The climate of Western Scotland is milder than that of Eastern Scotland due to the stronger maritime influence, as the prevailing winds blow from the sea. The warm Gulf Stream also has a strong influence on Western Scotland. With winds mainly blowing from the sea the annual mean temperatures are in the range 9.4 to 9.7 °C in coastal areas such as Ayrshire, Bute and Kintyre. This mean temperature is also reached in central Glasgow, due to the urban heat island effect. The annual mean temperature tends to fall towards the south and north parts of the region due to altitude, as well as inland where 8.0 to 9.3 °C is more typical. The lowest annual mean temperature for low lying areas in the UK is about 7 °C in the Shetlands.
The strong maritime control of temperature is best seen in the annual temperature range, the difference between the mean temperature of the warmest and coldest months. The smaller the temperature range, the greater the maritime influence. In parts of Galloway, Kintyre and the Hebrides the annual temperature range is about 9 °C, which is similar to that found in western Ireland; for comparison the annual temperature range increases to about 14 °C in the English Midlands as the maritime influence decreases.
The sea reaches its lowest temperature in February or early March so that on average February is the coldest month in some coastal parts of Ayrshire, the Rinns of Galloway, Kintyre and the Hebrides. In February the mean daily minimum temperature varies from about 1.6 °C in most of the islands, 0.7 to 1.6 °C along most of the Solway Firth and lowland inland areas, but less than -1.1 °C in parts of the Southern Uplands and central Highlands. Inland, where the influence of the sea is less, January is the coldest month with mean daily minimum temperatures between -3.4 and -0.9 °C.
Examples of extreme low temperatures that have occurred in the region are -18.3 °C at Glenlee (Dumfries and Galloway) in January 1940 and -24.8 °C at Carnwath (South Lanarkshire) on 11 January 1982. Unusually cold weather was experienced in the last week of December 1995 with, for example, a minimum temperature of -20 °C and a maximum of -12 °C at Glasgow airport on 29th. These very cold conditions coincided with the Christmas/New Year holidays and resulted in burst water pipes and severe flooding in unoccupied homes and commercial properties.
July and August are the warmest months in the region with mean daily maxima ranging from less than 14.6 °C on the highest ground to more than 18.5 °C in southern Dumfries and Galloway and the Clyde valley. These may be compared with 22.5 °C in the London area. Instances of extreme high temperatures are rare and are associated with hot air brought from mainland Europe on south easterly winds, accompanied by strong sunshine. The highest temperature ever recorded in the whole of Scotland was 32.9 °C at Greycrook (Borders) on 9 August 2003. The previous record of 32.8 °C was set almost a century before in Dumfries on 2 July 1908. Other examples of extreme high temperatures that have occurred in the region are 32.2 °C at Prestwick and Kilmarnock on 29 July 1948, and 31.7 °C at Buchlyvie (west of Stirling) on 12 July 1911.
The variation of mean daily maximum and minimum temperatures month by month, together with the highest and lowest temperatures recorded, is shown for Paisley and Tiree.
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 Western Scotland varies from less than 25 a year in the Hebrides to about 40 on the coast of the mainland to more than 80 a year over the higher ground of the Southern Uplands and Highlands. Ground frost averages range from less than 60 to over 140 days per year, with a similar distribution to air frost. However, valleys and hollows into which cold air can drain off hills are particularly prone to frost.
The average frequency of air and ground frost at Paisley and Tiree are shown in the following graphs. Although the summer months are usually free of air frost, a ground frost is possible in almost any month especially at sheltered inland sites.
