Encyclopædia Britannica, Ninth Edition/Climate
night. The great astronomer and geographer Ptolemy divided the surface of the globe, from the equator to the arctic circle, into climates or parallel zones, corresponding to the successive increase of a quarter of an hour in the length of midsummer-day. Within the tropics these zones are nearly of equal breadth; but, in the higher latitudes, they contract so much that it was deemed enough to reckon them by their doubles, answering consequently to intervals of half an hour in the extension of the longest day. To compute them is an easy problem in spherical trigonometry. As the sine of the excess of the semidiurnal arc above a quadrant is to unity, so is the tangent of the obliquity of the ecliptic, or of 23°28′, to the cotangent of the latitude. The semidiurnal arcs are assumed to be 91° 5212′, 93° 45′, 95° 3712′, 97° 30′, &c., and the following table, extracted from Ptolemy's great work, will give some general idea of his distribution of seasons over the surface of the globe. The numbers are calculated on the supposition that the obliquity of the ecliptic was 23° 51′ 20″, to which, according to the theory of Laplace, it must have actually approached in the time of Ptolemy. They seem to be affected by some small errors, especially in the parallels beyond the seventeenth, as the irregular breadth of the zone abundantly shows; but they are, on the whole, more accurate than those given by Varenius.
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Climate in its modern acceptation signifies that peculiar state of the atmosphere in regard to heat and moisture which prevails in any given place, together with its meteorological conditions generally in so far as they exert an influence on animal and vegetable life. The infinitely diversified character which climate displays may be referred to the combined operation of different causes, which are chiefly reducible to these four distance from the equator, height above the sea, distance from the sea, and prevailing winds, which may thus be regarded as forming the great bases of the law of climate.
Of these causes which determine climate incomparably the most potent is distance from the equator. The same sunbeam which, falling vertically, acts on a surface equal to its own sectional area is, when falling obliquely on the earth, spread over a surface which becomes larger in in verse proportion to the sine of the obliquity. Conse quently less and less heat continues to be received from the sun by the same extent of surface in proceeding from the equator toward the poles ; and this diminution of heat with the increase of obliquity of incidence of the solar rays is enhanced by the circumstance that the sun s heat, being partially absorbed in its passage through the atmosphere, the absorption is greatest where the obliquity is greatest, because there the mass of air to be penetrated is greatest. Hence arise the broad features of the distribu tion of temperature over the globe, from the great heat of equatorial regions, falling by easy gradations with increase of latitude, to the extreme cold of the poles. If the earth s surface were uniform, and its atmosphere motionless, these gradations would run everywhere parallel with the latitudes, and Ptolemy s classification of the climates of the earth would accord with fact. But the distribution of land and water over the earth s surface and the prevailing winds bring about the subversion of what Humboldt has termed the solar climate of the earth, and present us with one of the most difficult, as certainly it is one of the most important problems of physical science, viz., the determina tion of the real climates of its separate regions and localities, and the causes on which they depend.
The decrease of temperature with height is perceptibly felt in ascending mountains, and is still more evident in the snow-clad mountains, which may be seen even in the tropics. The snow-line marks the height below which all the snow that falls annually melts during summer. The height of this line above the sea is chiefly determined by the following causes by distance from the equator ; by the exposure to the sun s rays of the slope of the mountain, and hence, in northern latitudes, it is higher on the south than on the north slopes of mountains, other things being equal ; by situation with reference to the rain-bringing winds ; by the steepness of the slope ; and by the dryness or wetness of the district. Since, then, no general rule can be laid down for the height of the snow-line, it can only be ascer tained by observation. Speaking generally it sinks little from the equator to 20 N. and S. lat. ; from 20 to 70 it continues to fall equably, but from 70 it falls rapidly to 78, where it is at sea-level.
