Other overnight temperatures included -5.3C Kinbrace in the Highlands, -4.3C in Braemar in Aberdeenshire and -1.1C in Dyce in Aberdeen.
David Wheeler, a weather expert in Fair Isle, said low pressure north of Shetland had brought colder air down from Arctic regions to the islands.
He described the snow as a “shock”, and warned that climate change was altering movements of air and making low temperatures in May less unusual.
Wheeler said that at this time of year winds would normally be bringing up air from warmer latitudes.
He said milder weather was expected over the weekend, followed by a cooler period with the return of a northerly air flow.
