the mean temperature above which the normal crop-plants of cool temperate climates start to grow. While the choice of such a level is somewhat arbitrary and does not by any means deal satisfactorily with the physiological problems involved, it is convenient to use this convention for the purpose of making comparisons. We could thus estimate that for plants of the type named above, the growing season at Fort William would be about eight months, say 243 days, while at 2000 ft. it would be about 142 days, and at the summit of Ben Nevis it would be quite negligible.
One of the difficulties of using such a simple method of treatment is that the higher summer temperatures at low altitudes have also a strong and cumulative effect on the rate of plant growth, as indeed do other features of the temperature cycle, such as freedom from frost. Thus lower summer temperatures markedly reduce the intensity of growth and hence the total annual amount is also very greatly affected. The strong westerly winds on British mountains tend to regulate the temperature and in particular they help to maintain lower summer temperatures than obtain on continental mountains like the Alps. Prof. Gordon Manley has drawn attention to another difference associated with the temperature curves. In spite of their moderate size, British mountains become treeless at comparatively low levels, usually below 2,000 ft., and in the same way the zone up to which useful cultivation can extend is comparatively low, often less than 1000 ft. While this is partly due to the operation of other climatic factors, it is also associated with the nature of the annual temperature cycle. If we were to go to some place such as New England, where the mean annual temperature in the lowlands is of the same order as that in Northern Britain, about 46° to 47° F., it would be found that on the mountains, e.g. on Mount Washington, the treeless zone would not be reached below altitudes of some 5000 ft. Although in Switzerland the mean annual temperatures are more widely different from our own, a similarly high timber-line is to be found in the Alps. Prof. Manley points out that this feature can be associated with the temperature conditions, for the average July temperature on Dun Fell (2,735 ft.) in Northern England is almost the same, about 48° F., as that on Mount Washington in New England at 6,284 ft.
A biological explanation of this difference is seen in the form of the temperature curves, and to illustrate the fact an additional temperature curve is given in Fig. 10. This is a typical curve for the lowlands of New England (Vermont) taken from Prof. Manley’s paper. The effect of adjusting this for changes in altitude would be to lower it to an appropriate extent. To give the equivalent curve for a height of 4406 ft., that of Ben Nevis, would require a reduction throughout of 14·7° F. Even if this were done, a large part of the annual temperature cycle would remain above 42° F. There would be a growing season at this altitude of at least 60 days and a mean July temperature of 53·8° F. Thus a considerable amount of plant growth, even from crop-plants, would be possible under New England conditions, while none could be expected with the temperature cycles obtaining in the Western Highlands of Scotland.
This method of considering the matter emphasises the importance of the low summer temperatures in British mountains as an obstacle to plant growth and as a feature which distinguishes them from localities of comparable altitude in continental areas either in North America or in Europe. In fact if we wish to find a climate equivalent in summer to that of our high mountain zone in temperature and in humidity, we must go to places in Arctic regions, preferably to those near the sea and remarkable for their frequent summer fogs, like West Greenland. But even these only rarely attain the constantly high air humidity which was observed on Ben Nevis. This, it is true, probably represents the extreme in Britain, though, judging from the rainfall records, it must be closely paralleled on the other main mountain masses in the west. Farther east and notably in the Cairngorms, where rainfall is less, air humidity is probably more variable and hence more like the Arctic stations of which we have record.
It is rather striking that the low summer temperatures found on British mountains are not associated with the presence of permanent snow, although on the highest peaks drifts may persist throughout the summer on north-facing slopes and in deep gullies. Two such drifts are well known and almost permanent, one on the north face of Ben Nevis and the other in the great corrie of Braeriach (4,246 ft.) in the Cairngorms. The latter, after having been known for some fifty years, finally disappeared for a time in the summer of 1935.
On Ben Nevis the top is usually free from accumulated snow for about 75 days in the year, though some snow may fall on about one day in ten, even in July and August. Thus, though even the highest summits are below the permanent snow-line, they are evidently very near to it. In these circumstances it might be expected that the extent and duration of “snow-lie” in early summer would have a good deal of influence on the distribution of living organisms in the highest montane zone. No detailed study of this matter has, however, yet been made in Britain.
Just as the temperature conditions differ very greatly in Britain and in the Alps, so there is also a considerable difference in other conditions. Speaking generally, British uplands lie wholly within the range of altitudes in which rainfall rises as the height increases. At higher altitudes, however (above about 5,000 ft. in these latitudes), rainfall would diminish with further increases in elevation, and this is the condition obtaining in the Alps and Pyrenees. Further, the lower layers of air are denser as well as more humid. Hence they absorb light strongly, and so higher altitudes receive much larger proportions of the sun’s energy, particularly of the ultra-violet and blue rays.
Thus Alpine conditions imply not only lower precipitation of rain or snow, but also a clearer atmosphere and intense insolation. The average summer temperatures and the illumination are much higher in the Alps, but they are accompanied by the possibility of strong radiation at night and by the certainty of great diurnal and seasonal variations of temperature, in great contrast to the small variations of temperature observed on British mountains. It is evidently better to distinguish upland climatic conditions in Britain as montane rather than alpine, and, as we have already noted, there is a great similarity between these montane conditions in summer and the corresponding features of Arctic coastal regions.
RAINFALL
The influence of the high humidity that is characteristic of British hills is not easily assessed. Atmospheric humidity undoubtedly has a considerable direct effect on plant and animal life, so that most biologists would be able to point to facts of distribution, such as the greater abundance of mosses and lichens in the western hills, which can reasonably be attributed to greater air humidity. But climatic humidity expresses itself not only through its direct effects on the distribution of living organisms but indirectly by affecting the character of the soil, and in the British Isles these indirect effects are extremely important. The only climatic data available for examining them on a sufficiently extensive scale are the rainfall data, and these we must now consider to see how far it is possible to use them in defining climatic limits. In doing this it will be necessary to adopt the following rather rough method of analysing climatic effects.
In the southern part of the Pennines, and probably generally among their eastern foothills, the average annual loss of water by evaporation is equivalent to a rainfall of about 18 in. This figure has been obtained partly as the estimate of the average amount of water lost by evaporation from a 6–ft. Standard tank, and the figures given in Table 4 are actually monthly estimates of the average losses so obtained (as inches of rain). But a similar annual figure (about 18 in.) can be obtained by comparing the rainfall over a given river basin with the “run-off” down the river, and access to much unpublished data has shown that this figure is fairly representative for the eastern Pennines. The difference between rainfall and run-off (assuming no loss into the ground or taking an average over many years) gives the net amount lost by evaporation, and we shall assume it to be distributed seasonally as in the figures given. It may be noted in passing that the problem of estimating evaporation losses may be considerably more complex than this. Empirical formulae have been worked out for estimating these losses in which it is usually assumed that they increase with increasing rainfall as well as with rising temperature.
Table 4 gives, in addition to the monthly figures for evaporation, the average monthly rainfall, also in inches, for two adjacent stations. One of them, Doncaster, lying in the Plain of York and at an altitude of 25 ft., represents a typical lowland station in Eastern England, with an average rainfall of about 25 in. per annum. The other, Woodhead, lies