The climate of Fair Isle
Though Fair Isle lies in the north temperate zone, its climate is greatly influenced by its relatively small size and location. The climate of the island is truly oceanic in nature - and is often described as hyperoceanic (e.g. Birse 1974).
Due to the rain-shadow effect of the Scottish mountains rainfall is not excessive, but precipitation is frequent with measurable rainfall on 216 to 266 days each year.
The relative humidity is the highest in Britain and, except in late spring and early summer, cloudy conditions prevail.
Winters are generally mild, but frequently stormy. Summers are cool and often windy, with a high frequency of fog. During spring and summer Fair Isle often reports the lowest daytime maximum temperature in the British Isles - even though (in May and early June) it may at the same time also be the sunniest place in the country.
At almost 60° north, the climate is far from typical of what might be expected at this latitude, due to the ameliorating effect of the North Atlantic Drift. The mean annual temperature is 7.5° Celsius, with little variation between summer and winter. As sea temperatures do not reach their highest values until late summer, it is at this time when the maximum temperatures on Fair Isle often occur - mostly of the order of 16° or 17° Celsius.
In August 1975 20.2° Celsius - the highest temperature ever recorded on Fair Isle - was reported. This was almost equalled in July 1991 when 20.1° Celsius was recorded. During winter temperatures can fall as low as -2° or -3° Celsius, but are generally around 4° Celsius.
The lowest temperature recorded is -5.1° Celsius in February 1978, though -5.0° Celsius was also recorded in March 1992. Ground frosts are not a frequent phenomena - even in winter - but they are not unknown in summer, having been reported in every month of the year.
Air frosts are reported much less frequently and, due to a katabatic effect, virtually never under calm conditions. The lowest temperatures are always experienced with an out-break of cold northerly, Arctic air, when strong winds prevent the sea from exerting the usual ameliorating effect.
Under such conditions, temperatures may fall to between -4° and -5° Celsius, with the wind gusting in excess of 30 ms-1, and frequent snow showers often reducing visibility to less than 1000 metres. Because of its position close to one of the major depression tracks, the weather on Fair Isle is very unsettled.
Snow, which can be frequent - falling on as many as 70 days in a winter - does not usually lie for long, as a change of wind direction to the south always brings milder temperatures. Overall the prevailing wind direction is generally between south and west, but there is often much variation from day to day. During spring south-easterly winds are common, gradually tending towards a northerly direction by early summer.
Westerly winds are frequent in summer and autumn and, in winter, southerly winds are the prevailing wind direction. Fair Isle is probably one of the windiest lowland sites in the British Isles, experiencing gales (10 minute mean speed of 34 kt, 17.5 ms-1, or more) on an average of 57.8 days each year - with 112 gale days reported in 1979 but only 16 gale days in 1985. While periods of calm, or very light wind conditions, are very rare they can occur even in winter - but seldom last for more than an hour or two.
On clear nights in winter the aurora is often observed. Most frequently the display is limited to a glow, arc or arc with rays in which there is only a hint of colour. However, several times each winter, very active displays with much movement of coloured rays and curtains - often extending over much of the sky - do occur. Auroral activity varies from year to year with peaks of activity every eleven years.