PART V THE METEOROLOGICAL SITUATION LEADING UP TO SUNDAY 8TH AUGUST
From Wednesday 4th August, the general trend in the weather involved a shallow slow moving depression, tending to complex and fill, heading from the vicinity of the Bristol Channel towards the central North Sea. It produced a heavy thunderstorm in the South as the cold Atlantic air it brought with it, undercut the warm, humid stagnant air that had built up in the slow moving air over the continent and British Isles. However, as the front cleared, the air soon warmed up again and with the airflow still from the south, it was humid. Scattered thunderstorms recurred on Friday 6th, as a result, while conditions at sea were misty and sultry. The thundery depression was filling, nonetheless, while a weak cold front, trailing southwards from a shallow depression south of Spitzbergen straddled Scotland and Ireland (see Chart 1). Out in the Atlantic, a major Atlantic depression, with associated fronts, was moving westward between Cape Farewell and Iceland, with a secondary feature developing on its cold front.
Certainly, it looked as though dryer Atlantic air would flow over the country before the next depression arrived, and, in fact, an old occlusion (see Chart 1) lying down the eastern side of the United Kingdom, was sharpening on Saturday 6th into a cold front, with contrasting warm humid air to the east and a shallow low consequently developed to the east of Scotland. These developments are not shown on Charts 1 or 2 (Daily Telegraph) owing to the generalised nature of these weather maps, but were ascertained in discussion with RAF Honington over the telephone.
In fact the situation can be seen more clearly on Chart 3 (Meteorological Office) for Saturday 7th August. I shall use these charts after Friday 6th, as “actual situation” maps were not available (published one day in arrears).
I was not in hasty contact with RAF Honington, as I felt the potential developments, in the light of our needs, seemed favourable. Further, shipping forecasts were recorded and interpreted together with BBC 1 weather forecasts.
During Saturday, reports indicated that the cold front was progressing across the United Kingdom (see Chart 3) but it was still misty in the River Orwell, with thick fog off Harwich. The next major depression was still in situ, but its energy was pretty well spent, as its fronts had run up against the strong ridge of high pressure extending up to the Norwegian Sea from the Azores anticyclone.
However the secondary low was fast advancing towards northern Scotland, and it was this feature which was now the most important. Although the cold front pushed the humid air away and cleared the North Sea of fog, the secondary low collapsed the high pressure ridge ahead, bringing the wind from a light north westerly to a west to south westerly. Together with the northward bound anticyclone from the south, this tightened the pressure gradient sufficiently to give a consistent light to moderate breeze.
Decision time was Saturday evening. By that time, the cold front had passed through and pressure was falling generally as the ridge of high pressure progressively declined. In advance of the incoming depression, a moderate to fresh southwest/westerly looked probable, with good visibility. It was not clear how strong the breeze would be nor how long it would last. At any rate, it should be reasonably warm and sunny.
Further consultation took place with RAF Honington who were once again helpful. The view was that the wind would be south-west to west, force 3 to 4, for the following 24 hours. The north Scotland depression was filling but the anticyclone to the south (see Chart 3) was edging slowly north, maintaining the pressure gradient. Visibility was likely to be good/moderate and it would be sunny and warm, perhaps with some increasing high cloud late in the day, as a weakening warm front approached.
The weather was then predicted to be as good as could be hoped for, and in other respects, we were as well prepared as we could be, in these unusual circumstances. In principle, when George arrived back from the wedding, we decided to go if the weather forecast still gave us a clear run at 4 am the next morning. I had got a quantity of gear on board Merlin during the day and scrubbed her, so George went off to do the navigation. In any case, we agreed to plan for a 5 am start.