Memorandum by the British Chiefs of Staff
Cairo, 25 November 1943 CCS 410 Secret
The effect of weather on Operation OVERLORD
The following examination has been made regarding the limitations imposed by weather conditions on the postponement of Operation OVERLORD.
Suitable weather conditions are required for two phases of the operation, firstly, the assault for which a four-day fine weather period is required; secondly, the maintenance and buildup period for which suitable weather for a decreasing degree of beach maintenance is required for about three months.
In order to launch the assault a quiet spell of four days with winds of force 3 or less is desirable. Over ten consecutive years there were quiet spells for four or more consecutive days on the following number of occasions:
It will be seen that there is no serious deterioration in the chances of launching the assault between the months of May and September with the exception of July, where the incidence of a fine spell is only slightly less than in the month of June. It is therefore considered that, purely from the assault aspect, the operation could be postponed up to the month of September.
For tidal reasons the assault is limited in each lunar month to two periods of five or six days, which occur at times of full and new moon. The air lift can only be carried out in the full moon period. It therefore follows that if the full moon period is missed on account of the weather conditions being unsuitable, the assault must be postponed for 24 days. By sacrificing the air lift this postponement could be reduced to 10 days.
Air factors affecting the assault
a. For fully effective operation of air forces the following conditions must be satisfied:
|Minimum horizontal visibility||5 miles||5 miles|
|Minimum cloud base above ground level||3,000 feet||11,500 feet|
|Maximum wind at ground level||20 mph||20 mph (if airborne forces are used by day)|
|Minimum moon||5 days each side of full.|
|Moon 20° above horizon.|
b. If high level bombing is abandoned, the cloud conditions by day are then limited by the requirements of the fighter cover over shipping and beaches. These are 10/10 at not less than 5,000 feet.
c. The chances of obtaining these conditions are not yet available, but it is evident that they will lengthen the odds against launching the assault to some extent, although settled summer weather suitable for the landing will most probably be suitable for the air operations.
Maintenance and buildup period
COSSAC has stated that, making full use of every captured port, large and small, 18 divisions must be maintained over the beaches during the first month of the operations, 12 divisions during the second month, and a number rapidly diminishing to nil during the third month. It is believed that the use of MULBERRIES will approximately halve this commitment for beach maintenance. Therefore, during this period there will be at first a considerable, and later a gradually dwindling dependence on fine weather conditions. In assessing suitable weather for carrying out beach maintenance any day with wind of not more than Force 3 on shore and not more than Force 4 off shore has been accepted. In the OVERLORD area the average number of suitable days per month is as follows:
It is apparent from the above figures that a marked deterioration does not occur until October. Although the months of October, November, and December appear to provide a reasonable number of quiet days, it is considered that this proportion cannot be fully relied on owing to the severe weather which may occur during unsuitable days, thereby producing conditions of sea or swell which will render beach maintenance impracticable on the subsequent quiet day or days.
It is impossible to calculate what loss in expectation of suitable maintenance days can be accepted by COSSAC during the second and third months of the beach maintenance period without a very intimate knowledge of his maintenance and build-up plan; but it would appear that weather should be suitable for sufficient beach maintenance at least up to the end of September and possibly, in view of the dwindling commitment in this respect, up to the middle or end of October.
It is not possible to submit a firm recommendation on this subject, but from the limited facts available for this brief examination, there does not appear to be any overriding reason why the assault could not be carried out up to about the middle of July.
This means that the target date should be in the middle of June to allow for a postponement of 24 days in case weather conditions are unsuitable.
Thus if the target date is mid-June and the air lift is not sacrificed, only two periods of four or five days when Moon and Tide conditions are suitable will occur in 1944; and these must coincide with a four-day spell of fine weather.