Mean annual temperatures vary from about 11 °C in central London and along the south coast to about 9 °C over higher ground well inland. Over the UK, mean annual temperatures range from about 7 °C in the Shetlands to over 11 °C in Cornwall and the Channel Islands.
Temperature shows both a seasonal and a diurnal variation. Minimum temperatures usually occur around sunrise and maximum temperatures are normally 2 to 3 hours after midday.
January is the coldest month, with mean daily minimum temperatures varying from over 3 °C in London and along the coast to about 0.5 °C over the higher ground. Extreme minimum temperatures usually occur in December or January; examples include -13.8 °C at Marlborough, Wiltshire on 3 January 1979 and -18.2 °C at Lacock, Wiltshire on 13 December 1981. On the latter date, -11.8 °C was recorded at Heathrow (Greater London), whilst at St James's Park (Central London) the temperature fell no lower than -7.0 °C.
July is the warmest month, with mean daily maximum temperatures in the London area of 22.5 °C, the highest in the UK. Elsewhere in southern England the mean maxima are somewhat lower, and are close to 20 °C over the higher ground and along the south coast. Extreme maximum temperatures can occur in July or August, and are usually associated with heat waves lasting several days. Examples include that of late June/early July 1976, when 35.4 °C was recorded at North Heath, Sussex on 26 June and 35.6 °C at Southampton on 28 June, the heat wave of August 1990 when 36.5 °C occurred at Heathrow on 3rd and that of August 2003 when temperatures over 37 °C were recorded widely on 10th and a new UK record of 38.5 °C was set at Faversham, Kent. Heat waves are usually accompanied by warm nights, and notable examples include minimum temperatures of 23.9 °C at Brighton on 4 August 1990 (setting a UK record) and 23.2 °C at Ventnor, Isle of Wight on 10 August 2003.
From late spring through the summer, coastal areas can be affected by sea breezes, which result in lower maximum temperatures than further inland. In winter, coastal areas are generally milder than inland. However, in cold easterly winds places on the Kent and Sussex coasts can be just as cold as inland as the short sea track over the Dover Straits does not warm the cold continental air.
The variation of mean daily maximum and minimum temperatures month by month, together with the highest and lowest temperatures recorded, is shown for Heathrow and Hurn.
An 'air frost' occurs when the temperature at 1.25 metres above the ground falls below 0 °C, whereas incidence of a 'ground frost' refers to a temperature below 0 °C measured on a grass surface. The average number of days with air frost in Southern England varies from less than 30 a year in London and in areas bordering the Thames Estuary and the South Coast to more than 55 a year over the higher ground. Ground frost averages range from less than 70 days to over 110 days per year, with a similar distribution to air frost. However, those places into which cold air can drain are particularly prone to frost. Examples include the plain below the scarp slope of the Chiltern Hills in Oxfordshire where Benson, near Wallingford, averages about 55 air frosts and 110 ground frosts each year; here, only July and August are free of air frost and ground frost can occur in any month. In contrast, there is an urban heat-island effect associated with London, caused by the fabric of the buildings retaining heat from day time insolation. This is most conspicuous overnight in cold spells with light winds from late autumn to early spring, when temperatures in central London can be over 5 °C higher than in the outer suburbs and surrounding rural areas. The heat-island is also evident in summer heat waves.
The graphs show the average frequency of air and ground frost at Heathrow and Hurn. Although near to the coast, the topography and light soil at Hurn make it relatively prone to frost, with a ground frost possible in most months.
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