The number of hours of bright sunshine is controlled by the length of day and by cloudiness. In general, December is the dullest month and June the sunniest.
Sunshine duration decreases with increasing altitude, increasing latitude and distance from the coast. Industrial pollution and smoke haze can also reduce sunshine amounts but, since the Clean Air Act of 1956 and a decline in heavy industry, there has been an increase in sunshine duration over the London area particularly in the winter months.
Southern England includes the sunniest places in mainland UK, these being the coastal resorts of Sussex and Hampshire. The Isle of Wight also features in the list of high sunshine averages. On the coast average annual sunshine durations can exceed 1750 hours, but 1550-1600 hours is typical of most of the region with a decrease towards the north (e.g. 1450 hours over the higher Chilterns).
The graphs show the average monthly sunshine totals for Heathrow and Eastbourne, together with the highest and lowest totals recorded in the stated periods.
The highest known monthly sunshine total in the region is 383.9 hours at Eastbourne, East Sussex in July 1911, which is a UK record. In the dullest winter months, less than 20 hours have been recorded with none at all in central London in December 1890. High annual totals include the 2237 hours recorded at Bognor Regis in 1990, one of a remarkable series of years 1983-1991 when the West Sussex resort was the sunniest place on the mainland.
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