354 



NA TURE 



[Feb. II, 1886 



An annual rainfall of 40 to 60 inches covers extensivetraLls 

 of the liritish Islands ; a rainfall of at least 40 inches charac- 

 terising the climates of about a fourth part of the surface of 

 England, of about the half of Ireland, and considerably more 

 than the half of Scotland, the latter taken as a whole being by far 

 the rainiest of the three divisions of the United Kingdom. It 

 is to be noted that nowhere along the east coast of Great Britain, 

 or for some considerable distance inland, does the average rain- 

 fall anywhere reach 40 inches. In the east of Ireland, on the 

 contrary, the rainfall exceeds 40 inches in Wicklow, the south of 

 Down, and the middle districts of Antrim, which is probably 

 due to the south-westerly winds being diverted into a more 

 southerly direction in their passage through the Irish Sea. 



Over the whole of the west of Great Britain the rainfall 

 exceeds 40 inches annually, except from St. Bees' Head to 

 Dumfries, and from Holyhead to Lancaster, these districts being 

 largely protected from the rain-bringing winds by the Cumber- 

 land and the Welsh mountains respectively. It may also be 

 stated that the rainfall of the Orkney and the Shetland Islands 

 falls short of 40 inches, whereas in the Hebrides it exceeds that 

 amount. 



The shadings of blue on the map show in a striking manner 

 the extension eastwards of the areas of the 40 inches and up- 

 wards annual rainfall by the mountains of Sutherland, the 

 Grampians, the Cheviots, the Pennine Range, and the hilly 

 ground of the south-western counties of England. 



On the other hand, the breakdown at various intervals of the 

 mountainous or hilly plateau which may be regarded as extend- 

 ing along the west of Great Britain from Cape Wrath to the 

 Land's End has an equally striking influence on the distribution 

 of the rainfall, and as regards man's material interests is even 

 more important. Thus the opening of the Bristol Channel, 

 between Wales and the Cornish Peninsula, is the avenue through 

 which is spread a more generous rainfall over a large portion of 

 Central England than would otherwise have been the case. 

 Through the breakdown of the plateau between the Pennine 

 Range and North Wales another large portion of England, ex- 

 tending from Cheshire round by Derbyshire, and thence north- 

 ward through; Yorkshire, has its rainfall also very materially 

 increased. 



But the most remarkable of these breakdowns is the great 

 lowering of the water-parting between the Firths of Forth and 

 Clyde. Through the opening thus formed the south-westerly 

 winds pass freely, and overspread Dumbartonshire, Stirlingshire, 

 and the whole of Western Perthshire, precipitating over these 

 regions a rainfall truly western as regards its copiousness and 

 the direction of the winds with which it falls ; and through the 

 same breakdown there is extended, even eastward througli 

 Kinross-shire, a rainfall of fully 40 inches — an amount which 

 occurs noH'here else over comparatively level plains so far to the 

 east of the water-parting between eastern and western districts. 



Of the greatest importance is it to note the rainfall of Clydes- 

 dale, lying to the south of this breakdown. The amounts are, 

 in inches, 29-98 at Bothwell Castle, 30-54 at Dalziel House, 

 31-66 at Auchinraith, and 32-37 at JVIurdostoun. Now it is 

 simply the southerly element of the rain-bringing winds which 

 makes the rainfall of the Hamilton district of Clydesdale so 

 essentially different, both as respects its amount and the times of 

 its occurrence, from tliat of the Clyde below Glasgow. It may 

 be noticed here that when the rainfall of the west is in excess of 

 the average the rainfall of West Perthshire is also in excess ; and 

 on the other hand, when there is an excessive rainfall over the 

 Hamilton district, it generally occurs that the rainfall of eastern 

 districts is also in excess. The peculiarity of the rainfall of 

 Glasgow consists in this — that it lies midway between those 

 districts which are so differently circumstanced. 



The valleys of the counties of Kirkcudbright with Dumfries 

 and the intervening ridges lie awthart the course of the rain- 

 bringing winds, and show the inevitable result of a rainfall succes- 

 sively diminishing on advancing eastward. But on arriving in 

 Eskdale, the most easterly of these valleys, we meet with a 

 rainfall considerably in e-xcess of that of any of the valleys to 

 westward at the same elevations. The larger rainfall of Eskdale 

 is due to its lying more in the line of the Solway, and having 

 immediately to eastward a high mountainous region which the 

 south-westerly winds must cross in their passage to eastward. 