The following are a few of the more noteworthy of the exceptions. On the north side of the Himalayas it is about 4000 feet higher than on the south side, owing to the greater depth of snow falling on the south side and the greater dryness of the climate of Tibet, resulting in a more active" evaporation from the snows and stronger sun-heat on the north side, to which is to be added the comparative want of vegetation on the north side, thus favouring a more rapid melting of the snows. The snow-line is higher in the interior of continents than near their coasts, the rain fall there being less and the heat of summer greater ; and similarly, owing to the greater prevalence of westerly over easterly winds in many regions of the globe, it is higher on the east than on the west sides of continents. In South America the snow-line rises very considerably from tho equator to 18 S. lat. and more so, markedly, on the west than on the east slopes of the Cordilleras, because of the smaller amount of precipitation of the west side of this mountain range. It is as high in 33 as in 18 S. lat., but south of 33 it rapidly sinks owing to the heavy rains brought by the westerly winds which begin to prevail there. In the south of Chili it is 6000 feet lower than among the Rocky Mountains at the same distance from the equator, and 3000 feet lower than in the same latitudes in Western Europe. It is impossible to overestimate the importance of the snow-line as one of the factors of climate in its relations to the distribution of animal and vegetable life.
Glaisher, in his balloon ascents, made observations of temperature at different heights, the results of which may be thus summarized. Within the first 1000 feet the average space passed through for 1 was 223 feet with a cloudy sky and 162 feet with a clear sky; at 10,000 feet the space passed through for 1 was 4545 feet for the former and 417 feet for the latter; and above 20,000 feet the space with both states of the sky was 1000 feet nearly for a decline of 1. It must be noted, however, that these rates of decrease refer to the temperature of the atmosphere at different heights above the ground, which are in all probability altogether different from the rates of decrease for places on the earth s surface at these heights above the level of the sea the problem with which climatologists have to deal.
Observation shows, as might have been expected, that the rate at which the temperature falls with the height is a very variable quantity, varying with latitude, situation, the state of the air as regards moisture or dryness, and calm or windy weather, and particularly with the hour of the day and the season of the year. In reducing temperature observations for height, 1 for every 300 feet is generally adopted. In the present state of our knowledge this or any other estimation is at best no more than a rough approximation, since the law of decrease through its variations requires yet to be stated, being in truth one of the most intricate and difficult problems of climatology awaiting investigation at the hands of meteorologists. Among the most important climatic results to be determined in working out this problem are the heights at which in different seasons the following critical mean temperatures, which have important relations to animal and vegetable life, are met with in ascending from low-lying plains in different regions of the world, viz., 80, 75, 70, 65, 63, 60, 58, 55, 50, 45, 39 (the maximum density of fresh water), 32 (its freezing point), and 20.
perature in different seasons, and which ore due exclusively to differences of absolute height, though of the greatest possible practical importance, yet leave untouched a whole field of climatological research a field embracing the mean temperature of different hours of the day at different heights, for an explanation of which we must look to the physical configuration of the earth s surface and to the nature of that surface, whether rock, sand, black soil, or
covered with vegetation.Under this head by far the most important class of con ditions are those which result in extraordinary modifica tions, amounting frequently to subversions, of the law of the decrease of temperature with the height. This will perhaps be best explained by supposing an extent of country diversified by plains, valleys, hills, and table-lands to be under atmospheric conditions "favourable .to rapid cooling by nocturnal radiation. Each part being under the same meteorological conditions, it is evident that terres trial radiation will proceed over all at the same rate, but the effects of radiation will be felt in different degrees and intensities in different places. As the air in contact with the declivities of hills and rising grounds becomes cooled by contact with the cooled surface, it acquires greater density, and consequently flows down the slopes and accumulates on the low-lying ground at their base. It follows, therefore, that places on rising ground are never exposed to the full intensity of frosts at night ; and the higher they are situated relatively to the immediately surrounding district the less are they exposed, since their relative elevation provides a ready escape downwards for the cold air almost as speedily as it is produced. On the other hand valleys surrounded by hills and higu grounds not only retain their own cold of radiation, but also serve aa reservoirs for the cold heavy air which pours down upon them from the neighbouring heights. Hence mist is frequently formed in low situations whilst adjoining eminences are clear. Along low-lying situations in the valleys of the Tweed and other _rivers of Great Britain laurels, araucarias, and other trees and shrubs were destroyed during the great frost of Christmas 1860, whereas the same species growing on relatively higher grounds escaped, thus showing by incontestible proof the great and rapid increase of temperature with height at places rising above the lower parts of the valleys.
