Climate (Gaelic)

The Western Isles enjoy a Temperate Maritime climate - a climate without extremes, modified by the effect of the ocean.

Weather systems in the Northern Hemisphere in general move from west to east, so the prevailing weather in the Western Isles follows a 5000 kilometre sea track, all the way from the United States. The warm ocean current known initially as the Gulf Stream and further north as the North Atlantic Drift flows from the Gulf of Mexico past these islands ensuing that temperatures never vary by extremes, but also providing a plentiful supply of moisture ensuring the islands' reputation for rain.

There are of course exceptions: the most notable being occasional cold plunges from the Arctic in winter, when snow will sometimes lie for a day or two; or dry. sunny, settled spells in the late spring associated with high pressure extending from Scandinavia, brushing Atlantic depressions to north or south of the islands. The warmest weather experienced in the islands usually occurs in the summer if a continental anticyclone becomes established over eastern Europe and southeasterly winds bring warm air all the way from the Middle East or Southern Russia.

Temperature

August is, on average, the warmest month, when the mean maximum temperature reaches nearly 16°C. There is little variation along the length of the Western Isles with Benbecula being less than half a degree warmer, on average, than Stornoway in the North. Mean maximum temperatures during the coldest months, January and February, are about 7°C.

Hard frosts are a rarity due to the warming effects of the surrounding ocean with mean minimum temperature in winter well above freezing at 1.5 to 2°C. Occasionally lochs do freeze over but rarely to such an extent that it would be safe to skate or walk on the ice. While daily maximum temperatures in London in summer frequently exceed 30°C, in the Western Isles they rarely exceed a comfortable 20°C. Despite being 1300 kilometres further north than London mean minimum temperatures are about the same; however, the overnight minimum in London often falls below -10°C yet rarely falls below -5°C in the islands. The autumnal mildness also extends the growing season with lawns still growing well into November.

Precipitation

There is no shortage of rainfall in the Western Isles, with an annual rainfall of around 1200 millimetres. The rainfall, like the temperature, is evenly spread throughout the islands with Benbecula marginally wetter than Stornoway. The greatest amount of rain falls in the four months, October to January (approx 45% of annual total); April and May being relatively dry averaging around 63 millimetres per month. This spring rainfall usually occurs in the form of showers and nearly half the days of each of these months could be expected to be completely dry.

Fog is expected on only one or two days annually, but localised coastal mist can occasionally roll in from the sea, particularly in spring and summer.

Snow is not a significant factor in Western Isles weather with snow or sleet falling on no more than about 30 days a year. Lying snow is even more of a rarity, expected on no more than 6 or 7 days a year. This is particularly important to crofters overwintering sheep in the open. Drivers also appreciate this benefit as island traffic is rarely disrupted to any great extent by lying or drifting snow.

Wind

Wind is the element most associated with Western Isles weather. Mean monthly wind speeds range from about 12 knots (13mph) in July/August to about 16 knots (18 mph) in December/January though daily mean wind speeds over 30 knots (35 mph) with gusts in excess of 50 knots (58mph) are not uncommon even occasionally during the summer months. The highest gust recorded at Stornoway in recent years was 98 knots (113 mph) in February 1962.

The prevailing wind direction is southwesterly though some of the strongest winds are often from the northwest.

Cold, dry easterly winds, often persisting for a week or two, are a common feature in April if a high pressure area settles over Scandinavia. These winds have a particularly harsh effect on plant life just emerging from winter dormancy.

From mid-June there is a tendency for a mobile SW to W airstream to become established, characterised by moist but mild conditions carried along on a succession of Atlantic frontal systems.

As autumn progresses into winter the Southwesternly winds continue but with a tendency for winds to swing more frequently to NW or N bringing heavy showers, often of hail or even snow after the turn of the year.

Sunshine

Benbecula, on average, receives 1333 hours of sunshine annually while Stornoway receives 1237 hours. This compares with 1400 hours at London (Weather Centre) or 1332 hours at Edinburgh (Botanical Gardens). Thus the Western Isles are as sunny as the main population centres much further south. The annual distribution of sunshine, however, is different, in that the sunniest months in the Western Isles are April to June with July and August relatively dull months, whereas further south the sunnier conditions are maintained into July and August. The sunniest Western Isles day on record in the last 30 years was the 20th June 1977 at Benbecula when 16.9 hours of sunshine were recorded. On average 40 days with more than 9 hours of sunshine may be expected annually; however 80 days of no sunshine would also be expected.

Day Length

The Western Isles are at high latitude, so the seasonal variation in day length is very noticeable. In midsummer, there are barely two hours of darkness, whereas in midwinter it can be dark from 4pm to 9am.

Conclusion

In general, it may be said that the Western Isles have a mild, moist climate with few extremes. The rarity of snow and frost in winter and the absence of oppressive heat in summer combined with clear, fresh, unpolluted air is seen as a major advantage by residents and visitors alike.