969 



CLIMATE. 



CLIMATE. 



970 



the mountains of Europe, combined with those made by Humboldt in 

 South America, suggested the idea that the snow-line forms a regular 



curve (north and south) on the surface of the earth, in the following 

 way: 



Height in Feet 



Surface of the Earth. 



10 



20 



But more recent observation has shown that we are still very 

 imperfectly acquainted with the laws by which this important 

 boundary-h'ne in the atmosphere is determined. Mr. Pentland ascer- 

 tained, by a great number of observations, that in the Andes of South 

 America, between 14 and 17 S. lat., the snow-liiie does not occur on 

 mountains below the height of 17,000 feet. Posppig, in traversing the 

 Andes, near 11 S. lat., found that here also they were free from snow 



at an elevation several hundred feet higher than the snow-line under 

 the equator. Agreeably to these facts, and to a generalisation of Von 

 Buch which includes them, that the temperature of the summer 

 months determines the elevation of perennial snow, the snow-line 

 would thus be represented, not by a curve of continuous curvature, 

 but by one of this form : 



Height in Feet '-? 



Surface of the Earth. 



These observations on the elevation of the snow-line above the level 

 of the sea, are however only applicable to mountains which rise with a 

 steep ascent, and are not contiguous to table-lands. High table-lands 

 have a higher temperature than isolated mountains of the same height. 

 Humboldt observed that the elevated plains on which the towns of 

 Bogota, Popayan, Quito, and Mexico are built, have a much warmer 

 climate than they would have if elevation above the sea were the only 

 element that determined the temperature when the latitude is given. 

 In comparing the mean temperature of these plains with that of the 

 adjacent coast, he found that the thermometer, instead of descending 

 one degree for every 343 feet, had only sunk so much for every 400 

 feet of perpendicular elevation. He thinks that this difference may be 

 accounted for by supposing that the temperature of the atmosphere, 

 under these circumstances, is considerably raised by the reflection 

 and radiation of the sun's rays from a plain of considerable extent. 

 This supposition is confirmed by the circumstance of the different 

 heights at which the snow-line occurs on the southern and northern 

 declivities of the Himalaya Mountains. On the southern declivity of 

 that range, which rises rapidly from the low plains of the Ganges, 

 Mr. Webb found the snow-line at an elevation of about 13,000 feet, 

 corresponding pretty well with its latitude (about 30) ; but on the 

 northern declivity, which is about a degree farther north, the snow-line 

 ascends to 16,000 feet, an elevation which corresponds to that found 

 under the equator. On this side, however, the range, though very 

 steep, does not descend to a low country, but terminates in an immense 

 plain, the surface of which is about 10,000 feet above the level of the 

 sea. The great difference between the elevation at which the snow- 

 line occurs on the two declivities is thus partly to be explained by the 

 difference in the height of the plains which are respectively contiguous 

 to them ; but in part, also, by the excessive dryness of the climate of 

 Tibet, on the northern side of the range. 



Captain Gerard gives from 18,000 to 19,000 feet as the altitude of 

 the level of perpetual snow on the mountains of the Tibetan table- 

 land north of the upper valleys of the Indus and Sutlej. From the 

 more recent observations of Dr. Hooker, given in his ' Himalayan 

 Journals,' it appears that on the Donkia Mountain (23,176 feet high), 

 hi Sikkim, 28 N. lat. very nearly, the elevation of the snow-level is 

 not less than 19,000 feet ; and he is of opinion, that 40 miles further 

 north (in Tibet), its altitude is probably 20,000 feet. 



