Though Fair Isle lies in the north temperate zone, its climate is greatly influenced by its relatively small size and location. 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.
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. This was almost equalled in July 1991 when 20.1 Celsius was recorded. Air frosts are reported much less frequently and, due to a katabatic effect, virtually never under calm conditions.