Equatorial climate

“ This surprising fact seems most improbable to those who are not acquainted with the Protectorate, but it must be remembered that temperature and climate depend on elevation as much as on geographical position. The best-known equatorial countries—such as West Africa, Brazil, and the Malay Archipelago—are comparatively low and flat. It is only rarely, as in Bolivia and East Africa, that we find the much rarer combination of equatorial position and considerable altitude. ”

“ Land elevation, cooling crust and cooling oceans, sea restriction and consequent deepening, together with a great increase of land in northern latitudes, all conspired greatly to change the world's climate in all regions far removed from the equatorial zone. ”

“ It is immensely cooler, therefore, in the interior than at the coast; and the equatorial zone all over the world possesses a climate in every way superior to that of the borders of the temperate region. At night, in Equatorial Africa, it is really cold, and one seldom lies down in his tent with less than a couple of blankets and a warm quilt. The heat of New York is often greater than that of Central Africa ”

“ He has himself shown that the presence of large quantities of ice prevents "a warm and equable climate" however great may be the sun-heat; the ice therefore would not be melted, and there would be no increased flow of intertropical water to the Southern Ocean. The ocean currents are mainly due to the difference of temperature of the polar and equatorial areas combined with the peculiar form and position of the continents, and some one or more of these factors must be altered before the ocean currents towards the north pole can be increased. ”

“ Excepting some portions of the equatorial zone, where the seasons recur with tolerable constancy, our notions of the climate of a place are got at by taking an average among a great many successive seasons. It is true that our own islands afford an extreme instance of variability ”

“ Here alone, in the Oriental region, are found the most typical equatorial forms of life—organisms adapted to a climate characterised by uniform but not excessive heat, abundant moisture, and no marked departure from the average meteorological state, throughout the year. ”

“ Young supposes that an equable climate prevailed all over the globe in the antediluvian period. "Were the highest mountains transferred to the equatorial regions, the most extensive oceans removed towards the poles, and fringed with a border of archipelago,—while lands of moderate height occupied most of the intermediate spaces, between these archipelagos and the equatorial mountains; then a temperature, almost uniform, would prevail throughout the world." This "perpetual summer" would account for the prodigious quantities of animal and vegetable remains:—every region teemed with life. ”

“ Few days elapse without the occurrence of showers, which thus produce the most agreeable effect in reducing the temperature and cherishing vegetation. Without the continued influence of moisture, these regions would certainly exhibit a far less cheerful picture, and the climate prove much less congenial to the human frame. Heat in the equatorial regions is thus benignly attempered to the constitution of man. It will be found to prove infinitely less pernicious to his system [48] than it does some distance beyond the tropics, particularly in dry and arid climates. ”

“ As no such tropical and equatorial animals or plants have been known to exist there, nor even herbage of any kind, on which such animals might subsist, since the memory of man, there was a time anciently when the climate suited their growth, and of course was very much warmer than it is known to be now, its temperature then corresponding to the temperature of the present equatorial regions, as it produced and subsisted anciently the animals and plants which the tropical regions produce and subsist at the present time, and which cannot subsist in any other climates. ”

“ But the large masses of land, of uneven surface, which occur between the tropics, and the consequent inequalities of temperature, check the tendency of the intertropical atmosphere to a regular course, introduce derangement in its movements, so that it is only in the great open seas that the trade-winds are experienced. Still, it has been observed that, in some countries under and near the equator, constant easterly winds are found, which are no doubt identical in their cause with those that distinguish the equatorial regions of the ocean. ”

“ Influence of Winds on the Temperature of the Equator.—The distance from the northern to the southern tropics being considerably more than three thousand miles, and the area of the intertropical zone more than one-third the whole area of the globe, it becomes hardly possible for any currents of air to reach the equatorial belt without being previously warmed by contact with the earth or ocean, or by mixture with the heated surface-air which is found in all intertropical and subtropical lands. ”

“ Without going into meteorological details, some of which have been given in the preceding chapter, the main facts may be explained by the mode in which the great aerial currents are distributed. The trade winds passing over the ocean from north-east to south-west with an oblique tendency towards the equator, become saturated with vapour, and are ready to give out moisture whenever they are forced upwards or in any other way have their temperature lowered. The entire equatorial zone becomes thus charged with vapour-laden air which is the primary necessity of a luxuriant vegetation. ”

“ Therefore it is fair to suppose that much of the vapor which is taken up on that side of the equator is precipitated on this. The evaporating surface in the southern hemisphere is greater, much greater, than it is in the northern; still, all the great rivers are in the northern hemisphere, the Amazon being regarded as common to both; and this fact, as far as it goes, tends to corroborate the suggestion as to the crossing of the trade-winds at the equatorial calms. ”

“ Now study the configuration of the Southern American Continent from this cape to the Windward Islands of the West Indies, and take into account also certain physical conditions of these regions: the Amazon, always at a high temperature because it runs from west to east, is pouring an immense volume of warm water into this part of the ocean. As this water and the heat of the sun raise the temperature of the ocean along the equatorial sea-front of this coast, there is no escape for the liquid element, as it grows warmer and lighter, except to the north. ”

“ Therefore, the tropical Atlantic waters must have been reduced to a lower temperature during a frigid age than the tropical waters of the Indian Ocean or the western part of the tropical Pacific, as a large portion of the great equatorial current of the latter oceans, during its western movement, was exposed to the rays of a tropical sun for a much longer time, after being replenished by the cold waters of the high latitudes, than the tropical currents of the Atlantic ”

