As Shetland is entirely insular, and surrounded by ocean currents considerably warmer than those of other places in the same northern latitude, the climate is wonderfully equable, extremes of heat and cold being rare. The prevailing winds come from the south and west, laden with warm moisture from the Atlantic to temper the atmosphere. Added to that is the general “drift” of the Atlantic towards the British Isles. Of this warm current, Shetland gets its share.

When the air becomes laden with moisture, it is lighter than dry air, and in consequence the barometer is low. Wet and stormy weather usually follows. When the air is clear and dry, then the barometer is high, and good weather may be expected. The former is called cyclonic, and the latter anti-cyclonic types of weather. The atmospheric pressure varies as the storm moves onward; hence there is usually a falling barometer till the storm is over, succeeded by a rising barometer. The cyclone, as the storm is called, has two motions, a circular motion opposite to the hands of a clock, and a general forward movement as a whole, usually from some point west to some point east. In winter the deficiency of atmospheric pressure in the neighbourhood of Iceland accounts for the procession of westerly gales from the Atlantic during that season, causing the southerly type of wind to prevail over Shetland. In summer the area of low pressure is over Central Asia, which produces a northerly type of wind over the islands, and this corresponds with the dry season of the year.

Take a typical cyclone moving from the Atlantic, and passing, as it often does, between Shetland and Iceland. The weather may be unnaturally warm for the season of the year, the sky becomes overcast, the wind backs to the south or south-east, and the barometer falls rapidly. Other premonitory signs may be a halo or “broch” round the sun or moon; aurora (Merrie Dancers) high overhead; or a heavy swell in the sea. Wind and rain follow; after a time there may be a sudden lull; the wind shifts to the west or north; the barometer rises briskly and the clouds clear away, often with a strong gale of wind.

From November to February the prevailing winds are from the south and west, averaging about 5 days for each month, while October has an average of 7 days from the north. From March to June, the prevailing winds are from the north and north-east, which accounts for the cold spring and late summer, the average being about 4¾ days for each of these months. July is equally divided between west and north, having 5 days of each; while August has a total of 5 days from the north and September 6 from the west. The wind-rose shows that for the whole year the west wind prevails with 17 per cent. while the east has only 4. January has 7 days of west wind and only 1 of east; June has 6 days from the north-east, and 4 each from north and north-west. The average number of days in the year with calm weather is 27, with gales 37.

As might be expected from its insular position, the climate of Shetland is moist, and the annual rainfall is about 37 inches, while the number of rainy days in the year amounts to 251. This is the record for Sumburgh Head, extending over a period of thirty-five years; but of course the rainfall varies considerably in different parts of the islands. The wet period of the year embraces the months of October, November, December and January; while the dry period, if it can be called such, includes May, June and July—May being driest. Heavy falls of snow are not uncommon; but owing to the sea-breezes and changeable weather, snow seldom lies for any length of time.

The climate is somewhat cold, the mean temperature for the year being 44.7 degrees, while the mean monthly temperature varies from 38° in February and March, the coldest months of the year, to 53° in August, the warmest month. It is found that this mean annual temperature closely approximates to the temperature of the sea (from a depth of 100 fathoms to the bottom) between the Hebrides and Faroe. Thunderstorms are not frequent, and heat waves are usually followed by fog, which is very prevalent in summer and greatly impedes navigation round the coast. The month of May has an average of 8 days of fog, June 12, July and August 10 each, while the whole year shows a total of 75 days.

Winter in Shetland is dark and stormy; but this is to a certain extent compensated for by the long days of summer, from the middle of May to the end of July, when darkness is unknown. That season gives to the islands the poetic name of the “Land of the Simmer Dim.”