeared out of the mist and fog and quickly overran the British defense formations.
It was unfavorable in that the low clouds, mist and fog prevented the German Jagdstaffeln from taking off. The weather on the flanks was aparrantly better, as Jasta 35b, based in the 17 th Armee reported an air victory at 09.00 hours. By way of contrast, in the 2 Armee the Schusta were not even able to take off until around 10.30 hours, when they could fly under the low clouds as the fog lifted slightly. Thus the Schlachtgruppen could attack the British front lines as well as range far beyond them to attack reinforcements, artillery and reserve troops.
The Jagdstaffeln, at least in the 2 Armee, finally left the ground at 12.30 hours, when Jage Nr. 1 reported the destruction of two balloons at 12.55 and 13.10 hours.

Jos