We all know that the nights are long in winter, short in summer. For the Special Duties squadrons the changing seasons determined how far into Nazi-occupied Europe they could drop agents and supplies. In the winter they could fly to distant targets in Norway, Denmark, Czechoslovakia, Poland and southern France, but in summer their targets were limited to France and the Low Countries; special techniques were necessary to reach southern France.

Winter

From mid-November to mid-February any target within a Whitley’s range was possible. During these darkest months it was dark (i.e. after nautical twilight) for longer than than the longest possible operational endurance of the Whitley, about 11¾ hours. Poland was within range for a Halifax, but only just within reach for a Whitley, and that Whitley had to be equipped with long-range tanks and could carry only a light payload.

Take off time was largely determined by when the moon rose to illuminate the route to the target and the target itself: early in the first part of the moon period, near or after midnight under a waning moon. The agent needed to be able to see after his landing, too. (For more, see Moon Periods.)

Poor weather was normal in winter: sometimes it reduced the number of flyable nights within a moon period to a handful. Icing, more a promise than a threat, made flying in cloud extremely hazardous, and many operations were abandoned when the airscrews couldn’t be changed between fine and coarse pitch when the hydraulic control system froze. Fog often covered the route to the target, especially along river valleys used for navigation; without visible confirmation along their route the crews were unlikely to find their targets. The Whitley was draughty: heating inside the cockpit was occasionally adequate. In the rear, everyone froze. SD squadrons flew at much lower altitudes than the bombers – temperature drops by 1 degree for every 335 feet of altitude; at 12,000 feet it is 36°C colder than at sea level – so the SD crews escaped the worst effects suffered by the crews in the bomber squadrons.

Summer

In high summer the short nights dramatically reduced the radius of operation, i.e. the distance that agents could be delivered. Whereas a fighter might escape detection, a bomber was difficult to hide in daylight. Under the cover of darkness a large aircraft over Occupied Europe was relatively safe from attack, at least until 1942, but with the dawn it would become a sitting duck. It was essential to reach the open waters of the English Channel or the North Sea long before daybreak; better still to be back over England. In the high summer it never became truly dark, but for the few hours available operations to Holland, Belgium and northern France were possible. Eastern Europe and Scandinavia were out of reach, and operations to southern France would exit via the Atlantic coast into the Bay of Biscay, to be clear by daybreak.

Spring and Autumn

Spring and autumn provided the best compromise, with nights that were long enough for most targets to be reached, yet without the extremes cold of winter or the time restrictions of high summer. Yet the autumn became notorious for gales, fog and mist, while icing at altitude was encountered well into the spring. In October 1941, despite the increased resources available to the newly-formed 138 Squadron, only six nights of the 15 were flyable, but fifteen sorties were carried out and 21 operations completed.