This highly interesting subject has been admirably eluci dated by thenumerousmeteorologicalstationsof Switzerland. It is there observed in calm weather in winter, when the ground becomes colder than the air above it, that systems of descending currents of air set in over the whole face of the country. The direction and force of these descend ing currents follow the irregularities of the surface, and like currents of water they tend to converge and unite in the valleys and gorges, down which they flow like rivers in their beds. Since the place of these air-currents must be taken by others, it follows that on such occasions the temperature of the tops of mountains and high grounds is relatively high because the counter-currents come from a great height and are therefore warmer. Swiss villages are generally built on eminences rising out of the sides of the mountains with ravines on both sides. They are thus admirably pro tected from the extremes of cold in winter, because the descending cold air-currents are diverted aside into the ravines, and the counter-currents are constantly supplying warmer air from the higher regions of the atmosphere.
Though the space filled by the down-flowing current of cold air in the bottom of a valley is of greater extent than the bed of a river, it is yet only a difference of degree, the space being in all cases limited and well defined, so that in rising above it in ascending the slope the increased warmth is readily felt, and, as we have seen, in extreme frosts the destruction to trees and shrubs is seen rapidly to diminish. The gradual narrowing of a valley tends to a more rapid lowering of the temperature for the obvious reason that the valley thereby resembles a basin almost closed, being thus a receptacle for the cold air-currents which descend from all sides. The bitterly cold furious gusts of wind which are often encountered in mountainous regions during night are simply the out-rush of cold air from such basins.
The two chief causes which tend to counteract these effects of terrestrial radiation are forests and sheets of water. If a deep lake fills the basin, the cold air which is poured down on its surface having cooled the surface water, the cooled water sinks to a greater depth, and thus the air resting over the lakes is little if at all lowered in tem perature. Hence deep lakes may be regarded as sources of heat during winter, and places situated near their outlet are little exposed to cold gusts of wind, while places on their shores are free from the severe frosts which are peculiar to other low-lying situations. The frosts of winter are most severely felt in those localities where the slopes above them are destitute of vegetation, and consist only of bare rock and soil, or of snow. If, however, the slopes be covered with trees, the temperature is warmer at the base and up the sides of the mountain, the beneficial influence of forests consisting in the obstacle they offer to the descending currents of cold air and in distributing the cold produced by terrestrial radiation through a stratum of the atmosphere equalling in thickness the height of the trees.
Hence as regards strictly local climates, an intelligent knowledge of which is of great practical value, it follows that the best security against the severity of cold in winter is afforded where the dwellings are situated on a gsntle acclivity a little above the plain or valley from which it rises with an exposure to the south, and where the ground above is planted with trees. When it is borne in mind that in temperate climates, such as that of Great Britain, the majority of the deaths which occur in the winter months are occasioned or at least hastened by low temperatures, it will be recognized as of the most vital importance, especially to invalids, to know what are the local situations which afford the best protection against great cold. In truth, mere local situations may during periods of intense cold have the effect of maintaining a temperature many degrees above that which prevails close at hand a difference which must mitigate suffering and not unfrequently prolong life.
In addition to mere elevation and relative configuration of surface, the land of the globe brings about important modifications of climate in the degree in which its surface is covered with vegetation or is a desert waste. Of all surfaces that the earth presents to the influences of solar and terrestrial radiation an extent of sand is accompanied with the most extreme fluctuations of climate, as these are dependent on the temperature and moisture of the air ; whilst on the other hand, extensive forests tend to mitigate the extremes of temperature and distribute its daily changes more equably over the twenty-four hours.
