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 May or June the sunniest.
Sunshine duration decreases with increasing altitude, increasing latitude and distance from the coast. Local topography also exerts a strong influence and in the winter deep glens and north-facing slopes can be in shade for long periods. Industrial pollution and smoke haze can also reduce sunshine amounts, but the decline in heavy industry in the Clyde valley has resulted in an increase in sunshine duration particularly in the winter months.
The sunniest parts of Western Scotland are the Solway coast, Kintyre and the low-lying islands, where the average annual sunshine totals approach 1450 hours. Close to the other coasts, 1325 hours is typical whilst the averages decrease with altitude and to the north so that the Southern Uplands receive less than 1200 hours and the west Highlands less than 1100 hours. The sunniest places on mainland UK are along the south coast of England, with over 1750 hours per year on average, whilst the Channel Islands enjoy over 1900 hours.
The average monthly sunshine totals for Tiree and Eskdalemuir are shown, together with the highest and lowest totals recorded in the stated periods.
The graphs show that Tiree has more sunshine than Eskdalemuir in almost every month. Both places have least sun in December and most in May, due to the greater tendency for anticyclonic conditions in May often with easterly winds. The highest known monthly sunshine total in the area is 329 hours at Tiree in May 1975. The highest UK monthly total is 383.9 hours in July 1911 at Eastbourne, Sussex. In the dullest winter months less than 20 hours have been recorded on occasion.
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