The global logic of deluges and deserts: How the trade winds power the earth's climate extremes

“ As we shall hereafter see, the machinery of the earth's surface, all the life which it bears, its winds and rains—everything, indeed, save the actions which go on in the depths of the sphere—is determined by the heat and light which come from the sun. ”

“ The earth’s internal heat and the moon’s attraction are other minor agencies, but these may be neglected by comparison. The earth’s rotation also would be ineffectual in modifying the aërial movements, except for the coöperation of the sun. Without his influence the atmosphere, always stagnant, would simply rotate with the globe, at constant angular velocity and uniformly graded density at various levels. This evenness of density for any level is broken by the solar radiation increasing the temperature 364and moisture, otherwise the air would remain practically at a standstill. ”

“ The philosopher of civilization, has discovered that breath, the fan-wafted breeze, the air confined in vessels, the air moving in ventilation, that these are all parts of the great body of air which surrounds the earth, all in motion, swung by the revolving earth, heated at the tropics, cooled at the poles, and thus turned into counter-currents and again deflected by a thousand geographic features, so that the winds sweep down valleys, eddy among mountain crags, or waft the spray from the crested billows of the sea, all in obedience to cosmic laws. ”

“ Many slight actions, such as the movement of the hot and cold currents of the sea, the local air movements from the lands or from detached islands, somewhat perturb the trade winds, but they remain among the most permanent features in this changeable world. ”

“ Passing farther south we entered the wide belt of ocean over which the north-east trade wind blows. These almost unchanging winds, on both sides of the equator, known as “the trades,” are remarkable evidences of the goodness and wisdom of God. The beauty of this arrangement cannot but strike a thoughtful voyager most deeply. Without dwelling upon the fact that these and their partner winds are the great regulators of airs, clouds, and rains over the whole earth, we cannot but notice their great importance to commerce. ”

“ Approximately one-third of the Earth’s land surface is desert, arid land with meager rainfall that supports only sparse vegetation and a limited population of people and animals. Deserts—stark, sometimes mysterious worlds—have been portrayed as fascinating environments of adventure and exploration from narratives such as that of Lawrence of Arabia to movies such as “Dune.” These arid regions are called deserts because they are dry. They may be hot, they may be cold. They may be regions of sand or vast areas of rocks and gravel peppered with occasional plants. ”

“ The sun is perpetually raising enormous quantities of vapour from the ocean which the trade-winds carry to the equator: it is condensed when it rises with the air into the higher strata, and forms a ring of clouds along the southern side of the belt of equatorial calms that surrounds the earth, which, during the day, is perpetually pouring down torrents of rain, while the sun continually beating upon its upper surface dissolves the clouds into invisible vapour which is carried by the winds and condensed into rain on the extra-tropical regions. ”

“ God has so made it that as much water is incessantly rising into the air from the sea as is being poured into it. But how is it, then, that the streams and rivers do not dry up ? Whence comes that volume of water which they are continuously pouring into the sea? The mists and clouds which rise from the sea are driven over the land by the wind, and fall back on the earth in the form of either dew, fog, rain or snow. ”

“ RAIN-WINDS are the winds which convey the vapor from the sea, where it is taken up, to other parts of the earth, where it is let down either as snow, hail, or rain. As a general rule, the trade-winds (§ 126) may be regarded as the evaporating winds; and when, in the course of their circuit, they become monsoons, or the variables of either hemisphere, they then generally become also the rain-winds — especially the monsoons — for certain localities. Thus the southwest monsoons of the Indian Ocean are the rain-winds for the west coast of the. ”

“ The south-east trade-winds, with their load of vapour, whether great or small, take, after ascending in the equatorial calms, a north-easterly direction; they continue to flow in the upper regions of the air in that direction until they cross the tropic of Cancer. The places of least rain, then, between this tropic and the pole, should be precisely those places which depend for their rains upon the vapour which the winds that blow over south-east trade-wind Africa and America convey. ”

“ There is, therefore, a constant tendency with the air that these upper currents carry polarward to be crowded out, so to speak—to slough off and turn back. Thus the upper current is ever ready to supply the trade-winds, as they approach the equator, with air exactly at the right place, and in quantities just sufficient to satisfy the demand.
224. How is it drawn down from above?—This upper air, having supplied the equatorial cloud-ring (§ 514) with vapour for its clouds, and with moisture for its rains, flows off polarward as comparatively dry air. The dryest air is the heaviest. ”

“ A monsoon is a wind that blows from a continent toward the sea in winter, when the land is colder than the water, and in the opposite direction in summer, when the reverse conditions of temperature prevail. The pressure gradient is reversed with the seasons, and the wind varies accordingly.131 The most striking example of monsoon winds is found in southern Asia—where these winds are of special economic importance because they control the rainfall of India—but well-developed monsoons also occur in Australia and West Africa, over the Caspian Sea, on the coast of Texas, and elsewhere. ”

