A NEW THEORY OF WINDS AND STORMS..
TILE genesis of winds is commonly referred to the unequal degrees in which the atmospheric zones, from the equator to the poles, are subjected to solar heat. This inequality is supposed to cause a continual circulation of the atmosphere from the tropics to the polar regions and back again, the warm air rising and passing away North and South in an upper current, while the cold air flows in below to supply its place. Furthermore, it is supposed that these great aerial currents are variously deflected from their course not only by local inequalities of temperature but also by
the unequal rotatory velocities of different latitudes. To the former of these two influences are ascribed the variable winds; to the latter the steady trade winds. This is Hadley's theory, and it has passed unquestioned almost to the present day. Doubts of its accuracy, however, have recently been entertained in many quarters, and the author of the work' now before us undertakes to subvert it altogether.
He objects to it in the first place, because it is not proved, but merely assumed, that the heated air which rises from the earth's surface must overflow laterally in the higher regions, and no one asserts that the existence of the stream which is supposed to re- sult from this overflow has been actually ascertained ; on the contrary, the aerial currents that come under our observations give no countenance to such an hypothesis. Air raised by con- tact with the heated surface of the earth expands in all directions, laterally as well as vertically ; it does not penetrate the cooler air around it, but presses upon it, and upon the earth, the weight of its column remaining unchanged, and equally able as before to balance colder air at a distance. " Before the raised air could flow away, it would be necessary that it should be heavier than adjoining air as well as higher, which it is not, and therefore it cannot flow horizontally. And cold air could not pass from the polar regions under warmer air, and force it to rise, unless the cold column was high enough to give superior weight to enable it to press under the warm air." If this preponderance existed,. it would be indicated by the barometer; but, so far is this from being the case, that in Iceland near the arctic circle and over the ocean near the antarctic regions, the barometer stands lower than it does within the tropics. On the other hand, our author has failed, after much inquiry, to meet with any well-ascertained in- stance of diminished barometrical pressure due to the rising and flowing away of air over heated deserts. If Hadley's theory were true, its truth would be strongly exemplified in regions which are heated to a very high temperature every day, but air is not found flowing in from surrounding countries upon the Sahara, Arabia, and the desert of Bokhara, when they are thus heated. In April, when the Northern winds cease to blow over the plains of Hindustan, the sun shines brightly in a clear sky, and the air is from 10 degrees to 12 degrees warmer than during the South-west moonsoon. " But there is no strong wind blowing at this time, the weather being not only hot but calm, showing that surface-heating by the sun did not create an ascending cur- rent in the air to be fed by an influx of cold wind below." In fine, Hadley's theory will not sustain a comparison with the
* On Winds and Storms: with an Essay on Weather and its Varieties. By Thomas Hopkins, M.B.M.S. Published by Longman and Co,
observed facts; no direct evidence can be adduced in support of it, and it appears to have been generally adopted only upon the strength of loose analogies between the burning of fuel on the earth and the heating of its surface by the sun—two things between which there is at least as much dissimilarity as resem- blance. When a fire is burned in a grate, a chimney is needed to promote the ascent of the heated air by confining it laterally, and causing a strong indraught of the surrounding cool air beneath it; whereas half the globe is simultaneously heated by the sun, and has no chimney in which its warmed air is hindered from expanding in every direction but one. Make the fire on flat ground in the open air, and it will burn very slowly ; the draught being feeble, the warm air diffusing itself around, and rising but little ; thus approaching the state of the air that is heated by the direct action of the sun.
Since, then, mere heating by the direct action of the sun is not sufficient to account for aerial currents, some other cause must be sought for those disturbances of the atmosphere's static equili- brium which result in winds, storms, and their allied phenomena. The cause assigned by our author is the production of partial vacua by the condensation of watery vapour in the air, and the consequent rarefaction of the latter by the heat which is lost by the vapour in its reduction to the form of cloud or rain. The temperature of the atmosphere diminishes from below upwards in proportion to the elevation and the consequent diminution of pressure ; but the rate of cooling is very different for its two main constituents—the gases and the aqueous vapour. The former cool 5 decrees by expansion for every 500 yards of elevation, the latter only 1 degree ; but the vapour also undergoes a further cooling by parting with a portion of its heat to the colder gases through which it is expanding ; and this process continues until some of the vapour is condensed into cloud. Latent heat will thereby be set free, and increase the expansion of the gases, when they will be forced to still greater elevations by the influx beneath them of cooler and heavier air. "As the warmer gases ascend, they will be exposed to less incumbent pressure, and will expand and experience renewed cooling from expansion, when they will condense more vapour, which, being mingled with them, they are bearing to greater heights. In this way, alternate coolings and heatings take place in the open atmosphere, within ascending masses of air and vapour which are passing up a natu- ral chimney—the cold of the gases first condensing the vapour that is diffused through them, and then the freed heat of the con- densing vapour warming the gases ; and this double operation goes on until the cold of diminished pressure at great elevation converts much of the vapour into water." Meanwhile, a fall of the barometer will have indicated the diminished atmospheric pressure in the locality towards which the winds are moving to restore the balance.
