Chap29

[490]

CHAPTER XXIX.

BRITISH CLIMATES AND THEIR CAUSES—RAINFALL IN DIFFERENT AREAS—AREAS OF RIVER DRAINAGE.

BEFORE discussing the subject of rivers and rivergravels and alluvia, I now come to other phenomena connected with the physical structure of our island and its geography generally; and first, with regard to the rain that falls upon its surface. If we examine the best hydrographic maps of the Atlantic, we find on them numerous lines and arrows showing the direction of the flow of the ocean currents as first drawn by Captain Maury. One great current flows from the Gulf of Mexico, where the water in that land-locked area within the tropics is exceedingly heated; and flowing out of the gulf, it passes E. and NE. across the ocean, and so reaches the European area of the North Atlantic. So marked is the heat of this immense current that, in crossing from England to America, the temperature of the water suddenly falls some degrees. Twenty years ago, in crossing the Atlantic, I was in the habit early in the morning of taking the temperature of the water with one of the officers of the steamboat. We then found that at about five o'clock in the morning for several days, the temperature of the sea was always about four degrees above the temperature of the air, but quite suddenly, in passing out of the Gulf Stream, at the same hour of the morning, the temperature of

[Gulf Stream. 491]

the water was found to average about four degrees below that of the air.

Where in ocean current maps the arrows point southwards, there are cold streams of water coming from the icy seas of the north. One of these passes along the east coast of America, and coming from the North Sea, many an iceberg detached from the great glaciers of Greenland is floated from Baffin's Bay across the banks of Newfoundland into the Western Atlantic, as far south even as the parallel of New York. The western half of the North Atlantic is thus kept cool, and the water is often colder than the air.

The Gulf Stream occupies a very great width in the Atlantic, and approaches tolerably near to our own western coast, and the effect of this body of warm water flowing northward is to divert the isothermal lines (lines of equal temperature) far to the north, over a large part of the Atlantic area, and also of that of the western half of Europe. Thus a certain line runs across North America, about latitude 50°, representing an average temperature for the whole year of 32°. Across that continent it passes tolerably straight, but no sooner does it get well into the Atlantic than the Gulf Stream, flowing northwards, warms the air, and the result is that the line bends away to the far north above Norway; thus in the west of Europe producing an average warmer climate, for the whole year, than exists in corresponding latitudes in North America, the middle of Europe, and the interior of Asia. Our British climate, and all the west of Europe, becomes, as it were, abnormally warm, owing to the influence of the Gulf Stream, and we at once recognise this fact from the circumstance that trees of goodly size grow much further north on the west coasts of Europe than on the east

[492 Rainfall in Britain.]

coasts of North America. Another effect that the Gulf Stream produces, is to cause a great amount of moisture in the west of Europe, and if we consult a rain map of the British Islands, we see represented by different shades the average amount of rainfall in different areas-the darker the shade the greater the quantity of rain. The prevalent, winds in the west of Europe are from the SW. and therefore during a great part of the year, the south-west wind warm comes laden with moisture across the land from the sea where the Gulf Stream flows.

In the extreme south-west of England, in Cornwall, from 37 to 54 inches of rain falls every year; and the average for the county may be taken at about 43 or 45 inches. In Devonshire the rainfall varies from 31.75 and 32.6 at Sidmouth to 53.17 inches on Tavistock. In Somerset from 28.57 at Langport to 42 at East Harptree. In Dorset from 18.45 at Abbotsbury to 32.24 inches at Bridport. In Wiltshire from 28.59 at Swindon to 29.27 inches on Salisbury Plain. In Hampshire from 27 at Aldershot to 38 inches in Petersfield. In Sussex from 26.37 at Hastings to 29 inches at Chichester. In Kent and Surrey from 23.82 at Kew to 32.67 inches at Hythe. In Middlesex from 25.85 at Hampstead to 23.11 inches on Winchmore Hill. The rainfall in the western part of the south of England is therefore much greater than in the east.

In like manner in Pembrokeshire the annual rainfall varies from about 31 to 40 inches, and may be averaged at about 36 inches, and in Cardiganshire at Lampeter about 45.18 inches, in Glamorganshire at Cardiff about 42 inches, in Caermarthensh ire and Breconshire at about 40 inches, and in Montgomeryshire and Merionethshire at about 54 inches. In Caernarvonshire

[Rainfall in Britain. 493]

the fall is about the same, but at Beddgelert in 1870 it amounted to 101.58 inches, and in the Pass of Llanberis to 76.67, while at Caernarvon close by the sea the rainfall was only 38.02 inches. In Anglesea the average fall is about 34 1/2 inches.

