Mean annual temperatures over the region at low altitude vary from about 8.5 °C close to the Moray Firth and on the westernmost isles to about 7 °C on Shetland, but over the higher ground temperatures are generally lower so that on Ben Nevis (1344 metres) the annual mean is just below 0 °C. Elsewhere in the UK, mean annual temperatures reach over 11 °C in Cornwall and the Channel Islands. Within the region, significant variations in temperature arise from the combined effects of proximity to the coast and topography.
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 or February is the coldest month, with mean daily minimum temperatures varying from about 2 °C on west-facing coasts and in the Western and Northern Isles, to less than -1 °C over higher ground. Extreme minimum temperatures can occur in winter; examples include -27.2 °C at Altnaharra (Highland) on 30 December 1995 (the UK record low temperature, shared with Braemar, Grampian). Conversely, occasionally to the lee of high ground temperatures can reach up to 15 °C in winter when a south or SW airstream warms up after crossing upland - an effect known as a fohn wind.
July or August is the warmest month, with mean daily maximum temperatures at low levels around 19 °C in areas close to the Moray Firth. Elsewhere in northern Scotland the mean daily maxima are somewhat lower, and are less than 16 °C over the higher ground and the islands. Extreme maximum temperatures can occur in July or August, and are usually associated with heat-waves. On the larger Western Isles and in sheltered places, temperatures can exceed 28 °C. For example, 32.1 °C was recorded at Onich (Highland) on 1 August 1995. In contrast, over Shetland high maxima are not achieved and the record is only 25 °C, set at Baltasound on 2 July 1958.
The variation of mean daily maximum and minimum temperatures month by month, together with the highest and lowest temperatures recorded, is shown for Lerwick and Inverness.
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 Northern Scotland varies from less than 40 a year on west-facing coasts and the Western Isles to more than 80 a year over the mountains. Ground frost averages range from less than 90 days per year to over 140 days, with a similar distribution to air frost. However, those places into which cold air can drain are particularly prone to frost. Examples include Altnaharra (Highland) with an average of 73 air frosts per year.
In Northern Scotland the frost-free season is often as little as 3 months, as shown by the average frequencies of air and ground frost at Lerwick and Inverness.