Thus then heat, but not that heat which emanates immediately from the sun, is the prime disturber of atmospheric equilibrium, and the disturbance is always in proportion to the quantity of heat that is liberated in the conversion of vapour into water, and to the rapidity and continaity of the process. There are places where it goes on to a great amount, and these may be found by following the winds to their terminations. One of these great areas of condensation is the Eastern side of ihe tropical Andes. The rain there produced falls through the whole range of the tro- pics, and is copious and constant in that one of them in which the pre- sence of the sun, is for the time being furnishing the greatest abun- dance of vapour ; but always the winds are strongest when and where the most rain falls. To that point, they flow from distant parts, crossing the lowlands of South America, and nearly the whole breadth of the Atlantic ocean to fill up the partial atmos- pheric vacuum created over the Andes. In like manner are gene- rated the monsoons of India. The South-West monsoon blows to- wards the Ghauts and the Himalaya when the sun is in the Northern hemisphere, and those mountains are condensing the vapours freely raised by him from the Indian ocean. When the sun passes South of the equator, the mountains of Sumatra, Java, and other islands on the West side of the great Archipelago, become the rainmakers, and the termini of the North-West monsoon. Here, again, in both instances, it is always at the time that heavy rains are falling that the winds blow towards the rainy parts, and their strength is proportioned to the quantity of rain that falls.
Having discussed the origin of winds, Dr. Hopkins proceeds to solve the no less vexed question—what creates the great currents of the ocean ? The cause which is commonly assigned for these movements is the rotatory motion of the surface of the globe, in which the water, as the less cohesive body, is left a little behind the solid earth ; but he rejects this notion as visionary, because if the light air presses with sufficient force on the surface of the earth, to be taken round with it, there can be no sufficient reason to assume that the water on which the air presses is left behind. Besides, there are many currents which run with the rotatory mo- tion, apparently as readily as in opposition to it, and some even run in that direction faster than the surface of the land. Our author's conclusion is that the primary, oceanic currents are caused by winds moving the waters in the same direction as themselves, and that the velocity, depth, and length of the current are pro- portioned to the strength and continuity of the wind- " Slight and local breezes will put in motion the surface of water over which they pass to only a small extent; stronger and more extensive winds will be disposed to force the water to go farther, Put the surface water will always take with it that which is immediately below; and the stronger the wind, and the farther it extends, the greater will be the effect it will have on the water over which it is passing—putting it into more rapid motion, and making it take with itself the water which is beneath to a greater depth. In shallow places, friction on the bottom would retard the move- ment of that portion which rested on it t• but in deep seas there would be nothing to prevent lower water to great depths from moving with the upper but its own inertia and friction. To the extent, therefore, that the wind could overcome these powers' might water be put in motion, and this would be determined by the weight, velocity, and continuity of the wind."
Two branches of a great current, proceeding respectively from the neighbourhood of the Canary Islands and the Cape of Good Hope, meet at the Caribbean Sea, and flow through it to the Gulf of Mexico. Each branch is accompanied by a wind which is apparently the only force present to put the water in motion. "In the Southern Pacific Ocean a current is found, accompanied in its whole course by winds from the East that correspond with the breadth and velocity of the moving water, which increases in its rapidity as the wind becomes stronger. In the Northern Pacific also a current in the ocean is felt shortly after the North- East wind begins to blow over it, and as the wind increases in strength so does the current in velocity, until both arrive on the 1Vestern side of this wide ocean." In the Southern part of the Indian ocean a current runs from Australia towards and round the Cape of Gocd Hope, a wind accompanying it all the way. The current that sweeps through Torres Straits runs for one half of the year from the Indian ocean on the Western side of de great Archipelago, and during the other half from the Pacific on the Eastern side. The currents regularly change with, obey, and follow the winds, without being at all affected by the rota- tory velocity which is there nearly at its maximum, the locality being close to the equator.
Besides the primary oceanic currents which are set in motion by the winds, there are secondary currents, dependent on the former, and produced by a reaction of the force of gravitation, which the primary currents had partially overcome. Such re- fluxes of heaped-up waters, though not accompanied by winds, no more disprove Dr. Hopkins's theory of primary currents than the back water in a river with its up-hill course disproves the descent by gravitation of the water in the main channel :—The most celebrated of these secondary currents is the gulf stream which flows between Florida and the Bahama Islands and thence to the Canaries, and it is produced in the following way :—The waters which we have seen impelled by the South-East and North-East winds of the Atlantic through the Caribbean Sea and into the Gulf of Mexico cannot escape thence through the narrow outlet in the Florida channel until they rise to a considerable height above the mean level of the ocean, and then they rush through it, impelled by the pressure of the head accumulated in the Gulf. The immediate operating power in this rush is gravi- tation; but wind had to raise the waters to the requisite height to enable gravitation to act. " Wind is therefore the prime moving power."
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