The distribution of the rainfall over this hilly region and over 

 the valleys on each side of it is instructive. Thus at Kirkconnel 

 Hall, near Ecclefechan, it is, in inches, 39-64 ; Canonbie, 49-72 ; 

 Carlesgill, 58-00 ; Eskdalemuir, 63-30; Tudhope (on the ridge, 



1 96 1 feet high), 76-43 ; and on the east of the watershed at 

 Teviothead, 54 86 ; Borthwickbrae, 44-36 ; Il.awick, 33'S5 ; and 

 thence continues diminishing in descending the valley to 26-50 

 at .Springwood Park, near Kelso. The reason why the rainfall 

 of this region is thus distributed is that the air on the windward 

 side of the ridge being suddenly raised to a greater height in 

 crossing the range its temperature is continually reduced by mere 

 expansion, and copious precipitation follows ; whereas on the 

 leeward side, as the air descends to lower levels, its pressure (or 

 density) being increased, and its temperature also thereby in- 

 creased, it gradually becomes drier, and accordingly the rainfall 

 diminishes rapidly with the descent of the aerial current to the 

 lower plains. A similar distribution of the rainfall is seen in 

 crossing the Downs from Brighton to London, and over all other 

 regions similarly situated. 



It is the rapid increase of the temperature and drying of the 

 air as the wind passes from high and wide plateaus into lower 

 levels which determines the areas of least rainfall of the British 

 Islands. Accordingly the smallest average annual rainfall, varying 

 from about 22J to 25 inches, occurs in England, and overspreads 

 a large portion of the south-eastern counties extending from the 

 Plumber to the estuary of the Thames, exclusive of the higher 

 grounds of Lincoln and Norfolk, where the rainfall rises above 

 25 inches. In every other part of the British Islands the rainfall 

 is above 25 inches. The influence of the higher grounds of 

 Norfolk and Lincoln in swelling the rainfall, most probably by 

 increased falls with easterly winds, is very striking. Similarly 

 the rainfall of the Yorkshire Wolds is in excess of surrounding 

 districts. Between the valley of the Thames and the'Humber 

 the rainfall nowhere exceeds 30 inches, except near the Chiltern 

 Hills. 



It will be observed that the northern limit of the region mark- 

 ing off a rainfall under 25 inches annually is at the Humber, or 

 near where Great Britain suddenly shrinks in breadth. It is, 

 however, probable that the larger rainfall of the eastern part of 

 Yorkshire, as compared with what obtains further south, may 

 also, in part, be occasioned by causes analogous to those which 

 give Western Perthshire its large rainfall. 



In Scotland no rain-gauge gives an annual averse under 25 

 inches. In three districts, however, the averages are only slightly 

 in excess of 25 inches, and less than 27 inches : these districts 

 being (•) Lower Tweeddale from about Coldstream to Jedburgh ; 

 (2) the low-lying parts of East Lothian ; (3) the shores of the 

 Moray Firth from the mouth of the Spey round to Tain. It will 

 be seen that these districts are not only well protected by exten- 

 sive highlands from the rains of the south-westerly winds, but 

 also from the, in many cases, torrential downpours of south- 

 easterly winds. It is this double protection which gives the 

 driest of its climates to these parts of Scotland. 



In Ireland, on the contrary, only a small district round Dublin 

 shows a rainfall lessthan 30 inches, this district being well protected 

 by the Wicklow mountains from the rain-bringing winds ; and 

 as in that island there is no continuous mountain-mass stretching 

 north and south there is no such great difference of rainfall and 

 temperature shown between the eastern and western climates of 

 Ireland as in the case of Scotland and England. 



The narrowness of the strip round the east of Scotland where 

 the rainfall does not exceed 30 inches a year is an interesting 

 feature, of which the rain-ma'ps constructed for individual months 

 suggest the explanation. Of the rainfall of eastern districts the 

 larger proportion is due to easterly winds, and by much the 

 larger portion of these falls usually takes place neither on the 

 low-lying coasts, nor at any great distance inland, but in the 

 intermediate region at heights from about 250 feet and upwards. 

 The falls are also very heavy in low-lying valleys that open out 

 so as to face these rain-bringing easterly winds. Of these rains 

 the weather of October 18S0 afforded an excellent illustration. 

 The rains of that month fell with strong north-easterly winds, 

 and the foreshores, looking to the north-east, of the Firth of 

 Forth, the JVIoray Firth, and the Pentland Firth, had a monthly 

 rainfall above the average, being in some places more than double 

 the average ; whereas over the rest of Scotland the rainfall was 

 under the average, being over very extensive breadths from 70 to 

 98 per cent, less than the mean rainfall for October. 



One of the most marked features of the climates of the south 

 of England, to which many of our invalids are sent, is due to the 

 influence of the Downs on the rainfall. Over the whole of the 

 somewhat broad region occupied by the Downs the rainfall ex- 

 ceeds 30 inches, rising near Petersfield to 40 inches. Along the 

 south coast, and for a varying distance inland as determined by 



