coast of Scotland. The July isotherms tend to go east and west rather than north and south, but as the west coast of Scotland is reached they take a distinct dip south-westwards and show the West Highlands to have a mean summer temperature of 55–57° F. compared with 56–58° F. on the east coast. If we take the differences between annual mean summer and winter temperatures, there is only 14° F. of difference on the West Highland coast, compared with 20° F. at Dundee and 24° F. in London, Kent and East Anglia. These, of course, are sea-level temperatures. The Highlands show a completely different story as soon as you go uphill, and when you truly go inland into the Monaliadh or Cairngorm regions, conditions are much more extreme. Much work has been done on mountain climate in Britain by Dr. Gordon Manley, President of the Royal Meteorological Society. The twenty years’ work on Ben Nevis is available to me in summary and I give these figures as a comparison with the sea-level conditions which pertain on the coast only a few miles away. The mean temperature of the hottest month, July, was 41.1° F., and of the coldest month, February, 23.8° F., a range of 17.3° F. The extreme records were, Maximum 66° F. (28 June 1902) and Minimum 1° F. (6 January 1894).
FIG. 2a.—January Isotherms (reduced to sea level)
FIG. 2b.—July Isotherms (reduced to sea level)
FIG. 3.—Average annual rainfall in the Highlands Based on a map prepared in the Meteorological Office and reproduced by permission of His Majesty’s Stationery Office Crown copyright reserved
When we come to rainfall we see how easy it is to fall into a trap by generalizing that the west coast is wetter than the east. The beltof very high rainfall is not on the coast but a few miles inland, and even then it does not extend uniformly from north to south of the Highlands. Reference to Bartholomew’s Atlas of Scotland will show the monthly rainfall distribution in fair detail. Coming south from Cape Wrath, the strip of country with a rainfall over 100 inches a year does not start until Latitude 57.30° N. and then goes southwards and slightly south-eastwards to Latitude 56° N. There is a good area all round this strip with a precipitation of 60–100 inches, but the coastal promontories, especially in the north, and the Hebrides, receive only 40–60 inches of rain, a low figure which is not reached elsewhere in the true Highlands until the central area. A very few parts of the eastern Highlands receive as little as 30–40 inches. There is no doubt that Skye and the Inner Isles are partly responsible for the heavy precipitation on the mainland coast to the east of them.
These two climatic factors of temperature and rainfall are of immense importance in determining the vegetation of an area, but there are interrelations of these two which must always be taken into consideration when natural history is being studied. For example, something must be known of the rate of evaporation relative to the precipitation. The ratio between these two in any given situation has a big influence on the types of plants and to a lesser extent on the animals to be found there. The evaporating power of the air is measured by what is called the saturation deficit, which can be calculated from the difference in temperature between the wet and dry bulb thermometers when the temperature of the air is known. The saturation deficit is a measure of the humidity of the air. It is found in practice that there is often a constant variation in the saturation deficit during the day, and where such variation pertains, it may in itself influence the grouping of living things. The evaporation rate tends to be high in summer on the West Highland coast and for some way up the hills, but in the glens and on the high tops the evaporation rate is generally low. The annual average difference between wet and dry bulb readings on Ben Nevis was only 0.7° F., which must be quite the most humid climate in Great Britain.
Sunshine records in the Highlands are highly variable. Broadly speaking, the amount of sunshine is in inverse proportion to the rainfall, so the strip of the Highlands to which allusion has already been made as enduring 100 inches or more of rain enjoys least sun. It is also possible to draw a line bisecting the Outer Hebrides longitudinally, showing an annual total of sunshine of less than 1,200 hours on the west side and 1,200–1,300 hours on the east. I remember very well during several months on North Rona, equidistant 47 miles north of the Butt of Lewis and Cape Wrath, how much oftener it was possible to see Cape Wrath lighthouse quite clear than the north end of Lewis which would be shrouded in cloud. The resident in the Highlands knows well how much more sun there is on the little islands and promontories than on even the general run of coastal areas. The published figures probably do not show the true position because the number of sun gauges is few in the West Highlands. One of the surprises of the West is the local area of high sunshine records for the island of Tiree (Plate 25), the outermost of the Inner Hebrides. The island is low (beneath the waves, as Gaeldom described it in the old days) and has but little cloud-stopping or cloud-gathering power. The soil is mostly of shell sand or loam, so its moisture drains or evaporates quickly. It was not without good reason that in past days Tiree was known as the granary of the Isles. Only the south coast of England equals or exceeds Tiree’s record. Grain needs sunshine to ripen it and fill it, but apart from that agricultural fact the plant ecologists say it is hard to demonstrate the precise effect of differing amounts of sunshine on vegetation. It remains probable, nevertheless, that actual lack of sunshine or a very low summer figure would inhibit the growth of such plants as broom and harebells, and encourage others such as the bryophytes (mosses and liverworts).
Tradition has it that the climate of the Highlands and Islands has deteriorated in living memory and in the fifty to seventy years before that. The meteorologists are always telling us we are wrong in thinking the weather was better in “our young days.” All the same, in the North-West Highlands salt pans were in general use many years ago for evaporating sea water, but it is said the fall in the amount of sunshine—much more than the remission of the salt tax—was responsible for their use being discontinued.
It will be best for us to consider the climatic factor of snow when we come to note its effect in the higher mountainous regions. But let it be said that the Highlands as a whole do not suffer nearly so much snow as the Southern Uplands of Scotland or the Pennine Chain of England. Snow comes earlier and stays later on the tops of the hills because the factor of altitude is concerned, but the Atlantic mildness pervades much of the lower ground. Nearness to the sea is a considerable factor in determining how low the snow will come on a hill face and how long it will stay. This is brought home to anyone living offshore from the West Highland mainland and who has for a view a wide range of peaks stretching from three to twenty miles inland. Snow as a climatic factor can be of much importance in a region long after it has fallen if the catchment area of the snow is large. The Cairngorm hills provide an excellent example: their greatest accumulation of snow is at the end of April or even in early May, whereafter there is a steady melting which is not complete until August. It is in the dry month of June that the snow held on the spacious tops and plateaux of this region can maintain the water even in rapid-running rivers and affect the fish life down in the Spey and Dee Valleys. Obviously, snow has a great effect on plant and animal life in areas where it lies long, but we are here dealing with the general climate of the Highlands and must not be drawn away into discussion of local and micro-climates.
The shore line of the West Highland mainland is particularly free from snow. I have known years when the snow has not lain for more than a couple of hours in a whole winter—and then, like as not, it has been at the end of April or in the first week of May. That first week or so of May is regularly a wintry period, and is so well known in the North that it is called the Gab o’ May. Taken all in all, it is remarkable how little is the effect of snow on vegetation in the Highlands except on the summits and in those places where it drifts and packs. Its influence on the behaviour of animals may be profound, but of that more later.
Frost has a distribution in the Highlands somewhat like that of snow, except for the peculiar conditions which will produce spring or autumn frosts on the floor of a glen and not at a few hundred feet up the hillsides. That is a phenomenon, of course, which is known all too well in the fruit-growing districts of England. The shore line of islands