Physical Geography Of The Sea 1855/13
CHAPTER XIII. — THE WINDS.
Plate VIII., § 461. — Monsoons, 462. — Why the Belt of Southeast is broader than the Belt of Northeast Trade-winds, 463. — Effect of Deserts upon the Trade-winds, 466. — At Sea the Laws of Atmospherical Circulation are better developed, 470. — RAIN WINDS: Precipitation on Land greater than Evaporation, 472. — The Place of Supply for the Vapors that feed the Amazon with Rains, 473. — MONSOONS: How formed, 474. — Monsoons of the Indian Ocean, 475. — How caused, 476. — How the Monsoon Season may be known, 478. — The Distance to which the Influence of Deserts upon Winds may be felt at Sea, 479. — Why there are no Monsoons in the Southern Hemisphere, 482. — Why the Trade-wind Zones are not stationary, 483. THE CALM BELTS: Doldrums — a Zone of constant Precipitation, 486. — The Horse Latitudes, 488. — The Westerly Winds, 490.
461. PLATE VIII. is a chart of the winds, based on information derived from the Pilot Charts, one of the series of Lt. Maury’s Wind and Current Charts. The object of this chart is to make the student acquainted with the prevailing direction of the wind in every part of the ocean. The arrows of the plate are supposed to fly with the wind; the half bearded and half feathered arrows denoting monsoons or periodic winds; the dotted bands, the regions of calm and baffling winds.
462. Monsoons, properly speaking, are winds which blow one half of the year from one direction, and the other half from an opposite, or nearly an opposite direction. Let us commence the study of Plate VIII. by examining the trade-wind region; that, also, is the region in which monsoons are most apt to be found.
463. The belt or zone of the southeast trade-winds is broader, it will be observed, than the belt or zone of northeast trades. This phenomenon is explained by the fact that there is more land in the northern hemisphere, and that most of the deserts of the earth — as the great deserts of Asia and Africa — are situated in the rear, or behind the northeast trades; so that, as these deserts become more or less heated, there is a call — a pulling back, if you please — upon these trades to turn about and restore the equilibrium which the deserts destroy. There being no, or few such regions in the rear of the southeast trades, the trade-wind force prevails, and carries them over into the northern hemisphere.
464. By resolving the forces which it is supposed are the principal forces that put these winds in motion, viz., calorific action of the sun and diurnal rotation of the earth, we are led to the conclusion that the latter is much the greater of the two in its effects upon those of the northern hemisphere. But not to such an extent is it greater in its effects upon those of the southern. We see by the plate that those two opposing currents of wind are so unequally balanced that the one recedes before the other, and that the current from the southern hemisphere is larger in volume; i.e., it moves a greater zone or belt of air. The southeast trade-winds discharge themselves over the equator — i.e., across a great circle — into the region of equatorial calms, while the northeast trade winds discharge themselves into the same region over a parallel of latitude, and consequently over a small circle. If, therefore, we take what obtains in the Atlantic as the type of what obtains entirely around the earth, as it regards the trade-winds, we shall see that the southeast trade-winds keep in motion more air than the northeast do, by a quantity at least proportioned to the difference between the circumference of the earth at the equator and at the parallel of latitude of 9º north. For if we suppose that those two perpetual currents of air extend the same distance from the surface of the earth, and move with the same velocity, a greater volume from the south would flow across the equator in a given time than would flow from the north over the parallel of 9º in the same time; the ratio between the two quantities would be as radius to the secant of 9º. Besides this, the quantity of land lying within and to the north of the region of the northeast trade winds is much greater than the quantity within and to the south of the region of the southeast trade-winds. In consequence of this, the mean level of the earth’s surface within the region of the northeast trade-winds is, it may reasonably be supposed, somewhat above the mean level of that part which is within the region of the southeast trade-winds. And as the northeast trade-winds blow under the influence of a greater extent of land surface than the southeast trades do, the former are more obstructed in their course than the latter by the forests, the mountain ranges, unequally heated surfaces, and other such like inequalities.
465. As already stated, the investigations show that the momentum of the southeast trade-winds is sufficient to push the equatorial limits of their northern congeners back into the northern hemisphere, and to keep them, at a mean, as far north as the ninth parallel of north latitude. Besides this fact, they also indicate that while the northeast trade-winds, so called, make an angle in their general course of about 23º with the equator (east-northeast), those of the southeast make an angle of 300 or more with the equator (southeast by east). I speak of those in the Atlantic, thus indicating that the latter approach the equator more directly in their course than do the others, and that, consequently, the effect of the diurnal rotation of the earth being the same for like parallels, north and south, the calorific influence of the sun exerts more power in giving motion to the southern than to the northern system of Atlantic trade-winds.