ture of the air, attention is to bo given almost exclusively to the temperature of the extreme upper surface of the earth heated by the sun with which the air is in immediate contact. Badly conducting surfaces, such ^ as sand, will evidently have the greatest influence in raising the temperature of the air, for the simple reason that the heat produced by the sun s rays being conveyed downwards into the soil with extreme slowness must necessarily remain longer on the surface, in other words, remain in immediate contact with the atmosphere. Similarly at night, the cooling effects of terrestrial radiation being greatest on sandy surfaces, the climate of sandy deserts Ls characters; by nights of comparatively great cold. These daily alternations of heat and cold are still further intensified by the great dryness of -the air over extensive tracts of sand. In warm countries the surface temperature of sandy deserts often rises to 120, 140, or even to 200, and the shade temperature has been observed as high as 125. It is this hot air, loaded with particles of sand still notter, and driven onwards by furious whirlwinds, which forms the dreaded simoon of the desert ; and the irritating and enervating sirocco of the regions bordering the Mediterranean is to be traced to the same cause. It is in the deserts of Africa, Arabia, Persia, and the Punjab that the highest tempera ture on the globe occurs, the mean summer temperature of these regions rising to and exceeding 95. The extreme surface of loam and clay soils is not heated during day nor cooled during night in so high a degree as that of sandy soils, because, the former being better conductors, the heat or the cold is more quickly conveyed downward,
and therefore not allowed to accumulate on the surface.When the ground is covered with vegetation the whole of the sun s heat falls on the vegetable covering, and as none of it falls directly on the soil its temperature does not rise so high as that of land with no vegetable covering. The temperature of plants exposed to the sun does not rise so high as that of soil, because a portion of the sun s heat is lost in evaporation, and the heat cannot accumulate on the surface of the leaves as it does on the soil. Hence the essential difference between the climates of two countries, the one well covered with vegetation, the other not, lies in this, that the heat of the day is more equally distributed over the twenty-four hours in the former case, and there fore less intense during the warmest part of the day.
But the effect of vegetation on the distribution of the temperature during the day is most markedly shown in the cuse of forests. Trees, like other bodies, are heated and cojled by radiation, but owing to their slow conducting power the times of the daily maximum and minimum temperature do not occur till some hours after the same phases of the temperature of the air. Again, the effects of radiation are in the case of trees not chiefly confined to a surface stratum of air a very few feet in thickness, but as already remarked, are to a very large extent diffused through a stratum of air equalling, in thickness at least, the height of the trees. Hence the conserving influence of forests on climate, making the nights warmer and the days cooler, imparting, in short, to the climates of districts clad with trees something of the character of insular climates. Evaporation proceeds slowly from the damp soil usually found beneath trees, since it is more or less screened from the sun. Since, however, the air under the trees is little agitated or put in circulation by the wind, the vapour arising from the soil is mostly left to accumu late among the trees, and hence it is probable that forests diminish the evaporation, but increase the humidity, of climates within their influence. The humidity of forests is further increased by the circumstance that when rain falls less of it passes immediately along the surface into streams and rivers; a considerable portion is at once taken up by the leaves of the trees and percolates the soil, owing to its greater friability in woods, to the roots of the trees, whence it is drawn up to the leaves and there eva porated, thus adding to the humidity of the atmosphere.
Much has been done by Ur Marsh and others in elucidation of the influence on climate of forests and the denudation of trees, in so far as that can be done by the varying depths of lakes and rivers and other non- instrumental observations. Little comparatively has been done anywhere in the examination of the great practical question of the influence of forests on climate, by means of carefully devised and conducted observations made with thermometers, the evaporating dish, or the rain gauge. The most extensive inquiry on the subject yet set on foot has been for some years conducted in the forests of Bavaria under the direction of Professor Ebermeyer, and a like inquiry was begun in Germany in 1875, the more important results being that during the day, particu larly in the warm months, the temperature in the forest is considerably lower than outside in the open country, there being at the same time a slow but steady outflow of air from the forest ; and that during the night the tempera ture in the forest is higher, while there is an inflow of air from the open country into the forest. The mean annual temperature in the forest increases from the surface of the ground to the tops of the trees (where it is observed to approximate to what is observed in the open country), a result evidently due to the facility of descent to the surface of the cold air produced by terrestrial radiation, and to the obstruction offered by the trees to the solar influence at the surface. The mean annual temperature of the woodland soil from the surface to a depth of 4 feet is from 2 to 3 lower than that of the open country. A series of observations was begun at Carnwath, Lanarkshire, in Ih73, at two stations, one outside a wood, and the other inside the wood in a small grass plot of about 50 feet diameter clear of trees. From these valuable results have been obtained relative to the differences in the daily march of temperature and the different rates of humidity, the most important being the substantial agreement of the mean annual temperature of the two places. The estab lishment of a station, with underground thermometers, which it is proposed to erect under the shade of the trees close to the station in the cleared space, will furnish data which will not only throw new light on the questions raised in this inquiry, but also on the movements and viscosity of the air and solar and terrestrial radiation.
