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. [Illustration: Wind Roses showing the prevailing winds at Sumburgh Head] 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. [Illustration: Graph showing the average Rainfall and Temperature for each month of the year, from observations taken at Sumburgh Head, extending over 35 years]