466. That such is actually the case is rendered still more probable from this consideration: 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. The northwest winds of the southern are also — it may be inferred — stronger than the southwest winds of the northern hemisphere.
467. The investigations that have taken place show that the influence of the land upon the normal directions of the wind at sea is an immense influence. It is frequently traced for a thousand miles or more out upon the ocean. For instance, the action of the sun’s rays upon the great deserts and arid plains of Africa, in the summer and autumnal months, is such as to be felt nearly across the Atlantic Ocean between the equator and the parallel of 13º north. Between this parallel and the equator, the trade-winds are turned back by the heated plains of Africa, and are caused to blow a regular southwardly monsoon for several months. They bring the rains which divide the season in these parts of the African coast. The region of the ocean embraced by the monsoons is cuneiform in its shape, having its base resting upon Africa, and its apex stretching over till within 10º or 15º of the mouth of the Amazon.
468. Indeed, when we come to study the effects of South America and Africa (as developed by the Wind and Current Charts) upon the winds at sea, we should be led to the conclusion — had the foot of civilized man never trod the interior of these two continents — that the climate of one is humid; that its valleys are, for the most part, covered with vegetation, which protects its — surface from the sun’s rays; while the plains of the other are arid and naked, and, for the most part, act likefurnaces in drawing the winds from the sea to supply air for the ascending columns which rise from its overheated plains.
469. Pushing these facts and arguments still farther, these beautiful and interesting researches seem already sufficient almost to justify the assertion that, were it not for the Great Desert of Sahara, and other arid plains of Africa, the western shores of that continent, within the trade-wind region, would be almost, if not altogether, as rainless and sterile as the desert itself.
These investigations, with their beautiful developments, eagerly captivate the mind; giving wings to the imagination, they teach us to regard the sandy deserts, and arid plains, and the inland basins of the earth, as compensations in the great system of atmospherical circulation. Like counterpoises to the telescope, which the astronomer regards as incumbrances to his instrument, these wastes serve as make-weights, to give certainty and smoothness of motion — facility and accuracy to the workings of the machine.
470. When we travel out upon the ocean, and get beyond the influence of the land upon the winds, we find ourselves in a field particularly favorable for studying the general laws of atmospherical circulation. Here, beyond the reach of the great equatorial and polar currents of the sea, there are no unduly heated surfaces, no mountain ranges, or other obstructions to the circulation of the atmosphere — nothing to disturb it in its natural courses. The sea, therefore, is the field for observing the operations of the general laws which govern the movements of the great aerial ocean.
Observations on the land will enable us to discover the exceptions. But from the sea we shall get the rule. Each valley, every mountain range and local district, may be said to have its own peculiar system of calms, winds, rains, and droughts. But not so the surface of the broad ocean; over it the agents which are at work are of a uniform character.
471. 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. Peninsula (§ 139). In like manner, the African monsoons of the Atlantic are the winds which feed the springs of the Niger and the Senegal with rains.
472. Upon every water-shed which is drained into the sea, the precipitation may be considered as greater than the evaporation, for the whole extent of the shed so drained, by the amount of water which runs off through the river into the sea. In this view, all rivers may be regarded as immense rain-gauges, and the volume of water annually discharged by any one, as an expression of the quantity which is annually evaporated from the sea, carried back by the winds, and precipitated throughout the whole extent of the valley that is drained by it. Now, if we knew the rain-winds from the dry, for each locality and season generally throughout such a basin, we should be enabled to determine, with some degree of probability at least, as to the part of the ocean from which such rains were evaporated. And thus, notwithstanding all the eddies caused by mountain chains, and other uneven surfaces, we might detect the general course of the atmospherical circulation over the land as well as the sea, and make the general courses of circulation in each valley as obvious to the mind of the philosopher as is the current of the Mississippi, or of any other great river, to his senses.
