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Temperature

Temperature varies on a daily, seasonal and geographical basis. On a daily basis, the temperature is usually lower at night than by day, with the minimum temperature usually occurring shortly after dawn and the maximum temperature occurring two or three hours after midday. In urban areas, the minimum temperatures experienced tend not to be as low as those recorded in rural areas (also maximum temperatures are often higher). There are two main reasons for the 'urban heat island' effect: the materials used in buildings store heat; and the release of heat as a result of industrial and domestic energy consumption. Some sheltered low-lying areas have a greater incidence of frosts (and more severe frosts) than the surrounding areas — these are known as 'frost hollows'. The temperature varies according to the season because the axis of the Earth is tilted in relation to the plane in which it revolves around the sun. January is, on average, the coldest month and July the warmest. In January, the coldest areas are parts of the Aberdeenshire and Perth and Kinross regions of Scotland, and the least cold are the extreme south-west of England and the Channel Islands. The main factor determining the distribution of temperature is nearness to the coast, particularly the west coast; temperatures are lower inland than near the coast. In July, the warmest areas are around London and the coolest are in parts of Scotland. Areas near the coast are less warm than inland areas — the opposite to what happens in January — and the temperature decreases from south to north. For both January and July, the modifying influence of the sea on coastal regions (keeping temperatures up in winter but down in summer) is mainly felt in a region up to 15 to 20 miles from the coast.

Sunshine

Because of the variation in length of day from winter to summer, the duration of sunshine shows a marked seasonal variation. As a consequence, December is, on average, the month with least sunshine and June is the sunniest. In general, sunshine durations decrease with altitude and increasing latitude. Aspect also plays an important part; for example, south-facing slopes receive more sunshine than those facing north. Over the year as a whole, the sunniest places are flat areas near the coast. Some sites along the south coast, from the Isle of Wight eastwards, and the Channel Islands record over 40% of the maximum amount of sunshine possible in a year (1,800 hours out of 4,000). The Shetland Islands, on the other hand, only achieve about 24% of the maximum possible.