When the sun s rays fall on water they are not as in the case of land arrested at the surface, but penetrate to a considerable depth, which, judging from observations made by Sir Robert Christison on Loch Lomond, and from those made on board the " Challenger," is probably in clear water about 600 feet. Of all known substances water has the greatest specific heat, this being, as compared with that of the soil and rocks composing the earth s crust, in the proportion of about 4 to 1. Hence water is heated much more slowly by the sun s rays and cooled more slowly by nocturnal radiation than the land. It is owing to these two essential differences between land and water with respect to heat that climates come to be grouped into the three great classes of oceanic, insular, and continental climates.
The maximum densities of fresh and salt water, which are respectively 39"1 and 26 2 (when the sea-water is the average degree of saltness), mark an essential distinction between the effects of sheets of fresh and salt water on climate. The surface temperature of sea-water falls very slowly from 39 0l l to 28 4, its freezing point, because as it falls the temperature of the whole water through its depths must fall ; whilst from 39 l to 32 the surface temperature of fresh water falls rapidly because it is only the portion floating on the surface which requires to be cooled. If the bottom temperature of fresh water exceed 39 l the cooling takes place also very slowly, since in this case the water through all its depth must be cooled down to 39 l as well as that of the surface.
which is practically constant at all seasons, is not 47 8, the mean annual temperature of that part of Scotland, but 42", which happens to be the mean temperature of the cold .half of the year, or that half of the year when terrestrial radiation is the ruling element of the tempera ture. Thus, then, there is an immense volume of w T ater at the bottom of this lake at a constant temperature 5 8 below that of the mean annual temperature of the locality. From this follow two important consequences, viz. (1) during each winter no inconsiderable portion of the cold produced by terrestrial radiation is conveyed away from the surface to the depths of the lake, where it therefore no longer exercises any influence whatever on the atmosphere or on the climate of the district in lowering the tem perature ; and (2) this annual accession of cold at these depths is wholly counteracted by the internal heat of the earth. In corroboration of this view it may be pointed out that the water of the Rhone as it issues from Lake Geneva is 3 7 higher than that of the air at Geneva. Thus, the influence of lakes which do not freeze over is to mitigate in some degree the cold of winter over the district where they are situated. This is well illustrated ori a large scale by the winter temperature of the lake region of North America. The influence of the sea is exactly akin to that of lakes. Over the surface of the ground slanting to the sea-shore the cold currents generated by radiation flow down to the sea, and the surface-water being thereby cooled sinks to lower depths. In the same manner no inconsiderable portion of the cold produced by radiation in all latitudes over the surface of the ocean and land adjoining is conveyed from the surface to greater depths. The enormous extent to which this transference goes -on is evinced by the great physical fact disclosed to us in recent years by deep sea observations of temperature, viz., that the whole of the depths of the sea is filled with water at or closely approaching to the freezing point of fresh water, which in the tropical regions is from 40 to 50 lower than the temperature of the surface. The with drawal from the earth s surface in high latitudes of such an enormous accumulation of ice-cold water to the depths of the sea of tropical and subtropical regions, rendered possible by the present disposition of land and water over .the globe, doubtless results in an amelioration to some extent of the climate of the whole globe, so far as that may be brought about by a higher surface temperature in
polar and temperate regions.Oceanic climates are the most equable of all climates, showing for the same latitudes the least differences between the mean temperatures of the different hours of the day and the different months of the year, and being at all times the least subject to violent changes of temperature. So far as man is concerned, oceanic climates are only to be met with on board ship. The hygienic value of these climates in the treatment of certain classes of chest and other complaints is very great, and doubtless when better understood in their curative effects they will be more largely taken advantage of. It is, for instance, believed by many well qualified to form an opinion that they afford absolute, or all but absolute, immunity from colds, which are so often the precursors of serious complicated dis orders.