This subject will be resumed in the article SNOW, PERENNIAL. Its 

 relations to that of climate in general have been thus summarily stated 

 by Prof. Forbes, in his work on Norway, pp. 205-206 : " The occur- 

 rence of perpetual snow at a certain height above the sea, in even the 

 warmest regions in the globe, has in all ages excited the curiosity of 

 geographers and naturalists. Regarded at first as a very simple indi- 

 cation of the depression of temperature as we ascend in the atmosphere, 

 it has been carefully studied and applied (often erroneously) to the 

 ' determination of clinlate. Closer examination has shown that the 

 presence of perennial snow in other words, a predominance of all the 

 causes tending to its accumulation over those which tend to its waste 

 or fusion is, indeed, a very complicated fact, and cannot be taken as 

 the simple expression of any one of the elements of climate. The 

 snow-line is far from having invariably a mean temperature of 32, as 

 was at one time supposed. Under the equator it is about 35 [in the 

 Sikkim Himalaya probably 20, according to Dr. Hooker] ; in the Alps 

 and Pyrenees about 25 ; and in latitude 68, in Norway, it is (according 

 to Von Buch) only 21. .... The mean temperature of the snow-line 

 diminishes towards the Pole, because, for a given mean temperature 

 of the whole year, the summer is far hotter in proportion. Also, places 

 at" which the temperature of the summer is low, are those which have 

 a moderated or coast climate ; but there also the fall of rain and snow 

 is most abundant, whilst in excessive or continental climates the 

 precipitations are comparatively small." 



Temperature is still affected by such circumstances and phenomena 

 as the nature of the soil, the prevailing winds, the quantity of moisture, 

 the electrical state of the atmosphere, and the physical character of the 

 adjacent countries and seas. 



Where the soil has been to a great extent cleared and brought into 

 cultivation, the air is much drier and warmer in summer than in those 

 tracts which, for want of cultivation, remain covered with swamps and 

 marshy grounds. This, according to Darby, is the case in the culti- 

 vated parts of the United States, in which he asserts, contrary to the 

 vulgar notion, that in the cleared and cultivated tracts the summer 

 temperature has been raised, and that of winter diminished. The 

 latter circumstance would seem a natural consequence of clearing the 

 surface and exposing it during the winter months to the full influence 

 of the north-west winds. In fact, as the country becomes more open, 

 the range of the thermometer increases. As far as we know, no 

 attempts have been made to ascertain to what amount such differences 

 in the soil affect the temperature of a country, though our knowledge 

 seems to be quite sufficient to assign the reasons for the existence of 

 such a difference. 



The effect of the winds on the temperature of a place is still more 

 obvious. It is a common observation, that the thermometer is more or 

 less raised or depressed by every change of the wind. But there is a 

 great difference in this respect between the lower and higher altitudes. 

 In the former, a change of wind rarely raises or depresses the thermo- 

 meter more than a few degrees ; while in the higher latitudes it 

 frequently happens that in a few hours a change of ten or twelve 

 degrees, and even more, takes place. Captain Scoresby mentions an 

 instance of this near the polar ice. On a sudden veering of the wind 

 to the north, the thermometer fell, in sixteen hours, 34 degrees, from 

 + 32 to 2. But changes as great as this, so far as such changes 

 are measured merely by degrees, occur in the United States of North 

 America, at some distance from the Atlantic. Again, even in the lower 

 latitudes, the change of the wind has a greater effect on the tempe- 

 rature on high table-lands than on low plains. Mr. Dunn states, that 

 on the table-land of Guatemala, it sometimes, though very rarely, hap- 

 pens, that during the period of the northern winds a cold current 

 produces a difference of 20 degrees in a few hours. A similar pheno- 

 menon has never been observed on the low coasts between the 

 tropics. 



It is a very common observation, that both cold and heat are more 

 intense when the sky is clear than when it is overcast with clouds. 

 Hence it may be inferred that countries, whose atmosphere is more 

 loaded with clouds and vapours than that of others in the same parallel, 

 must have warmer winters and colder summers. This difference is 

 observed all over the world, and from it partly arises the difference of 

 climate in maritime and continental countries. It is even observed in 

 countries which are at no great distance from one another, as in 

 England and Holland. In England, the mean temperature of the sum- 

 mer and winter is said not to differ more than about 22 degrees of 

 Fahrenheit, while on the opposite shores of Holland it amounts to 

 27 degrees, the winters there being a little colder and the summers 

 somewhat warmer than with us. On this, however, we must observe 

 that there is no very great propriety in comparing the mean tempe- 

 rature of a country of the fonn and position of England with one of 

 the form and position of Holland. If two points under the same 

 latitude, and both near the sea, were taken on the opposite coasts of 

 England and Holland, the difference, whatever it might be, would 

 obviously be owing to' the difference in the tracts of country con- 

 tiguous to the respective places, to the winds, and to the hygrometrical 

 state of the atmosphere. Before any safe conclusion could be drawn, 

 all these conditions should be known. There appears no doubt, how- 

 ever, that the difference in the mean temperature of the two seasons 

 increases as we proceed farther east iii the European continent ; and it 



