Mean annual temperatures depend very much on altitude and, to some extent, proximity to the coast. The coldest waters around the UK are found off NE England with sea surface temperatures varying from about 5 °C in winter to 13 °C in summer (compared to a range of 8 °C to 18 °C off SW England). This, coupled with extensive areas of upland, leads to a prevailing coolness with mean annual temperatures typically in the range of 8.5 °C to 9.5 °C in the lower parts. 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. January is usually the coldest month, with mean daily minimum temperatures varying from below -0.5 °C over the highest ground to about 1.5 °C along the coast and in South Yorkshire. Minimum temperatures usually occur around sunrise and extreme minima have been recorded in winter, often in January or February. The lowest known temperature recorded in the region was -21.1 °C on 5 January 1941 at Houghall, a pronounced 'frost hollow' in the Wear valley near Durham.
July and August are the warmest months, with mean daily maximum temperatures ranging from about 21 °C in South Yorkshire to less than 16 °C in the higher Pennines. The highest July mean maxima occur in the London area (22.5 °C) whilst the lowest occur in the Shetlands (15 °C). Maximum temperatures are normally 2 or 3 hours after midday. Extreme maximum temperatures can occur in July or August, but are less common in NE England than areas further south. However, one example was the heat wave of 3 - 4 August 1990 when temperatures of 33 °C occurred widely.
The variation of mean daily maximum and minimum temperatures month by month, together with the highest and lowest temperatures recorded, is shown for Durham and Sheffield.
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. In NE England, the average number of days with air frost varies from about 30 a year on the coast to about 55 inland and over 90 in the higher Pennines. Ground frost occurs on average on about 80 days per year on the coast and over 135 days on high ground, with a similar distribution to air frost.
The graphs show the average frequency of air and ground frost at Durham and Sheffield. Although the summer months are usually free of air frost, ground frost may occur at any time of the year, especially at sites in inland valleys.