473. These investigations as to the rain-winds at sea indicate that the vapors which supply the sources of the Amazon with rain are taken up from the Atlantic Ocean by the northeast and south east trade-winds; and many circumstances, some of which have already been detailed (§ 226), tend to show that the winds which feed the Mississippi with rains get their vapor in the southeast trade-wind region of the other hemisphere. For instance, we know from observation that the trade-wind regions of the ocean, beyond the immediate vicinity of the land, are, for the most part, rainless regions, and that the trade-wind zones may be described, in a hyetographic sense, as the evaporating regions (§ 32). They also show, or rather indicate as a general rule, that, leaving the polar limits of the two trade-wind systems, and approaching the nearest pole, the precipitation is greater than the evaporation until the point of maximum cold is reached.
And we know, also, that, as a general rule, the southeast and northeast Trade-winds which come from a lower and go to a higher temperature are the evaporating winds, i.e., they evaporate more than they precipitate; while those winds which come from a higher and go to a lower temperature are the rain-winds, i.e., they precipitate more than they evaporate. That such is the case, not only do researches indicate, but reason teaches, and philosophy tells. These views, therefore, suggest the inquiry as to the sufficiency of the Atlantic, after supplying the sources of the Amazon and its tributaries with their waters, to supply also the sources of the Mississippi and the St. Lawrence, and of all the rivers, great and small, of North America and Europe. A careful study of the rain-winds (§ 32), in connection with the Wind and Current Charts, will probably indicate to us the “springs in the ocean” which supply the vapors for the rains that are carried off by those great rivers. “All the rivers run into the sea; yet the sea is not full; unto the place from whence the rivers come, thither they return again.”
474. Monsoons (§ 462) are, for the most part, formed of trade winds. When a trade-wind is turned back or diverted by overheated districts from its regular course at stated seasons of the year, it is regarded as a monsoon. Thus the African monsoons of the Atlantic (Plate VIII.), the monsoons of the Gulf of Mexico, and the Central American monsoons of the Pacific, are, for the most part, formed of the northeast trade-winds, which are turned back to restore the equilibrium which the overheated plains of Africa, Utah, Texas, and New Mexico have disturbed. When the monsoons prevail for five months at a time, for it takes about a month for them to change and become settled, then both they and the trade-winds, of which they are formed, are called monsoons.
475. The northeast and the southwest monsoons of the Indian Ocean afford an example of this kind. A force is exerted upon the northeast trade-winds of that sea by the disturbance which the heat of summer creates in the atmosphere over the interior plains of Asia, which is more than sufficient to neutralize the forces which cause those winds to blow as trade-winds; it turns them back; and were it not for the peculiar conditions of the land about that ocean, what are now called the northeast monsoons would blow the year round; there would be no southwest monsoons; and the northeast winds, being perpetual, would become all the year, what in reality for five months (§ 474) they are, viz., northeast trade-winds.
476. The agents which produce monsoons reside (§ 475) on the land. These winds are caused by the rarefaction of the air over large districts of country situated on the polar edge, or near the polar edge of the trade-winds. Thus the monsoons of the Indian Ocean are caused by the intense heat which the rays of a cloudless sun produce during the summer time upon the Desert of Cobi and the burning plains of Central Asia. When the sun is north of the equator, the force of his rays, beating down upon these wide and thirsty plains, is such as to cause the vast superincumbent body of air to expand and ascend.
There is, consequently, a rush of air, especially from toward the equator, to restore the equilibrium; and in this case, the force which tends to draw the northeast trade-winds back becomes greater than the force which is acting to propel them forward. Consequently, they obey the stronger power, turn back, and become the famous southwest monsoons of the Indian Ocean, which blow from May to September inclusive.
477. Of course, the vast plains of Asia are not brought up to monsoon heat per saltum, or in a day. They require time both to be heated up to this point and to be cooled down again. Hence there is a conflict for a few weeks about the change of the monsoon, when neither the trade-wind nor the monsoon force has fairly lost or gained the ascendency. This debatable period amounts to about a month at each change. So that the monsoons of the Indian Ocean prevail really for about five months each way, viz., from May to September from the southwest, in obedience to the influence of the overheated plains, and from November to March inclusive from the northeast, in obedience to the trade-wind force.
478. The monsoon season may be always known by referring to the cause which produces these winds. Thus, by recollecting where the thirsty and overheated plains are which cause the monsoons, we know at once that these winds are rushing with greatest force toward these plains at the time that is the hottest season of the year upon them.
