Page images
PDF
EPUB
[blocks in formation]

THET

THE term climate is used to express the combination of temperature and moisture which prevails in any particular region. The most prominent causes of diversity of climate

are

-the heat of the sun, the relative position of land and water, and the elevation of land above the level of the sea. To these may be added, as producing considerable, though less marked effects, the nature of the soil, the prevailing winds, the position of mountain ranges, and the currents of the ocean.

The sun is the grand agent in diffusing heat over the surface of the globe; the temperature of any part, mainly, if not wholly, depending on its exposure to his rays; this being, however, modified by circumstances, the power of the sun's rays being great in proportion to the vertical direction in which they strike the earth. Whenever the sun is above the horizon of any place, that place is receiving heat; when below, parting with it, by the process called radiation. Whenever, therefore, the sun remains more than twelve hours above the horizon of any place, and consequently less than twelve hours beneath, the general temperature of that place will be above average; and when the reverse occurs, it will be below average. As the earth revolves on its axis, successive portions of its surface are heated by the solar rays; and, owing to "the simple yet stupendous contrivance of the inclination of the axis of the earth to the plane of its orbit," the northern and southern hemispheres are brought alternately

more directly under the solar influence; and not only do we thus enjoy the varieties of day and night, and the grateful change of the seasons, but by this means the heat of the intertropical zone is moderated, and the limits of the temperate regions greatly extended.

The division of the earth into five zones, the torrid, the two temperate, and the two frigid, may convey a general idea of the temperature of these respective portions of the earth's surface; and, in fact, did the temperature of any place depend solely on the heat of the sun, this arrangement, or any other, dividing the earth's surface into parallel bands, might present a correct indication of the temperature of each zone. The power of the sun's rays is, however, so modified by other circumstances, that great varieties of temperature are observed to prevail in countries situated in the same parallels or degrees of latitude. Thus, by some observations recently made at New Archangel, on Sitka Island, situated immediately off the western coast of North America, in lat. 57°, it appears that the mean annual temperature at that place is nineteen degrees higher than at Nain, on the coast of Labrador, in the same parallel of latitude. The difference between the winter temperature of the two stations is yet greater, amounting to no less than thirty-six degrees. At Sitka, the snow never lies long on the ground, but usually disappears as soon as it falls. In Labrador, on the other hand, there are hardly two months of the year perfectly free from ice and snow; and the latter accumulates on the plains near the sea to the depth of twenty or twenty-four feet. In the interior of the continent, at Fort Chippewyan, in the same parallel, the summers are much hotter than at Sitka, and the winters more severe even than at Labrador, the thermometer sometimes falling to 20° or 30° below zero. Nor is this remarkable difference of climate confined to America; it is also observable in the Old World, of which the respective climates of Kamtschatka and Great Britain may be mentioned as instances.

This variation of climate appears, in great measure, to be attributable to the different arrangement of land and water, which has been already mentioned as one of the principai causes of diversity of climate.

The waters of that great "climate agent," the ocean, are of very equal temperature, and have, therefore, a tendency to preserve a similar mean, or equal temperature, wherever their influence extends, moderating alike both heat and cold. Thus, when a cold wind passes over the sea, it becomes warmed, and the cold of winter will be moderated. A hot wind, on the contrary, becomes cooled in passing over the sea, and the summer temperature of the adjacent regions will be lowered. And hence we find that the climate of islands, and of countries bordering on the sea, differs considerably from that of the interior of continents, the former being usually characterized by milder winters and more temperate summers. Such countries are said to possess an insular climate. When, on the other hand, any region experiences great severity of cold in winter, and at the same time, a considerable degree of heat in summer, it is said to possess an excessive climate. Thus, Sitka has an insular climate, the summer temperature being low, and the winter moderate; but Fort Chippewyan presents an instance of an excessive climate. Labrador, from its littoral or coast situation, might seem entitled to an insular climate, but the difference between its climate and that of Sitka, is supposed by M. Baer to arise from the configuration of the land; Sitka being completely screened from the influence of the circumpolar regions by the peninsula of Alyaska and the Aleutian Isles, which in general form a range of high land, protecting it from the north and east, and preventing the ice of the arctic regions from drifting to its shores; whilst, at the same time, it is open to the influence of the warm currents flowing from the equatorial regions of the Pacific. Labrador, on the contrary, has no intervening land to protect it from the north, or from the icebergs which are formed in Baffin's Bay, and from whence a current sets southward, bearing these icy masses past its shores. A similar cause is also assigned, by M. Baer, for the temperate climate of the western shores of the Old World; that being preserved from extreme cold by the protection afforded by Nova Zembla, and a submarine ridge extending from thence to Spitzbergen, which forms a barrier to the ice accumulated on, and borne down by the great rivers of Siberia.

