The hilly nature of the terrain in Wales and its proximity to the Atlantic tends to encourage cloud cover. Even so, the south-western coastal strip of Pembrokeshire manages an average annual sunshine total of over 1700 hours, which is comparable to the 1750 hours achieved by many places along the south coast of England. The dullest parts of Wales are the mountainous areas, with average annual totals of less than 1200 hours. These figures compare with values of less than 1100 hours a year in the Shetland Islands to over 1900 hours in the Channel Islands.
Mean monthly sunshine totals reach a maximum in May or June, and are at their lowest in December. The key factor is, of course, the variation in the length of the day through the year, but cloud cover plays a part too.
The graphs show the average monthly sunshine totals for Valley and Swyddffynnon , together with the highest and lowest totals recorded in the stated periods.
The highest known monthly sunshine total in Wales is 354.3 hours recorded at Dale Fort (Pembrokeshire) in July 1955. The highest UK monthly total is 383.9 hours at Eastbourne in July 1911. In the dullest winter months, less than 20 hours have been recorded - with none at all in December 1890 in central London.
"