The nearest approach to such climates on land is on very small islands such as Monach, which is situated about seven miles to westward of the Hebrides, in the full sweep of the westerly winds of the Atlantic which there prevail. The mean January temperature of this island, which is nearly in the latitude of Inverness, is 43 - 4, being 1 8 higher than the mean of January at Ventnor, Isle of Wight, 8 higher than that of Jersey and Guernsey, and almost as high as that of Truro. Again, Stornoway, being situated on the east coast of Lewis on the Minch, an inland arm of the Atlantic, has thus a less truly insular position than Monach. Its climate is therefore much less insular, and accordingly its mean temperature in January is 38 7, or 4-7 lower than that of Monach. From its position near the Moray Firth, on the east of Scotland, Culloden occupies a position still less insular ; hence its January temperature is only 37 l, being l-6 less than that of Stornoway, and 6 - 3 less than that of Monach. On the other hand, the mean temperature of July is 55-0 at Monach, 57 8 at Cullodeu, GT O at Guernsey, and G2 G at Ventnor. Thus the conditions of temperature at these stations are completely reversed in summer, for while in January Monach is l - 8 wanner than Ventuor, in summer it is 7 G colder. Since the prevailing winds in the British Isles are westerly, places on the east coast are less truly insular than are places similarly situated on the west, whence it follows that the winter and summer climates of the east coast approach more nearly the character of inland climates than do those of the west.
The facts of the temperature at such places as Monach in Scotland and Valentia in Ireland disclose the existence of an all but purely oceanic climate along the coasts, particular^ of the west, so distinct and decided, and extending inland so short a distance, that it would be impossible to represent it on any map of land isothermals of ordinary size. The only way in which it can be graphically represented is by drawing on the same map the isothermals of the sea for the same months, as Petermann has done on his chart of the North Atlantic and continents adjoining. Such maps best lead to a knowledge of the true character of our seaside climates.
Though it is Impossible to overestimate the climatological importance of seaside climates, as evinced by their curative effects on man, and their extraordinary influence on the distribution of animal and vegetable life, it must be con fessed that we are yet only on the threshold of a rational inquiry into their true character. Undoubtedly the first step in this large inquiry is the establishing of a string of about six stations at various distances from a point close to high-water mark to about two miles inland, at which observations at different hours of the day would be made, particularly at 9 A.M. and 3 and 9 P.M., of the pressure, tem perature, humidity, movements, and chemistry of the air.
Our large towns have climates of a peculiar character, which may be said to consist chiefly in certain disturbances in the diurnal and seasonal distribution of the temperature, an excess of carbonic acid, a deficiency of ozone, and the presence of noxious impurities. Systematic inquiries into the condition and composition of the air of our large towns have been instituted this year (1876) in Paris and Glasgow, in which the ozone, ammonia, nitric acid, and germs present in different districts of these cities are regularly observed. There yet remain to be devised some means of making truly comparable thermometric and hygrometric observa tions in different localities, including the more densely- peopled districts, for the investigation of what we may call the artificial climates peculiar to each district. While such an inquiry, at least in its earlier stages, must necessarily be regarded as a purely scientific one, it may fairly be expected to lead sooner or later to a knowledge of the causes which determine the course of many epidemics- why, for instance, diphtheria is more frequent and more fatal in the new than in the old town of Edinburgh, and why in some parts of Leicester diarrhoea is unknown as a fatal disease, while in other parts of the same town it rages every summer as a terrible pestilence among infants and ultimately suggest the means by which they may be stamped out when they make their appearance.