“ The propelling power of the winds.—If we would discover the seat of those forces which produce this difference in the dynamical status of the two great aerial oceans that envelop our planet, we whould search for them in the unequal distribution of land and water over the two hemispheres. In one the wind is interrupted in its circuits by the continental masses, with their wooded plains, their snowy mantles in winter, their sandy deserts in summer, and their mountain ranges always. ”

“ But there are other subordinate and minor influences which are continually taking place in the atmosphere, and which are also calculated to alter the place of this calm belt, and to produce changes in the thermal status of the air which the trade-winds move. These are unusually severe winters or hot summers, remarkable spells of weather, such as long continuous rains or droughts over areas of considerable extent, either within or near the trade-wind belts. It is tremblingly alive to all such influences, and they keep it in continual agitation ”

“ The most conspicuous and important aërial current on the globe is the permanent double vortex playing between the equator and the poles. The heated air of the equatorial belt, uplifted by expansion, overflows beneath the isothermal layer toward the north and south, thereby increasing the pressure in the higher latitudes sufficiently to generate a surface inflow along the earth, and thus maintaining a perpetual closed circulation which is felt all over the globe. ”

“ Now if we liken this belt of calms to an immense atmospherical trough, extending, as it does, entirely around the earth, and if we liken the north-east and south-east trade-winds to two streams discharging themselves into it, we shall see that we have two currents perpetually running in at the bottom, and that, therefore, we must have as much air as these two currents bring in at the bottom to flow out at the top. What flows out at the top is carried back-north and south by these upper currents, which are thus proved to exist and to flow counter to the trade- winds. ”

“ The cause of the trade winds is supposed to be the joint influence of the higher temperature of the torrid zone and the rotation of the earth on its axis. On the equator, and extending sometimes a few degrees on either side, is a tract where light easterly winds, calms, and squalls, with thunder, lightning, and inundating rains, prevail. ”

“ When the rarefied equatorial air has travelled some thirty or forty degrees of latitude along the upper regions of the atmosphere towards the poles it becomes cooled, and is ready to descend again, between the latitudes of 30° and 60°, to supply the place of the lower air, drawn off towards the equator by the Trade-winds. ”

“ In the belt of the northeast trades the weight of the air for a given area is greater than in the doldrums, the wind blows with remarkable uniformity both of direction and force, the sky is normally clear, and rain infrequent except when the warm moist air is forced upward either by local storms or on coming in contact with high land. The trade-winds blow across the West Indies, Mexico, and much of Central America. ”

“ But because the earth revolves from west to east, the course of the trade winds is oblique to the equator, those in the northern hemisphere blowing from northeast to southwest, those in the southern from southeast to northwest. ”

“ If it be admitted that all the easterly character of the Trade-winds is due to the difference of velocity between the rotation of the torrid zone of the earth from west to east, and that of the air impressed only with the slower rotatory motion to the east of the temperate zone, it will follow, that, if this difference of velocities between the earth and the air in contact with it be diminished or annihilated, the easterly character of these winds will be diminished or annihilated likewise. ”

“ This prevalence of westerly winds beyond the tropics is readily explained by the same reasoning which has been applied to the Trades blowing within them. The swift moving air of the torrid zone, on being rarefied and raised up, flows along towards the poles, and in a direction from the equator, above the cooler and slower-moving air, which, as I have already described, is drawn along the surface of the earth from the temperate regions beyond the tropics. ”

“ I know of no law of nature or rule of philosophy which would forbid the supposition that the air which has been brought along as the northeast trade-winds to the equatorial calms does, after ascending there, return by the counter and upper currents to the calm zone of Cancer, here descend and reappear on the surface as the northeast trade-winds again. ”

“ Beyond the trade-wind belt, toward the poles, the movements of the atmosphere are dependent in part on the counter trades which descend to the surface of the earth in latitudes higher than that in which the surface or trade winds flow. Thus along our Atlantic coast, and even in the body of the continent, at times when the air is not controlled by some local storm, the counter trade blows with considerable regularity. ”

“ Thus, though we have tallied the air, and put labels on the wind, to "tell whence it cometh and whither it goeth," yet there evidently is an agent concerned in the circulation of the atmosphere whose functions are manifest, but whose presence has never yet been clearly recognized, When the air which the north-east trade-winds bring down, meets in the equatorial calms that which the south-east trade-winds convey, and the two streams rise up together, what is it that makes them cross? where is the power that guides that from the north over to the south, and that from the south up to the north? ”

“ Let A represent the equator, the arrows near the surface of the circle the two trade-winds, and the two cross arrows, two particles of wind in the act of crossing. The difficulty is to conceive how these particles should cross, without mixing with each other, and losing their identity; or why they should not turn back, as well as continue their course. What law of nature is it, that makes the particles of atmosphere which have come from the north pole, so separate and distinct from those which have come from the south pole, as to prevent the two from fusing, and becoming one? ”

“ In the mean time, the trade-winds north and south are pouring into this cloud-covered receiver, as the calm and rain belt of the equator may be called, fresh supplies in the shape of ceaseless volumes of heated air, which, loaded to saturation with vapour, has to rise above and get clear of the clouds before it can commence the process of cooling by radiation. ”

