Trade wind

“ 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. ”

“ The Trade Winds The trade winds at the surface blow from the belts of high pressure toward the equatorial belts of low pressure. Because of the rotation of the Earth, the moving air is deflected toward the west. Therefore, the trade winds in the Northern Hemisphere are from the northeast and are called the northeast trades, while those in the Southern Hemisphere are from the southeast and are called the southeast trades. The trade-wind directions are best defined over eastern ocean areas. ”

“ The southeast trade-winds, with their load of vapor, whether great or small, take, after ascending in the equatorial calms, a northeasterly 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 vapor which the winds that blow over southeast trade-wind Africa and America convey. ”

“ True, the S. E. trade which enters the belt of rains, and issues out on the north, a S. E. upper current or counter-trade, keeps that course until it arrives at the northern limit of the surface trade, when, in obedience to another law, which we shall notice, it gradually decends near the surface, curves to the eastward, and becomes the S. W. current which passes over us. And so we have the S. E. trade-wind of the South Atlantic, with its moisture, warmth, electricity, and polarity, over, and perhaps sometimes around us, dropping the electric rain which makes glad our fields ”

“ When a trade-wind is turned out of its course, it is regarded as a monsoon. For instance, the summer sun, beating on the interior plains of Asia, creates such intense heat in the atmosphere that it is more than sufficient to neutralize the forces which cause the trade-winds to blow. They are, accordingly, arrested and turned back. The great general law of the trades is in this region temporarily suspended, and the monsoons are created. ”

“ 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. ”

“ In the east the trade wind, arriving fresh from the sea, lessens this effect, and makes the air pleasant, but as it sweeps across the hundreds of miles of burning soil it becomes itself heated, and as it gets farther west adds to the discomfort, instead of correcting it. ”

“ Even the experienced observers on islands and the alert officers on shipboard might not see the real implications in the weather messages they prepare. In the enormous reaches of the belt of trade winds, where the tremendous energy of the sun’s heat and the irresistible force of earth rotation dictate that the winds shall blow as steady breezes from the northeast, somebody might put in his report, for example, that there was a light wind coming from the southwest. ”

“ The land and sea breezes express more than double the amount of wind force which the direct heat of the sun is capable of exerting upon the tradewinds. I say more than double, because in the land and sea breezes the wind-producing power acts alternately on the land and on the sea—in opposite scales of the balance; whereas in the trade-winds it acts all the time in one scale—in the sea scale ”

“ Upon an examination of the laws that govern the direction of the trade-winds in the Atlantic and Pacific oceans, it is found that there is a strong easterly wind continuously blowing across the Atlantic towards Florida, Mexico and Yucatan. There is also a strong westerly wind invariably blowing across the North Pacific, over the regions between the fortieth and fiftieth parallels of latitude, from Japan towards the north-west coast of North America. ”

“ To the north of the trade-wind belt is a belt of prevailingly high barometrical pressure, light variable winds, narrower and less well defined than the doldrums, which encircles the earth in the region of the Tropic of Cancer. ”

“ North or north-west winds are, however, as a rule never of long continuance, and indeed frequently continue only a few hours, because so soon as the first burst is over, the trade-wind, upon whose limits it has encroached, soon begins to drive it before it, under the influence of an air-wave from the southward, and ships which, with the view of suffering as little as possible from north or north-west winds, keep as far from the lighthouse as possible, have nothing to dread from even a heavy "norther," if all proper precautions are taken, and their anchors and cables hold. ”

“ But the heat, oppressive and exhausting at times, is nevertheless tempered by the sea and land breezes, which blow daily, alternating with intervals of calm between them. Besides these daily breezes the trade wind blows regularly from the east during the fine season, when the sky is constantly blue and cloudless. ”

“ Regarding the trade-wind as a great current set up in the atmosphere, it is conceivable that the heating and consequent expansion which must occur as the sun acts upon it, tends to increase the rate of flow at the bottom of the aërial stream, while the cooling which ensues as the sun’s heat is withdrawn, has the contrary effect. ”

“ 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 trade wind, blowing on the eastern coast of South America, and proceeding from the east, crosses the Atlantic ocean, and therefore reaches the land charged with the vapours accumulated in its passage. ”

“ The trade-wind, at this season of the year, is saturated with steam from the ocean which it has crossed; and the least disturbance in its temperature, from ascending hot air or descending cold, precipitates the steam in a sudden splash of water, out of a cloud, if there happens to be one near ”

“ The south-east wind, which blows upon the southern coasts of the chain of islands, is sometimes violent, always strong through the straits which separate them from each other, and this appears to be more and more the case as we go eastward. Here, also, upon the northern coast, we find land breezes, yet the trade-wind often blows so violently that they have not sufficient power to force it beyond the coast. ”

“ Currents and winds are disorganized by the close-hugging lands. Here the steady northeast trade wind is replaced by the alternating air currents of the northeast and southwest monsoons, which at a very early date573 enabled merchant vessels to break away from their previous slow, coastwise path, and to strike a straight course on their voyage between Arabia or the east coast of Africa and India.574 Moreover, this northern half of the Indian Ocean looks like a larger Mediterranean with its southern coast removed. ”

“ Trade-Wind Belts.—The trade belts near sea-level are characterized by fair weather, steady winds, infrequent light rains or even an almost complete absence of rain, very regular, although slight, annual and diurnal ranges of temperature, and a constancy and regularity of weather. ”

“ All our surface-winds, except the light, peculiar W. S. W. wind which is felt where the counter-trade is in contact with the earth, and which is a part of it, and perhaps the genuine N. W. wind which is very peculiar, are incidents of the trade, and are due to its conditions and attractions. We have already said this was true of the easterly wind and scud of a storm—it is alike true of all. The storm winds east of the Alleghanies are usually, though not always, from the eastward. ”

“ We can feel our way across on the trade-wind—unless we hit something, of course. You see, it blows almost out of the east on this side, and 'll haul more to the sou'east and south'ard as we get over. By the wind first, then we'll square away as we need to. ”

“ Lieutenant Maury, in an able and original article upon the circulation of the atmosphere, conceives the bands of comparative calms at the northern limits of the trades, which he appropriately terms [Pg 198] the “Calms of Cancer,” to be nodes in the circulation of the atmosphere, and that the upper or counter-trade here decends and becomes a surface wind from the S. W., as the N. E. trade is a surface wind; and that an upper current from the poles approaches and descends at the same node, to make the N. E. trade. ”

“ The trade stratum is a continuous unbroken sheet, and its descent must displace a portion of the surface atmosphere. A portion of it is impelled forward, aiding in the precedent elevation of the barometer, and a portion is attracted backward, into the space from which a like portion had been previously attracted by the passing storm cloud, forming the easterly wind. ”

“ In the region approaching nearest to the northern or southern lines of the equinoxes, rain falls throughout the year, thanks to the opposing trade winds, which by neutralising each other often preserve the stillness of the atmosphere, and enable the local vapours to condense and precipitate themselves on the spot. ”