The effect produced on climate by the relative position of land and water, is exhibited on a large scale in the difference of climate observable in the northern and southern hemispheres. The space occupied by the waters of the ocean is much greater in the southern than in the northern hemisphere, the consequence of which is, that a much more equable temperature prevails in the former than in the latter. Thus, in Tasmania, or Van Diemen's Land, which is situated about the same distance south of the equator, as Rome is to the north of that line, the winters are milder than at Naples, and the summers not warmer than at Paris.

It is, however, a remarkable fact, that in high latitudes in the southern hemisphere, the cold is much greater than in similar parallels in the northern hemisphere. Thus, in the island of Georgia, which is in the same latitude as Yorkshire, the ground is covered with snow at all seasons of the year. This greater severity of climate has been supposed to arise, partly from the absence of any large surface of land in the temperate region of the southern hemisphere, capable of reflecting the heat of the sun, and partly from the occurrence of some yet undiscovered elevated land within the south polar circle; to which may be added the want of any protecting ridge to screen it from the cold of the latter regions, and the floating icebergs, which are borne from thence by the currents of the ocean.

The elevation of land above the level of the sea is another leading cause of the diversity of climate. We have already seen that the air becomes rarefied, or less dense, as we ascend above the level of the sea; we also find a gradual decrease of temperature, in proportion to the elevation of land above that level, (this decrease being about one degree of Fahrenheit for every 656 feet,) until we attain a point where perpetual congelation takes place. This point, which is called the snowline, or line of perpetual snow, varies in different latitudes, and even in similar latitudes under different conditions; but, generally speaking, a gradual decrease in the elevation of the snow line above the sea level takes place, as we approach the frozen regions of the polar circles. The limit of the snowline within the tropics is about 16,000 or 17,000 feet above

the level of the sea; and in the northern hemisphere this line descends to the level of the sea, at about latitude 80°. In the southern hemisphere, as we have just seen, the shores of the island of Georgia, situated in latitude 56°, are bound in perpetual frost; it will therefore be evident that the same rules will not apply to both hemispheres; little, however, is yet known on this subject relative to the southern hemisphere ; but the following table will convey a general idea of the snow-line in the northern hemisphere, though, owing to local variations, this can only be regarded as an approximation to the truth:

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

In considering the above table, we shall perhaps feel some surprise, at finding the snow-line higher at the distance of twenty degrees from the equator, than at the equator itself. This, however, is readily explained by the circumstance that, at the equator, the sun is never more than twelve hours above the horizon; whereas, near the tropics, the longest days are thirteen hours and a-half in length; and, as the sun at that period is vertical or nearly so, the summer heat, on which the line of perpetual snow depends, is greater than directly under the equator.

The gradual decrease of temperature at increasing elevations above the level of the sea, has a marked and very beneficial effect, in diversifying the climates over the surface of the globe, and in fitting the earth for the production and maintenance of an almost endless variety of plants and animals. All countries situated between the tropics are subject to an

« PreviousContinue »