“ We had reached the horse latitudes, those detestable regions embracing the debatable area between the limits of the north-east and south-east trade winds. Here you may have such an exhibition of what the skies are capable of in the matter of rain as nowhere else in the world. For days together the weather will consist of squalls—not much wind in them as a rule—from all points of the compass, but rain—well, one might almost as well be living beneath an ocean of which the bottom is given to falling out occasionally. ”

“ The latent heat liberated in the processes of condensation from and under the cloud-ring, true cause of the trade-winds—Evaporation under this cloud-ring is suspended almost entirely. We know that the trade-winds encircle the earth; that they blow perpetually; that they come from the north and the south, and meet each other near the equator ”

“ The whole system of trade-winds, equatorial and tropical calms, with the cloud ring, follow the sun in declination; consequently in its journeys back and forwards it annually travels over 1000 miles of latitude, and regulates the dry and rainy season in the tropical parts of the earth. ”

“ The reader, who has now been a passenger with us for some time, has watched the trade-winds, as he has crossed the tropics, and has fanned himself and panted for breath, when we have been working our tedious way through the calm-belts. He has seen how this system of trade-winds and calm-belts wanders up and down the earth, from north to south, and south to north, drawn hither and thither by the sun. ”

“ The trade winds in two belts on the equatorial sides of the Horse Latitudes heat up as they move toward the Equator. These dry winds dissipate cloud cover, allowing more sunlight to heat the land. Most of the major deserts of the world lie in areas crossed by the trade winds. The world’s largest desert, the Sahara of North Africa, which has experienced temperatures as high as 57° C, is a trade wind desert. ”

“ Coastal deserts generally are found on the western edges of continents near the Tropics of Cancer and Capricorn. They are affected by cold ocean currents that parallel the coast. Because local wind systems dominate the trade winds, these deserts are less stable than other deserts. ”

“ It is a remarkable fact that the countries in the extra-tropical regions of the north that are situated to the north-east of the south-east trade-winds of South Africa and America—that these countries, over which theory makes these winds to blow, include all the great deserts of Asia, and the districts of least precipitation in Europe. ”

“ All the great deserts are in the northern hemisphere, and the land surface is also much greater on our side of the equator. The action of the sun upon these unequally absorbing and radiating surfaces in and behind, or to the northward of the northeast trades, tends to retard these winds, and to draw large volumes of the atmosphere, that otherwise would be moved by them, back to supply the partial vacuum made by the heat of the sun, as it pours down its rays upon the vast plains of burning sands and unequally heated land surfaces in our overheated hemisphere. ”

“ Here the deserts often reach the sea, as on the western coasts of South America, Africa and Australia. Cold ocean currents, with prevailing winds along-shore rather than on-shore, are here hostile to rainfall, although the lower air is often damp, and fog and cloud are not uncommon.
Monsoon Climate.—Exceptions to the general rule of rainier eastern coasts in trade-wind latitudes are found in the monsoon regions, as in India, for example, where the western coast of the peninsula is abundantly watered by the wet south-west monsoon. ”

“ When any portion of the earth’s surface is periodically more heated above its normal temperature, or average for the year, than the neighboring region, the resulting abnormal temperature gradient causes a periodic surface wind tending toward the excessively heated place, and a counter wind above. That is, the cooler and heavier column of air sinking and uplifting the lighter, results in a lowering of the common center of gravity of the two 386columns of air, and thus furnishes the driving power of the wind. ”

“ It is toward this district that the wind is sucked in, and the attraction is sufficient to draw a portion of the south-east trade-wind across the line into the northern hemisphere. In our winter the region where the rarefaction is greatest is the continent of Australia; and accordingly, in its turn, it sucks the north-east trade-wind of the northern hemisphere across the equator. Thus we see that in the region which extends from the coast of Australia to the centre of Asia we have monsoons, or winds which change regularly every six months. ”

“ In India the prediction of great droughts has long been held to be one of the most important questions that can be attacked by the weather bureau of that country, and eminent men have worked upon it for twenty years past. The progress of their studies has gradually led us to see that the moist air of the southwest monsoon from which the rain falls has come from an unexpectedly great distance, namely, from the southern Indian Ocean. As you see on this globe before you, during the Asiatic winter season we have northeast trade winds here, and southeast trade winds there, south of the equator ”

“ The south-east wind, thus encouraged, changes its course, first to the north, and then to the north-east, and blows stronger and stronger as the season advances, and the heat accumulates over the deserts; until at last the south-east trade-wind of the southern hemisphere has become the south-west monsoon of the northern hemisphere! This monsoon prevails from about the 1st of May to the 1st of November, when the sun has again passed into the southern hemisphere, and withdrawn his heat from the great deserts. ”

“ On the continent of North America we have monsoon influences similar to those of Asia, but not nearly so strong, because the extent of the continent, and consequently the annual range of temperature, are not so great. They are, for the most part, not sufficiently strong to completely overcome and reverse the current of the general circulation of the atmosphere, and so to produce a real monsoon, but they cause great differences between the prevailing directions of the winter and summer winds. ”