479. The influence of these heated plains upon the winds at sea is felt for a thousand miles and more. Thus, though the Desert of Cobi and the sun-burnt plains of Asia are, for the most part, north of latitude 30º, their influence in making monsoons is felt south of the equator (Plate VIII.). So, too, with the great Desert of Sahara and the African monsoons of the Atlantic; also, with the Salt Lake country and the Mexican monsoons on one side, and those of Central America in the Pacific on the other. The influence of the deserts of Arabia upon the winds is felt in Austria and other parts of Europe, as the observations of Kriel, Lamont, and others show. (See Appendix H.)
480. It would appear, therefore, that these desert countries exercise a powerful influence in checking, and consequently in weakening, the force of the northeast trade-winds. There are no such extensive influences at work checking the southeast trades. On the contrary, these are accelerated; for the same forces that serve to draw the northeast trade-winds back, or retard them, tend also to draw the southeast trade-winds on, or to accelerate them. Hence the ability of the southeast trade-winds to push themselves over into the northern hemisphere.
481. Hence, also, we infer that, between certain parallels of latitude in the northern hemisphere, the sun’s rays, by reason of the great extent of land surface, operate with much more intensity than they do between corresponding parallels in the southern; and that, consequently, the mean summer temperature on shore, north of the equator, is higher than it is south: a beautiful physical fact which the winds have revealed, in corroboration of what observations with the thermometer had already induced meteorologists to suspect.
482. It appears, from what has been said (§ 474), that it is the rays of the sun operating upon the land, not upon the water, which causes the monsoons. Now let us turn to Plate VIII., and examine into this view. The mo§soon regions are marked with halfbearded and half-feathered arrows; and we perceive, looking at the northern hemisphere, that all of Europe, some of Africa, most of Asia, and nearly the whole of North America, are to the north, or on the polar side of the northeast trade-wind zone; whereas but a small part of Australia, less of South America, and still less of South Africa, are situated on the polar side of the zone of southeast trade-winds. In other words, there are no great plains on the polar side of the southeast trade-winds upon which the rays of the sun, in the summer of the other hemisphere, can play with force enough to rarefy the air sufficiently to materially interrupt these winds in their course. But, besides the vast area of such plains in the northern hemisphere, on the polar side of its tradewind belt, the heat of which is sufficient (§ 479) to draw these trade-winds back, there are numerous other districts in the extratropical regions of our hemisphere the summer heat of which, though it be not sufficient to turn the northeast trade-winds back, and make a monsoon of them, yet may be sufficient to weaken them in their force, and, by retarding them (§ 480).draw the southeast trade-winds over into the northern hemisphere.
483. Now, as this interference from the land takes place in the summer only, we might infer, without appealing to actual observation, that the position of these trade-wind zones is variable; that is, that the equatorial edge of the southeast trade-wind zone is farther to the north in our summer, when the northeast trades are most feeble, than it is in winter, when they are strongest.
484. We have here, then, at work upon these trade-wind zones, a force now weak, now strong, which, of course, would cause these zones to vibrate up and down the ocean, and within certain limits, according to the season of the year. These limits are given on Plate VIII. for spring and autumn. During the latter season these zones reach their extreme northern declination, and in our spring their utmost limits toward the south.
485. THE CALM BELTS.—There is between the two systems of trade-winds a region of calms, known as the equatorial calms. It has a mean average breadth of about six degrees of latitude. In this region, the air which is brought to the equator by the northeast and southeast trades ascends. This belt of calms always separates these two trade-wind zones, and travels up and down with them. If we liken this belt of equatorial calms to an immense atmospherical trough, extending, as it does, entirely around the earth, and if we liken the northeast and southeast 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 the 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. Using still farther this mode of illustration: if we liken the calm belt of Cancer and the calm belt of Capricorn each to a great atmospherical trough extending around the earth also, we shall see that in this case the currents are running in at the top and out at the bottom (§ 101).
486. The belt of equatorial calms is a belt of constant precipitation. Captain Wilkes, of the Exploring Expedition, when he crossed it in 1838, found it to extend from 40 north to 120 north. He was ten days in crossing it, and during those ten days rain fell to the depth of 6.15 inches, or at the rate of eighteen feet and upward during the year. In the summer months this belt of calms is found between the parallels of 80 and 140 of north latitude, and in the spring between 50 south and 40 north. (Vide Plate VIII.)