prevailing winds are the simple result of the relative distri bution of atmospheric pressure, their direction and : being the flow of the air from a region of higher towards a region of lower pressure, or from where there is a surplus where there is a deficiency of air. Since climate is pract! cally determined by the temperature and moisture of t wr, and since these are dependent on the prevailing winds wmci come charged with the temperature and moisture of the regions they have traversed, it is evident that isobaric charts, showing the mean pressure of the atmosphere, form the key to the climates of the different regions of the globe, particularly those different climates which are found to prevail in different regions having practically the same latitude and elevation. This principle is all the more important when it is considered that the prevailing winds determine in a very great degree the currents of the ocean,
which exercise so powerful an influence on climate.Since winds bring with them the. temperature of the regions they have traversed, southerly currents of air are warm winds, and northerly currents cold winds. Also since the temperature of the ocean is more uniform than that of the land, winds coming from the ocean do not cause such variations of temperature as winds from a continent. As air loaded with vapour obstructs both solar and terrestrial radiation, when clear as well as when clouded, moist ocean winds are accompanied by a mild temperature in wintsr and a cool temperatura in summer, and dry winds coming from continents by cold winters and hot summers. Lastly, equatorial currents of air, losing heat as they proceed in their course, are thereby brought nearer the point of satura tion, and consequently become moister winds ; whereas northerly currents acquiring greater heat in their progress become drier winds.
It follows from these relations of the wind to temperature and moisture that the S.W. wind in the British Isles is a very moist wind, being both an oceanic and equatorial current ; whereas the N.E. wind, on the other hand, is peculiarly dry and parching, because it is both a northerly and continental current. Owing to the circumstance of atmospheric pressure diminishing from the south of Europe northwards to Iceland, it follows that S.W. winds are the most prevalent in Great Britain ; and since this diminution of pressure reaches its maximum amount and persistency during the winter months, S.W. winds are in the greatest preponderance at this season ; hence the abnormally high winter temperature of these islands above what is due to mere latitude. The msan winter temperature of Lerwick, Shetland, in respect of latitude alone would be 3, and of London 17, bat owing to the heat conveyed from the warm waters of the Atlantic across these islands by the winds, the temperature of Shetland is 39" and of London 38. In Iceland and Norway the abnormal increase of temperature in winter is still greater. This influence of the Atlantic through the agency of the winds is so pre ponderating that the winter isothermals of Great Britain lie north and south, instaad of the normal east and west direction.
This peculiar distribution of the winter temperature of the British Isles has important bearings on the treatment of diseases. Sinca the temperature of tha whole of jthe eastern slope of Great Britain is the same, it is clear that to those for whom a milder winter climate is required a journey southward is attended with no practical advantage, unless directed to the west coast. As the temperature on the west is uniform from Shetland to Wales, Scotland is as favourable to weak constitutions during winter as any part of England, except the south-west, the highest winter temperatures being found from the Isle of Wight westward round the Cornish peninsula to the Bristol Channel ; and from Carnsore Point in Ireland to Galway Bay the tempera ture is also high.
The height and direction of mountain ranges form an important factor in determining the climatic characteristics of prevailing winds. If the ranga be perpendicular to the winds, tha effect is to drain the winds which cross them of their moisture, thus rendering the winters colder and tha summers hotter at all places to leeward, as compared with places to windward, by partially removing the protecting screen of vapour and thus exposing them more effectually to solar and terrestrial radiation. To this cause much of the observed difference between the west and east climates of Great Britain is due. In Ireland, on the other hand, where the mountains are not grouped in ranges running north and south, but in isolated masses, the difference between the climates of the east and west is very much less. In the east of the United States the prevailing winds in summer are S.W., and as the Alleghanies lie in the same direction the temperature is little affected by these mountains, and the rainfall is pretty evenly dis tributed on both sides of the range.
In its climatological relations the distribution of rain over the globe presents us with a body of facts which lead, when intelligently interpreted, to a knowledge of the laws regulating the distribution of plants more quickly and cartainly than do the facts of temperature. It is to the prevailing winds we must look for an explanation of the rainfall, the broad principles of the connection being these: 1, The rainfall is moderately large when the wind has traversed a considerable extent of ocean ; 2, if the v.-inds advance into colder regions the rainfall is largely increased, and if a range of mountains lie across their path the amount precipitated on the side facing the winds is greatly augmented, but diminished over regions on the othej side of the range ; 3, if the winds, though coming from the ocean, have not traversed a considerable extent of it, the rainfall is not large ; and 4, if the winds, even though having traversed a considerable part of the ocean, yet on arriving on the land proceed into lower latitudes, or regions markedly warmer, the rainfall is small or nil. It is this last consideration which accounts for the rainless character of the summer climates of California, of Southern Europe, and of Northern Africa.
