The number of hours of bright sunshine is controlled by the length of day and by cloudiness. The day is shortest in December and longest in June and so in general December is the dullest month and June the sunniest. The south west of England has a favoured location with respect to the Azores high pressure when it extends its influence north eastwards towards the UK particularly in summer. Subsidence associated with high pressure reduces cloud cover and in spring and summer when the sea is cool relative to the air there is little convective cloud over the sea. Coastal areas are then favoured by high sunshine amounts. Inland and especially near hills, convective cloud often forms and acts to reduce sunshine amounts.
Coastal areas in the south west have average annual sunshine totals above 1600 hours, the south (English Channel) coast being more favoured than the north (Bristol Channel) coast. The Channel Islands are the sunniest part of the UK with some places exceeding 1900 hours per year. This may be compared with the 1100 hours per year recorded on the Shetland Islands. Inland the annual sunshine totals are mainly between 1400 and 1600 hours.
The graphs show the average monthly sunshine totals for St. Mawgan and Yeovilton, together with the highest and lowest totals recorded in the stated periods.
The highest known monthly sunshine totals in the area are for June 1925 when 334.8 hours were recorded at Long Ashton and 381.7 at Pendennis Point in Cornwall. The highest UK monthly total is 383.9 hours for July 1911 at Eastbourne, only a few hours above the Cornish record. In the dullest winter months less than 20 hours have been recorded on occasion.
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