487. This calm belt carries with it the rainy seasons of the torrid zone, always, in its motions from south to north and back, arriving at certain parallels at stated periods of the year; consequently, by attentively considering Plate VIII., one can tell what places within the range of this zone have, during the year, two rainy seasons, what one, and what are the rainy months for each locality. Were the northeast and the southeast trades, with the belt of equatorial calms, of different colors, and visible to an astronomer in one of the planets, he might, by the motion of these belts or girdles alone, tell the seasons with us. He would see them at one season ging north, then appearing stationary, and then commencing their return to the south. But, though he would observe (§ 131) that they follow the sun in his annual course, he would remark that they do not change their latitude as much as the sun does his declination; he would, therefore, discover that their ex tremes of declination are not so far asunder as the tropics of Cancer and Capricorn, though in certain seasons the changes from day to day are very great. He would observe that these zones of winds and calms have their tropics or stationary nodes, about which they linger near three months at a time; and that they pass from one of their tropics to the other in a little less than another three months. Thus he would observe the whole system of belts to go north from the latter part of May till some time in August. Then they would stop and remain stationary till winter, in December; when again they would commence to move rapidly over the ocean, and down toward the south, until the last of February or the first of March; then again they would become stationary, and remain about this, their southern tropic, till May again.
488. THE HORSE LATITUDES. — Having completed the physical examination of the equatorial calms and winds, if the supposed observer should now turn his telescope toward the poles of our earth, he would observe a zone of calms bordering the northeast trade winds on the north (§ 100), and another bordering the southeast trade-winds on the south (§ 106) These calm zones also would be observed to vibrate up and down with the trade-wind zones, partaking (§ 130) of their motions, and following the declination of the sun. On the polar side of each of these two calm zones there would be a broad band extending up into the polar regions, the prevailing winds within which are the opposites of the trade-winds, viz., southwest in the northern and northwest in the southern hemisphere.
489. The equatorial edge of these calm belts is near the tropics, and their average breadth is 100 or 120. On one side of these belts (§ 101) the winds blow perpetually toward the equator; on the other, their prevailing direction is toward the poles. They are called (§ 101) the “horse latitudes” by seamen.
490. Along the polar borders of these two calm belts (§ 129) we have another region of precipitation, though generally the rains here are not so constant as they are in the equatorial calml. The precipitation near the tropical calms is nevertheless sufficient to mark the seasons; for whenever these calm zones, as they go from north to south with the sun, leave a given parallel, the rainy season of that parallel, if it be in winter, is said to commence. Hence we may explain the rainy season in Chili at the south, and in California at the north.
491. THE WESTERLY WINDS. — To complete the physical examination of the earth’s atmosphere, which we have supposed an astronomer in one of the planets to have undertaken according to the facts developed by the Wind and Current Charts, it remains for him to turn his telescope upon the southwest passage winds of the northern hemisphere, pursue them into the arctic regions, and see theoretically how they get there, and, being there, what becomes of them. From the parallel of 40º up toward the north pole, the prevailing winds, as already remarked, are the southwest passage winds (Plate VIII.), or, as they are more generally called by mariners, the “westerly” winds; these, in the Atlantic, prevail over the “easterly” winds in the ratio of about two to one. Now if we suppose, and such is probably the case, these “westerly” winds to convey in two days a greater volume of atmosphere toward the arctic circle than those “easterly” winds can bring back in one, we establish the necessity for an upper current by which this difference may be returned to the tropical calms of our hemisphere (§ 109). Therefore there must be some place in the polar regions (§ 112) at which these southwest winds cease to go north, and from which they commence their return to the south, and this locality must be in a region peculiarly liable to calms. It is another atmospherical node in which the motion of the air is upward, with a decrease of barometric pressure. It is marked P, Plate I. If we now return to the calm belt of the northern tropic, and trace theoretically a portion of air that, in its circuit, shall fairly represent the average course of these southwest passage winds, we shall see (§ 113) that it approaches the pole in a loxodromic curve; that as it approaches the pole, it acquires, from the spiral convolutions of this curve which represents its path, a whirling motion, in a direction contrary to that of the hands of a watch; and that the portion of atmosphere whose path we are following would gradually contract its gyrations, until it would finally ascend, turning against the hands of a watch as it whirls around In the southern hemisphere a like process is going on; only there, the northwest passage wind would, as it arrives near the antarctic calms, acquire a motion with the sun, or in the direction of the hands of a watch.