13-YEAR PERIOD
The temperature figures given in the table are for mean monthly temperatures and they bring out very clearly the striking difference in temperature conditions which higher altitude entails. At the summit, the mean monthly temperatures are at or well below the freezing point of water for eight months in the year. Even during the four “summer” months, June to September, the mean monthly temperatures barely rise above those experienced during winter at the foot of the mountain. The temperature conditions are therefore severe.
It may justifiably be urged that this represents the extreme case among British mountains and that we need a more general method of representing the usual effects of temperature. Roughly speaking, an increase of altitude of 300 ft. entails a fall in the mean temperatures of about 1° F. Assuming now that 2,000 ft. represents an approximate lower limit to the mountain zone in Britain, we can obtain representative temperatures at this altitude by taking the average of the Ben Nevis and the Fort William temperatures and adding 0·6° F. to reduce the values approximately to those at 2,000 ft. The results are included in Fig. 10.
It is interesting to note, however, that essentially similar results can be obtained for different parts of the British uplands using the varying records of temperatures made at various altitudes and calculating from them the probable values at 2,000 ft. The following table summarises the mean temperatures so obtained for January and July:
Table 3
The Dun Fell and Moor House stations are two set up in the Northern Pennines by Prof. Gordon Manley, for which the data are less complete, though it will be seen that they suggest that the temperature conditions are essentially similar to those at Braemar, which represents the Eastern Scottish Highlands. The conditions at 2,000 ft. are generally similar therefore, with lower summer temperatures in the west. They may perhaps be regarded as sub-Arctic, resembling those just above sea-level in South Iceland.
The graphs in Figure 10 thus serve to illustrate what are for practical purposes the upper and lower limits of temperature for the British mountain climate. In effect, the increases in altitude produce little relative change in the levels of summer and winter temperatures but they sink, as it were, the whole temperature curve, in relation to any temperature level which may be chosen. Such a level, for example, is that represented in the graph by the horizontal line at 42° F. This level is given, because it is a temperature level which has been used
FIG. 10.—Mean monthly temperature in °F. at Fort William and at the summit of Ben Nevis (4,406 ft.)—continuous lines. The broken line gives calculated figures at 2,000 feet. The circles are summer temperatures in West Greenland and the crosses are data for Vermont (U.S.A.).
by meteorologists to represent 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,
