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.
In general, 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 industrial areas of Merseyside and Greater Manchester.
Average annual sunshine durations over NW England range from around 1200 hours in the higher Pennines to about 1500 hours on the coast. On the Isle of Man, the range is from about 1300 hours at high altitude to 1550 hours on the coast. These figures compare with values of less than 1100 hours a year in the Shetland Islands to over 1750 hours along the south coast of England and over 1900 hours in the Channel Islands. The tendency for convective cloud to develop over inland areas in summer leads to values that are lower than coastal sites.
The graphs show the average monthly sunshine totals for Ringway and Ronaldsway, together with the highest and lowest totals recorded in the stated periods
The highest known monthly sunshine totals in the region are 336.1 hours at Blackpool and 327.0 hours at Ronaldsway, both 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.
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