“ If, then, in the latitude of Melville Island, 75° N., a high temperature, and consequent humidity, prevailed at that period when we know the arctic seas were filled with corals and large multilocular shells, how could plants of tropical forms have flourished? Is not the bright light of equatorial regions as indispensable a condition of their well-being as the sultry heat of the same countries? ”

“ We readily perceive that the effect of the oceanic circulation upon the temperatures of different regions is not only great but widely contrasted. By taking from the equatorial belt a large part of the heat which falls within that realm, it lowers the temperature to the point which makes the district fit for the occupancy of man, perhaps, indeed, tenable to all the higher forms of life. This same heat removed to high latitudes tempers the winter's cold, and thus makes a vast realm inhabitable which otherwise would be locked in almost enduring frosts. ”

“ As the sun advances northward in the spring, the southeast trades creep up toward and across the equator, the volume of that portion of the equatorial current which is diverted to the northward of Cape San Roque is gradually increased, and this increase is soon felt far to the westward, in the Yucatan and Florida straits. Figures fail utterly to give even an approximate idea of the amount of heat thus conveyed from the tropics to the north temperate zone by the ceaseless pulsations of this mighty engine of oceanic circulation. ”

“ From this equatorial zone, the heart of the globe, come forth two rivers of tepid water that heat the coasts of the north. They are the two currents that issue from the Gulf of Mexico and the Java Sea. Their enormous liquid masses, fleeing ceaselessly from the equator, govern a vast assemblage of water from the poles that comes to occupy their space, and these chilled and fresher currents are constantly precipitating themselves on the electric hearth of the equator that warms and salts them anew, renewing with its systole and diastole the life of the world. ”

“ The adjacent air, both on the north and south, being cooler, and, of course, heavier, would rush in to supply the place of the heated air. This air coming from the regions beyond the tropics would, in its turn, be heated, and rise on reaching the warmer equatorial regions, giving place to a fresh supply, which, it is easy to see, must be furnished by the descent of that portion of air formerly heated at the equator, raised into the cold regions of the sky, and forced into a regular circuit by fresh elevations of heated air. ”

“ When the north-east and south-east trades meet and produce the equatorial calms (§ 212) , the air, by the time it reaches this calm belt, is heavily laden with moisture, for in each hemisphere it has travelled obliquely over a large space of the ocean. It has no room for escape but in the upward direction (§ 223) . It expands as it ascends, and becomes cooler; a portion of its vapour is thus condensed, and comes down in the shape of rain. Therefore it is that, under these calms, we have a region of constant precipitation. ”

“ But beyond 45° S. observations are wanting, and we are left to reason and conjecture. That the southern hemisphere should, till within a certain distance of the pole, be warmer in winter and cooler in summer, may be explained by the fact that the southern hemisphere has more water; that water being more equable than land in its temperature, produces more equable climates; that the vapour which is taken up from trans-equatorial seas and condensed into rains for cis-equatorial rivers conveys with it a vast amount of heat which the southern hemisphere receives from the sun. ”

“ Land breezes from the west coast of Africa scorching hot.—It is easier to obstruct and turn back the current in a sluggish than in a rapid stream. So, also, in turning a current of air first upon the land, then upon the sea—very slight alternations of temperature would suffice for this on those coasts where calms would prevail were it not for the land and sea breezes, as for instance, in and about the region of equatorial calms; there the air is in a state of rest, and will obey the slightest call in any direction; not so in regions where the trades blow over the land, and are strong. ”

“ Blowing with regular force on the surface of the sea, they cause it to move in the same direction in which they are travelling; and, this motion once acquired, the ocean stream keeps up its course far beyond where its original propelling power directly acted upon it. The ‘Great Equatorial Current’ is produced by the south-east trade, the Gulf Stream, as I’ve just explained to you, by the western or passage winds ”

“ This great flood of thermal waters spreads out in the Northern Atlantic, imparting to Europe a climate corresponding to countries twenty degrees south of it on this side of the ocean. There is, of course, an under-current from the Arctics to the equator, exactly compensating this enormous northward flow of the surface-waters. The same process and effect are repeated in the Pacific Ocean; and the great Japan Stream robs the southern hemisphere, for the benefit of our Pacific States, only in a degree less than does the Gulf Stream for the benefit of Europe. ”

“ Rising up at the equator, it did not leave all its moisture with the cloud-ring, but, retaining a part, conveyed it through the cloud region, above the north-east trades, to this calm belt, where there was a descent and a crossing. The fact that these dry places are all within or on the equatorial side of this calm belt, while countries abounding with rains and well watered with running streams are to be found all along its polar side, is clearly indicative of a crossing. Upon no other supposition can we account for the barrenness on one side, the fertility on the other. ”

“ Contrary to the rule in the Equatorial seas, we find in the Polar-ocean an increase of temperature from the surface downwards, in consequence of the warmer under-currents, flowing from the south northwards, and passing beneath the cold waters of the superficial Arctic current. ”

“ The result is that this heated equatorial zone is constantly sending up warm air caused by the inrush of colder air, which is heavier than the air at the equator, expanded by the heat. The warm air at the equator is forced up into the higher regions of the atmosphere, and here it overflows each way, north and south, causing a current of air in the upper regions counter to that of the lower. ”

“ Land as a Barrier to Ocean Currents.—We have seen already the great importance of elevated land to serve as condensers and ice-accumulators; but there is another and hardly less important effect that may be produced by an extension of land in high latitudes, which is, to act as a barrier to the flow of ocean currents. In the region with which we are more immediately interested it is easy to see how a comparatively slight alteration of land and sea, such as has undoubtedly occurred, would produce an enormous effect on climate. ”