Mean annual temperatures over the region depend very much on altitude and, to some extent, proximity to the coast. Over the lower lying areas inland the average varies from around 10 °C in Cheshire to 9 °C in the Solway Plain near Carlisle and there is an approximate decrease of 0.5 °C for each 100 metres increase in altitude. The highest values occur along the coasts of the Isle of Man, Merseyside and Lancashire, whilst the lowest occur at the higher altitudes such as the Lake District fells and Pennines. 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 on the mainland, with mean daily minimum temperatures varying from below 0 °C over the highest ground to about 2 °C along the coasts and in Merseyside. However, on the Isle of Man the coldest month is February with a mean daily minimum temperature around 3 °C. This lag is due to the modifying effect of the Irish Sea, which is coldest in late February or early March. 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 at Ambleside (Cumbria) in January 1940. On the Isle of Man the lowest recorded has been -11.7 °C at Douglas in February 1895.
July is the warmest month, with mean daily maximum temperatures approaching 21 °C in Cheshire, but only 17 °C to 18 °C on the Isle of Man and in upland areas such as the Pennines and Lake District. The highest July mean daily 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. For example, on 3 July 1976 a temperature of 33.7 °C was recorded at Blackpool and in the heat wave of early August 2003 32.9 °C was achieved at Southport and Crosby, Merseyside.
The variation of mean daily maximum and minimum temperatures month by month, together with the highest and lowest temperatures recorded, is shown for Ringway and Ronaldsway.
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 on the Isle of Man varies from about 15 to 40 a year and in NW England it varies from about 20 in Merseyside to over 75 a year in the higher Pennines and Lake District. Ground frost occurs on average on 50-100 days per year on the Isle of Man and from about 75 to over 130 days on the mainland, with a similar distribution to air frost.
The graphs show the average frequency of air and ground frost at Ringway and Ronaldsway. 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.
