montane areas is likely to be of especial interest. We shall expect to find that the biological character of the unstable areas is widely different from that found elsewhere and perhaps in some respects reminiscent of a condition that was more widespread in post-glacial times.
CLIMATE
THE differences between upland habitats and those of the lowlands are only partly structural. Partly they are climatic and this aspect must now be considered. British mountains are only of moderate size but they lie near the sea and across the path of the strong Atlantic breezes from the west. For this reason, wind and cloud and rain play a large part in the weather conditions and they combine to give a characteristic “atmosphere” to British mountain scenery, something of which is conveyed in the photograph of Glen Einich in Pl. VIII. Equally familiar to inhabitants and noticed by many visitors is the building up of evening cloud after sunset (see Pl. XXIX), while even in the finest weather the day is likely to break beneath a curtain of morning mist, well shown in the charming photograph of Llyn Padarn, (see Pl. 10). The visual impressions we thus carry with us can readily be confirmed from the precise data collected by meteorologists, and to them we may now turn.
We are fortunate in having detailed records which enable us to assess these effects over long periods and thus to present them as the main features of mountain climate in Britain. They were made between 1884 and 1903, when an observatory was maintained near the top of Ben Nevis (4,406 ft.), and though they thus give the extreme climatic limits for British mountains, they enable other more scattered observations to be checked and utilised.
In the first place, the records confirm the impression that strong winds are frequent. During thirteen years, an average of 261 gales a year with wind velocities exceeding 50 miles an hour was recorded at the summit of the mountain. This large number should be compared with the conditions at sea-level, when, even on the exposed western seaboard, few places average annually more than forty winds of such a velocity. The comparison between montane and lowland conditions may, however, be made in another form. A more recent estimate of wind-speeds has been made on Crossfell (2,930 ft.), a much lower summit in the Northern Pennines. There, it was estimated that the average wind velocity was at least twice that prevailing in the adjacent lowlands, a result comparable to similar estimates on Ben Nevis.
The Ben Nevis records also serve to illustrate the cloudiness of the mountain sky, for during the years of observation the summit was clear of mist and cloud for less than 30 per cent of the time and, as the table shows, had correspondingly low figures for exposure to sunshine (Table 2). These are, however, only different aspects of a more fundamental feature, the great humidity of the atmosphere. The average relative humidity of the air on Ben Nevis was 94 per cent of saturation with water vapour, showing little variation throughout the year, except in June, when it fell temporarily to 90 per cent, still an exceptionally high average figure.
As might be expected, this high atmospheric humidity was associated with high rainfall. Over a long period this averages 161 in annually at the summit, and it was rather higher during the thirteen years of comparative observations given in Table 2. The maximum recorded was 242 in. in 1909, and as much falls on Ben Nevis during the three “dry” months, April, May and June, as would represent the whole annual rainfall in Eastern England. High rainfall is, of course, a general feature of British mountains. Thus there is the well-known example of the Seathwaite District in Cumberland where Stye Head Tarn, east of Great Gable (2,900 ft.) has an annual average of 153 inches with a recorded maximum of 250 inches in 1928. The computed average for Glas Llyn (2,500 ft.), 500 yards north-east of the summit of the Snowdon ridge, is 198 inches. The Snowdon summit, Y-Wyddfa, in fact, competes with the head of Glen Garry (in Western Inverness), east of Sgurr na Ciche (3,140 ft.), for the distinction of being the wettest place in the British Isles. Both are considered to have an average annual rainfall of some 200 inches. Ben Nevis or Scafell and its Pike, have more of the character of isolated peaks, so that the prevalent winds can slip around them and less rain results.
The last feature of the Ben Nevis records to which attention must be directed is the range of temperatures, also given in Table 2, where they are compared with those at Fort William (at the base of the mountain).
In this table, the figures given at the foot of the columns for the year are averages in the case of temperature, and annual totals for hours of sunshine and rainfall. As there are many summits between 2,000 and 2,900 ft. to the south and west of Fort William, the rainfall there is already much higher than it would be on the outermost seacoast, and sunshine records are accordingly lower, so that the contrast between the lowland and montane conditions is much diminished.
Table 2 METEOROLOGICAL DATA OVER THE SAME 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