The weather

Meteorology was a developing science during the war years, there were daily flights over Europe by fast high flying aircraft such as Mosquitoes bringing back the most up to date information possible about weather conditions ready for the experts on the ground to translate into meaningful data.

Weather conditions, especially adverse weather conditions could significantly affect initially whether an operation was "on" or scrubbed, and then the success or otherwise of not only bombing accuracy but also the survival of the Lancasters and their crews.  Bright moonlit nights were initially thought to be favourable because it aided navigation; landmarks such as rivers and lakes, coastlines, dark patches of forest and so on were all the more discernible when the moon was full, but it also favoured the hunter; the glint of moonlight on the perspex of a gun turret or canopy could be a give away to a bomber's position.  A thin layer of cloud over the target area proved lethal too, light from burning fires and searchlights below made the cloud into a light blanket which Nachtjager named "the shroud" for the bomber stream could be clearly seen silhouetted against it.