In Staffordshire, further from the west coast and from the mountains, the average rainfall is about 23 inches, in Leicestershire about 19 inches, in Bedfordshire about 16 inches, and in Norfolk about 24 to 25 inches. In this southern half of England the rainfall therefore evidently decreases from west to east. Lancashire is a rainy county. At Manchester the rainfall varies from 32.59 to 36.77 inches, at Bolton 44.21 to 49, and at Coniston it is as high as 64 inches, but that is in the Cumbrian region of Lancashire. In Cumberland the annual rainfall varies from about 22 at Cockermouth, on the low ground near the coast, to 154 inches at Seathwaite, in the heart of the mountains, and in 1871 it is stated to have been still higher, and perhaps the average rainfall of the whole of that mountain region may amount to about 70 inches annually. As we pass eastward it decreases, but on the highest grounds of Yorkshire and Northumberland there are places where it rises from 51 to 56 inches, while in the lower ground at Holbeck, Leeds, it falls to about 22.85, at Newcastle to about 24, and at North Shields on the coast to about 26 inches.

In Scotland the same kind of observation holds good with regard to the rainy character of the west. In Argyleshire the lowest rainfall in 1870 was 42 inches at Inverary, and the highest 109.20 inches at Lochgoilhead. The average rainfall for the whole county, and in the islands, may perhaps be estimated at from 55 to 60 inches. At Portree in Skye, in 1871, it amounted

[494 Rainfall in Britain.]

to 104.26 inches. The mountainous character of the country produced that result, for in the Isle of Lewes in. the same year the rainfall at Stornoway was only 31.79 inches, while at Cromarty on the east coast of Scotland about 26 inches of rain fell. In parts of Aberdeenshire the average fall is from 24 to 33.5 inches, and in Fife the fall is from about 20 to 30 inches, in Midlothian from 29 to 37, and in Haddingtonshire from 23 to 25. The same rule of decrease of rainfall therefore prevails in Scotland that prevails in England, and it is needless to multiply instances. The area, therefore, of Great Britain varies much in the fall of rain, and the average temperature of the western area is raised and rendered agreeable by the influence of the Gulf Stream. So much is this the case, that certain garden plants grow through the winter in Wales and the west of England, and even in the far north-west of Scotland, which the winter cold of Middlesex kills. I have seen bamboo canes growing in the open air in a garden in Anglesea all the year round, and common fuchsias on the shores of Loch Erribol in Sutherland.

Now the watery vapour in the air that rises from the heated water of he Gulf Stream, is carried to the British coast by the prevalent west and south-west winds, and is partly intercepted on its passage eastward by the mountains which rise in the west of Ireland and Great Britain. Everyone who has visited Cumberland and Wales knows how rainy these regions are compared with the centre and east of England. The reason is, that the air laden with moisture from the Atlantic rises with the winds against the western flanks of the mountains into the colder regions of the atmosphere, and the air also expanding at these heights, rain is precipitated there and upon adjacent lands. The same is the case in Scotland, where the Highland mountains

[Areas of Drainage. 495]

on the west produce a like effect; and thus, partly because it is the first land that the wind laden with moisture reaches, and partly because of the mountains, it happens that a greater amount of rain is precipitated in the western than in the eastern parts of our Island.

If we examine our country with regard to special areas of drainage, we find, that they are exceedingly numerous. In Scotland the rivers that run into Moray Firth drain an area of about 2,500 square miles; the Spey, which runs into the German Ocean, nearly 1,200 square miles. The Tay drains an area formed by the Grampian mountains and part of the Old° Red Sandstone of 2,250 square miles. The Forth, including its estuary, drains an area of about 2,000 square miles. The Clyde, not including the greater part of its estuary, drains an area of 1,580 square miles, the Tweed 1,870 square miles.

In England, the Tyne drains 1,100 square miles, the Tees, 774. If we take the Trent and the Ouse as draining one area, the immense extent, for such a country as ours, of about 9,550 square miles are drained into the Humber. The Witham, the Welland, the Nen, and the Great Ouse, flowing into the old bay of the Wash, drain 5,850 square miles. The Thames drains an area of about 6,160 square miles; and if we include all the estuary, about 10,000. The Severn drains an area of 8,580 square miles. The Avon that enters the sea at Christchurch drains 1,210 square miles; the Ex, 643; the Towey, in Caermarthenshire, 506; the Dee, 862; the Mersey, 1,748; the Ribble, 720; and the Eden, 995; and if we take all the rivers that run into the Solway Firth, including the Eden, the area drained amounts to nearly 3,000 square miles. This leads to the question of the origin of river valleys and their different geological dates.