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Scotland

Underline
Introduction

Details available for: sunshine - rainfall - wind - temperature - snow - visibility

See also England - Wales - Northern Ireland climates.

Sunshine

Generally, Scotland is more cloudy than England, due mainly to the hilly nature of the terrain and the proximity of low-pressure systems from the Atlantic. Even so, parts of Angus, Fife, the Lothians, Ayrshire, and Dumfries and Galloway average over 1,400 hours of sunshine per year. This compares favourably with the coastal areas of Northern Ireland and the north of England, though not perhaps with the annual totals of over 1,700 hours achieved along the south coast of England. The dullest parts of Scotland are the more mountainous areas, with an annual average of less than 1,100 hours of sunshine over the mountains of the Highland region.

Mean daily sunshine figures reach a maximum in May or June, and are at their lowest in December. Wind and cloud play their part, but the key factor is, of course, the variation in the length of the day through the year. The relatively high latitude of Scotland means that although winter days are very short, this is amply compensated by long summer days with an extended twilight. On the longest day there is no complete darkness in the north of Scotland. Lerwick, in Shetland, has about four hours more daylight (including twilight) at midsummer than London.

Facts and Figures (bright sunshine)

Maximum duration in a month: 329 hours at Tiree (Argyll and Bute) in May 1946 and May 1975.

Minimum duration in a month: 0.6 hours at Cape Wrath (Highland) in January 1983.

Sunshine Graph

Rainfall

There is a general misconception that the whole of Scotland experiences high rainfall. In fact, rainfall in Scotland varies widely, with a distribution closely related to the topography, ranging from over 3,000 mm per year in the western Highlands (comparable with rainfall over the mountains of the English Lake District and Snowdonia in Wales) to under 800 mm per year near the east coast (comparable with the Midlands of England). (Note that rainfall also includes snow, which is melted and measured as rainfall.)

Typically, measurable rainfall (an amount of 0.2 mm or more) occurs on over 250 days per year over much of the Highlands, decreasing to around 175 days per year on the Angus, Fife and East Lothian coasts. In comparison, the driest part of Britain, along the Thames Estuary in south-east England, averages around 150 days per year with measurable rainfall.

The frequency of thunderstorms in Scotland, around three to nine days per year, is relatively low compared with an average of nine to 15 days over England. The number of thunderstorms can vary widely from year to year, but in general the northern and eastern coasts of Scotland average only three or four days with thunder per year, whilst inland values range from nine in the south to six in the north.

Facts and Figures

Maximum in a day (09-09 UTC): 238 mm at Sloy Main Adit, Loch Lomond on 17 January 1974.

Rainfall Graph

Winds

The most common direction from which the wind blows in Scotland is the south-west, but the wind direction often changes markedly from day to day with the passage of weather systems. There is a close relationship between surface isobars (lines joining points of equal air pressure) and the wind speed and direction over open, level terrain. However, in mountainous areas local topography also has a significant effect, with winds tending to blow along well-defined valleys.

Over land, the roughness of the ground causes a decrease in the mean wind speed compared with that which occurs over the sea, with the size of the decrease depending on the nature of the terrain. In major towns and cities the overall mean speed is considerably reduced by the buildings, but local funnelling may occur and the wind may gust to about the same speed as in open country. It is the gusts which cause much of the damage to buildings and trees during major storms. In general, wind speed increases with height, with the strongest winds being observed over the summits of hills and mountains.

Since many of the major Atlantic depressions pass close to or over Scotland, the frequency of strong winds and gales is higher than in other parts of the United Kingdom. Over low ground, the windiest areas are the Western Isles, the north-west coast and Orkney and Shetland with over 30 days with gales per year in some places. A day of gale is defined as a day on which the mean wind speed at the standard measuring height of 10 m above ground attains a value of 34 knots (39 miles per hour, 17.2 metres per second) or more over any period of 10 minutes during the 24 hours.

Facts and Figures

Highest gust recorded at a low-level site: 123 knots (142 m.p.h.) at Fraserburgh (Aberdeenshire) on 13 February 1989.

Highest gust recorded at a high-level site: 150 knots (173 m.p.h.) at Cairngorm Automatic Weather Station (on the border of Highland and Moray at an altitude of 1,245 m AMSL) on 20 March 1986.

Wind graph

Temperature

Over Scotland the mean annual air temperature at low altitude ranges from about 7 °C on Shetland, in the far north, to 9 °C on the coasts of Ayrshire and Dumfries and Galloway in the south-west. Normally temperature decreases by approximately 0.6 °C for each 100 m rise in height so that over the high ground temperatures are generally colder. For example, Braemar (at 339 m above mean sea level) has an annual mean temperature of 6.4 °C, while the corresponding value on Ben Nevis (at an altitude of 1,344 m) is -0.3 °C.

