may, as has been said, register no more than 2° F. of frost all winter, but occasional bad years such as the early 1940’s may show up to 14° F. of frost. I remember seeing very thin pancake ice forming on the flat-calm sea of the Anchorage of Tanera in January 1941. Tanera is well out to the Minch and the sea is of fairly high salinity there.
Wind is a climatic factor of great variability and of exceedingly great importance in Highland natural history and the topography of the area. It would be entirely wrong to call the Highlands windswept; it cannot even be said truly that the coastal areas are windswept, for here on the shores of Loch Sunart where I am writing, the trees round the house are of beautiful symmetry and of great height, yet only 150 yards away on a shingle beach the wind blows much harder. The summits of the hills are windswept and the outer coasts are still more windswept, and it should be understood that the outer coasts of the North-West Highlands are the windiest part of Great Britain; much windier, for example, than the Shetland Isles or the outer Norwegian coast, or Valencia Island off south-west Ireland. The gales above 4,000 feet are worse than on the coast; the meteorological data from the Ben Nevis Observatory period gave an average of 261 gales a year of more than 50 m.p.h.
On the outer coasts of the Highlands gusts of 100 m.p.h. occur from time to time, and in certain places, where the configuration of the hills governs the play of wind, there are freak gusts and up and down draughts of excessive strength. In December 1938, I was going over the hill of North Rona during a three-day southerly gale to fetch water from the well on the southern cliff face. The wind was not so bad as it had been in the night, but I had to go on hands and knees over the ridge at 300 feet, from which there was an unbroken downward sweep to the sea on the south. When I reached the edge of the 70-foot cliffs which were at an angle of 30° from the vertical, I saw the turf at the edge of the cliff being lifted like the edge of a blanket, and the outer fringes of it were being torn off and flung inland just as a blanket would wear in a wind. All this is common enough in the islands; I had seen it before. But a few yards inland I saw two bare patches in the turf where two boulders had rested for years; the black surface of the bare patch was a good inch below the turf, and the boulders themselves—a foot to eighteen inches across and about eight inches high—were rolled uphill a distance of about three feet. The seals might well have shifted them had they been there, but no seals wandered in that part of the island. Only the force of the wind could have moved those stones, and as I still cannot believe that any wind we know could turn up the dead weight of a boulder well set with a flat bottom in the turf, presumably the cliff being set at that angle had the effect of multiplying the force of the wind at the upper edge; and a fairly large area of turf must have been lifted and stretched in some of the gusts, with the result that the boulders would be thrown out of their sockets uphill. The effect of wind is much less farther inland from the sea. The tree line on the coasts may be no more than 200 feet—assuming that trees will grow at all—whereas it is 1,800 feet on the western side of the Cairngorms. The prevailing wind in the Highlands is from the south-west. Such winds come off the relatively warm waters of the North Atlantic Drift and are laden with moisture. The weather is rarely cold during the time they blow. If the observer is far enough out from the high hills to see what is really happening in the sky, he will note that the south-westerly gales are predictable from the movements of the clouds before the wind is felt at sea level, or he may see a great bank of cloud out to the west in the Atlantic the night before. The south-westerly gales are gusty even out on the coasts, but they are so moisture-laden that one’s sense of smell is heightened: earth and sea have a beauty of their own at such a time through the scents they convey. The observer is watching the sky for signs of the end of the gale and sees a break of blue sky for a moment; then he notices that the clouds are no longer moving from the south-west but from the west. Soon he feels the wind to be coming from the west at sea level and the clouds are moving from north-west. Finally the wind veers farther to north-west and falls light in the north. That is the end of the gale.
The trough of low barometric pressure is left behind and the recording needle marks a steady rise and then levels off as the wind reaches the north. These gales have a closely similar pattern: in winter they may last several days; or only twenty hours in summer, sometimes completing the pattern day after day, beginning in the early morning with short gusts which are the forerunners and falling light in the late evening. It is the West Highland coast which shows up these gales as if under a magnifying glass. They will be mere breezes a few miles inland or at the head of a sea loch. Only their raininess will be felt there. The barograph shows less pronounced movement also, back in from the coast.
It is obvious that the effect of wind on the outer coasts is very great, for there is not only the period of great gales in the winter, when there are no leaves on the shrubs and no leafy vegetation at ground level, but there is the continual wearing of the summer period. I have seen leaves die from shaking through three days of blowing. Such trees as exist take on a distorted appearance, not one branch or twig managing to survive on the windward side of the trunk. It is a remark commonly heard that a tree has been bent by the constant action of the wind, but this is not true. Distortion is brought about by the continual lack of survival of all growth on one side. The distal ends of twigs are killed and the tree develops more and more a fuzziness of short annual shoots from the main stem. The influence of wind on the coastal region is further complicated by the spray which it may carry, for most broad-leaved plants object to a deposition of salt on their foliage. This is a subject we shall touch on later in the book.
North winds are relatively uncommon in the Highlands, but are recognized as bringers of snow in winter, snow which sets up its own train of events in natural history. A north wind in June or July means the best of sunny weather, but in August the north wind brings rain. East winds blow most regularly in spring, but gales from the south-east occur as well in the West Highlands. They are very cold for the district, as the south wind can be as well, for the air has come over a mountainous region where it must become chilled. The southeasters are dry winds and have a desiccating effect on the autumn herbage, sufficient to curtail the grazing season in some years. The south winds of summer mean a leaden sky and rain.
To conclude, the climate of the Scottish Highlands and Islands is rapidly changeable and far from uniform. The coastal climate is maritime or oceanic, but in the Central and Eastern Highlands it is more continental—more extreme temperatures, less wind, less rain and drier air. The meteorological tendency to drier air in the central region and the Dee Valley is further added to by the capacity of the ground to drain rapidly.
RELIEF AND SCENERY
LET US look at a physical map of Scotland and allow ourselves to make a tour of the Highlands as observers of country rather than as naturalists making detailed studies of habitats. We shall then be able to see those habitats with eyes wider open for the comparative sense we shall have gained. It will be convenient to divide the Highlands into five zones to which we cannot fairly give definite boundaries, though the zones themselves are significant in natural history. The divisions are my own and do not carry the weight of the acceptance of a committee of biologists. I should call them:
1 The southern and eastern Highland fringe which is in effect a frontier zone.
2 The Central Highlands, which may be likened to a continental or alpine zone.
3 The Northern Highlands, a zone with sub-Arctic or boreal affinities.
4 The West Highlands south of Skye, which may be called the Atlantic or Lusitanian Zone.
5 The Outer Hebrides and islands of Canna, Coll, Tiree, and such small islands as the St. Kilda group, the Treshnish group, the Flannans, North Rona and Sula Sgeir; an oceanic zone.
THE SOUTHERN AND EASTERN HIGHLAND FRINGE
FIG. 4.—Generalized relief features of the Highlands By courtesy of the Land Utilisation Survey of Great Britain
This zone follows the