The region extending from Alaska to Lower California presents more sudden transitions of climate, and climates more sharply contrasted with each other, than any other portion of the globe, this arising from the contour of its sur face and the prevailing winds. A direct contrast to this is offered by the United States to the east of the Mississippi, a region characterized by a remarkable uniformity in the distribution of its rainfall in all seasons, which, taken in connection with its temparature, affords climatic conditions admirably adapted for a vigorous growth of trees and for the great staple products of agriculture. India and the region of tha Caspian Saa and the Caucasus Mountains also present extraordinary contrasts of climate in all seasons, due to the prevailing winds, upper as well as lower winds, tha relative distribution of land and water, and the physical configuration of the surface of the land.
has been the average climate of localities and regions. There are, however, it need scarcely be added, vital elements of climate of which such a discussion can take no cognizance. These are the deviations which occur from the seasonal averages of climate, such as periods of extreme cold and heat, or of extreme humidity and dryness of air, liability to storms of wind, thunderstorms, fogs, and extraordinary downfalls of rain, hail, or snow. An illustration will show tha climatic difference here insisted on. The mean wintar temperature of the Southern States of America is almost the same as that of Lower Egypt. Lower Egypt is singularly free from violent alternations of temperature as well as frost, whereas these are marked features of the winter climate of the States bordering on the Gulf of Mexico. Robert Russell, in his Climate cf America, gives an instance of the temperature falling in Southern Taxas with a norther from 81 to 18 in 41 hours, the norther blowing at the same time with great violence. A temperature of 18° accompanying a violent wind may be regarded as unknown in Great Britain.
It is to the cyclone and anticyclone (see Atmosphere) we must look for an explanation of these violent weather changes. Climatically, the significance of the anticyclone or area of high pressure consists in the space covered for the time by it being on account of its dryness and clearness more fully under the influence of solar and terrestrial radiation, and consequently exposed to great cold in winter and great heat in summer; and of the cyclone or area of low pressure, in a moist warm atmosphere occupying its front and southern half, and a cold dry atmosphere its rear and northern half.
The low areas of the American cyclones, as they proceed eastward along the north shores of the Gulf of Mexico, are often immediately followed to west and north-westward by areas of very high pressure, the necessary consequence of which is the setting in of a violent norther over the Southern States. Since similar barometric conditions do not occur in the region of Lower Egypt, its climate is free from these sudden changes which are so injurious to the health even of the robust. Since many of the centres of the cyclones of North America follow the track of the lakes and advance on the Atlantic by the New England States and Newfoundland, these States and a large portion of Canada frequently experience cold raw easterly and northerly winds. The great majority of European storms travel eastward with their centres to northward of Farö, and hence the general mildness of the winter climate of the British Isles. When it happens, however, that cyclonic centres pass eastwards along the English Channel or through Belgium and North Germany, while high pressure prevails in the north, the winter is characterized by frosts and snows. The worst summer weather in Great Britain is when low pressures prevail over the North Sea, and the hottest and most brilliant weather when anticyclones lie over Great Britain and extend away to south and eastward.
Low pressures in the Mediterranean, along with high pressures to northward, are the conditions of the worst winter weather in the south of Europe. A cyclone in the Gulf of Lyons or of Genoa, and an anticyclone over Germany and Russia, have the mistral as their unfailing attendant, blowing with terrible force and dryness on the Mediterranean coasts of Spain, France, and North Italy, being alike in its origin and in its climatic qualities the exact counterpart of the norther of the Gulf of Mexico. It follows from the courses taken by the cyclones of the Mediterranean, and the anticyclones which attend on them, that also Algeria, Malta, and Greece are liable to violent alternations of temperature during the cold months.
The investigation of this phase of climate, which can only be carried out by the examination of many thousands of daily weather charts, is as important as it is difficult, since till it be done the advantages and hazards offered by different sanataria cannot be compared and valued. It may in the meantime be enough to say that no place anywhere in Europe or even in Algeria offers an immunity from the risks arising from the occurrence of cold weather in winter at all comparable to that afforded by the climates of Egypt and Madeira. See Atmosphere, Meteorology, and Physical Geography.(a. b.)