To a large extent, winter temperature in the British Isles is influenced by the surface temperature of the surrounding sea, and as the North Sea is cooler than the waters off the west coast, the east coast is generally slightly cooler in winter than the west coast.

In general, January and February are the coldest months. The daytime maximum temperatures over low ground in Scotland in January and February average around 5 to 7 °C, but on rare occasions in the lee of high ground, temperatures can reach up to around 15 °C when a moist south or south-westerly airflow warms up after crossing the mountains, an effect known as the föhn after its more dramatic manifestations in the Alps.

The coldest nights are those on which there is little wind, skies are clear, and there is a covering of snow on the ground. The lowest temperatures occur inland, away from the moderating influence of the sea, in valleys into which the cold air drains. It was under such conditions that the temperature fell to -27 °C, the lowest recorded in Britain, at Braemar in Aberdeenshire on 10 January 1982 and also more recently at Altnaharra in Highland Region on 30 December 1995. Coastal areas do not experience such cold nights; as an example the lowest temperature recorded at Lerwick on Shetland in the thirty years 1961 to 1990 is only -8 °C.

In summer, the effect of latitude on the amount of heat received from the sun plays a major role in determining the temperature. Thus, temperatures in Scotland are generally a few degrees cooler than in England. For example, the average daily maximum temperature at Glasgow in July is 19 °C compared with 22 °C in London.

July and August are normally the warmest months in Scotland. The highest temperatures normally occur inland, away from the moderating influence of the cooler sea. The highest air temperature recorded in Scotland was 32.9 °C at Greycrook (Scottish Borders) on 9 August 2003.

Facts and Figures

Air temperature (measured under standard conditions at 1.25 m above the ground).

Highest recorded 32.9 °C at Greycrook (Scottish Borders) on 9 August 2003.

Lowest recorded -27.2 °C at Braemar (Aberdeenshire) on 11 February 1895 and 10 January 1982. Minus 27 °C was also recorded at Altnaharra (Highland) on 30 December 1995.

Snow

As temperature generally decreases with height, precipitation which reaches the ground as rain at low-level sites may fall as snow over higher ground. Consequently, there is a marked increase with altitude in the number of days with snow falling and also the number of days with snow lying on the ground. The average number of days with sleet or snow falling in Scotland ranges from around 20 or less near the west coast to over 100 days in the Cairngorm Mountains and some other high peaks.

Snow rarely lies on the ground at sea level before November or after April. On low ground in the Western Isles and in most coastal areas of Scotland, snow lies on average for less than 10 days a year, although this increases to around 15 to 25 days for coasts in the north and north-east. However, over the mountains snow typically lies for more than 50 days a year. Serious difficulties with roads blocked by snow are, fortunately, not common on low ground, but some higher roads in Scotland are regularly affected each year.

In heavy snow there can be quite extensive drifting of the snow, especially over the higher ground. Snow is deposited on leeward slopes with exposed areas often left relatively bare. Snow deposited in natural hollows, such as high-level corries, can persist for some considerable time (an effect utilised to good effect by the development of the skiing industry in Scotland) and a few of these high-level, north-facing, localised snow beds are semi-permanent, only disappearing in very occasional summers. On the highest summits, such as Ben Nevis, snow cover typically persists for around six or seven months of the year.

Facts and Figures

Snow graph

Visibility

Scotland often enjoys excellent visibility, since the greater part of the country is remote from the industrial and populous areas of Britain and mainland Europe. Even in the industrial areas of central Scotland the move away from coal fires for domestic heating and the decline in traditional heavy industry has reduced the incidence of smoke haze caused by local air pollution. Frequently, however, cloud with a very low base can shroud high ground with hill fog. Extensive hill fog often results, especially in the west, when a moisture-laden south-westerly airstream covers the country. The resulting low visibility, especially if further reduced by drizzle, can pose a hazard for hill walkers and motorists.

Radiation fog may form overnight in low-lying inland areas on clear, calm nights, particularly in winter. Sea fog from the North Sea, known locally as 'haar', sometimes ruins what would otherwise be a fine day on or near the east coast, or in the Northern Isles, between April and September. Both these types of fog tend to break up and disperse during daytime, although inland, particularly in winter, mist and fog does sometimes persist. At Edinburgh, the midday visibility is less than 1,000 m on 3% of December days.