MIST OVER ST. DAVID’S* By F. W. M. RUCK

During 1942 and 1943 I was working near St. David’s Head a t the western extremity of Pembrokeshire. On several occasions the headland and surronnd- ing area were quickly covered by a layer of low stratus cloud. The phenomenon was entirely local, the remaining countryside being in brilliant sunshine, the usual conditions being as follows :-

Stratus rloud wouhl commence to form near the coastline, and, within half an hour or less, a contillnous layer of cloud would cover the area and persist for several hours, usually fornlilip in the early afternoon and clearing before everiing, horizontal visibility (where tho cloud reached the ground) being only a few yards. ‘l‘lin disc of the sun could be wen for short intervals.

A clear sky and SW wind of about 10 mi/lir, fairly warm.

On one such occasion, a Plying Fortress flew into the t,op of Carn Llidi, a local hill ; there were no survivors.

As this cloud effect appeared to be a local phenomenon, I thought i t was worthy of further investigation ; unfortunately, the War heing in operation,

Fig. 1. Arm of fog mid S t . Dnvitl’s Head

I was unable to obtain weather niaps for these occasions. A glance a t t h . contour map (Fig. 1) shows that the peninsula is about 10 miles long, 5 miles

wide, and consists of a plateau varying from 200 f t above sea-level at, tllr * I’rizc \tiirtiii-z cmtrb for Xir iY:lpiw Slraw Cornpetitmn. 1 ! 1 4 ~

3f;o

Cliffs, to about 400ft above sea-level further inland. There is also high ground exceeding 500 f t in height near St. David’s Head and near Fishgixard. The peninsula is open to the Atlantic Ocean.

A few weeks later the stratus began to form under similar conditions, RO

I cycled to the foot of Pen Berry, and commenced the climb through dense mist to the summit-573 feet above sea-level. Visibility was only a few feet, the grass and bushes dripping wet, and drops of moisture formed on my clothes and face. As I neared the summit I had glimpses of the sun, and when I was a few feet from the top I was in bright sunshine.

I was standing on a rocky “ island ’) of about 30 feet diameter, surrounded by the billowing tops of white cloud. The sun shone from a blue sky. Two miles to the south-west was another small “island”, the top of Carn Llidi. About three miles out to sea, the cloud abruptly ceased. To the north-east the summits of Cam Fawr, Carn Rolch and Bwlch-y-Rhos stood out above cloud-level. This appeared to be about the northern limit of the cloud. The mist extended for some distance over the sea in a southerly direction, the edge being very definite, as the details of the coastline at St. Anne’s Head, 13 miles away, stood out very clearly in the afternoon sunshine.

The top of the mist was not flat, but fairly lumpy, some of the “ cumulus heads ” extending about 40 feet above the main cloud-level. Small gaps appeared now and then in the surrounding cloud and I could momentarily see fragments of fields about 300ft below me. The sun cast my shadow on the cloudbank; it was sometimes about three times natural size (it varied with the cloud movement) and was surrounded by a bright light or halo effect.

I was suddenly enveloped in a billow of white mist, t h e sun disappeared, i t was damp, silent, and quite cold-visibility being almost nil. After a few minutes the mist cleared from the summit and I was able to enjoy the scene again. I stayed there for an hour or more, and, although the wind was blowing fairly strongly, the area covered by the cloud did not appear to vary much.

I made several subsequent excursions to the top of Pen Berry under similar circumstances, and noticed that the phenomenon was very similar to that seen on the first occasion.

The contrast was remarkable.

I “ Science Today ” A concise weekly newsletter, covering all branches of science. Recent

issues have included an article on glaciers, which mentioned a number of researches and explorations not otherwise accessible ; and reports of work on overwintering in insects, flame temperatures (showing an unexpected con- nection with small particle scattering), and the recent more exact delimitation of the size of active radio-emitting areas on the sun. Published from 104, Clifton Hill, London, N.W.8 ; annual subscription 30s.

361