THE ATMOSPHERE AND ITS CURRENTS.

It has been mentioned as a part of the planetary constitution of our globe, that a gaseous envelope environs its mass, the atmosphere, which requires the attention of the astronomer, on account of its influence in displacing the celestial bodies, and contributing to their visibility, by refracting and reflecting the rays of light. This elastic fluid is the scene of interesting phenomena, and performs important functions in the economy of nature. Besides being essential to the life of man, and the animal races, whose existence would terminate in a few minutes without the respiration of it; the exhalation of moisture from the surface of the earth is mainly owing to the common air we breathe, which receives and sustains the vapors formed into clouds, distributes them over different regions by its incessant motions, and tempers by its currents those extremes of heat and cold to which various localities are subject. It is in these last-named offices that the atmosphere demands the notice of the physical geographer. The consideration of its actual constitution does not belong to his province, but a general view of the fluid may be appropriate before we proceed to those agitations and changes which are in constant action, and upon which the welfare of organized beings so materially depends.

The atmosphere is, then, an integral portion of the earth, a body of air revolving with the solid mass upon its axis, the higher strata, of course, increasing in velocity with the distance from the axis of revolution. From hence a conclusion may be drawn respecting its height, for an absolute limit is put to its elevation by this feature of its physical condition. There is a point where the centrifugal force, or the tendency to fly off from the centre, will counterbalance the centripetal, or the gravitation toward the centre, and beyond that point the latter will be vanquished. It is obvious that no portion of the atmosphere can extend beyond the point where the two influences balance, or are in equilibrium, and the projectile force becomes greater than that of gravitation, or its projection into space would follow. At the distance of 6.6 radii from the centre of the earth, or at an elevation of 22,200 miles, about the eleventh part the distance of the moon, this point is fixed, beyond which it is impossible for the atmosphere in the smallest quantity to extend. This consideration is only of importance to show that physical laws rigidly restrict it within finite bounds, for any portion of air at that distance must have a tenuity which is utterly inconceivable. The indications of the height of the atmosphere drawn from its weight, as shown by the barometer, reduce its elevation within a vastly circumscribed limit. A column of the whole circumambient air is nearly equal in weight to a similar column of mercury of thirty inches, or of water of thirty-four feet, which would give it an elevation of but 27,000 feet, or rather under five miles, if its density were uniform. But the elasticity of the air causes it to expand with the diminution of its own pressure, which becomes less at every step from the surface of the earth; and owing to this expansion we must place the limit to its height at a far greater distance than that suggested by the simple barometrical measurement of its weight. A pretty common opinion prevails that its extreme boundary does not exceed forty or fifty miles, and we have sensible evidence on the high lands of the globe, that for all the purposes serviceable to vegetable and animal life, the atmospheric zone is of very contracted elevation. It is a well-known property of the air that the temperature diminishes with its height, a circumstance referable to the general physical law, that as the density of gases decreases they acquire an increased capacity for heat. The higher, therefore, a body ascends in the atmosphere, the greater is the quantity of heat abstracted from it, the surrounding fluid becoming more rare. Hence the perpetual snow, and the piles of glaciers, that crown the summits of mountains, at whose base the orange and the citron bloom, and man pants in the fierce sultriness of a torrid climate.

But while the atmosphere may be considered generally as an aerial zone of the earth, the companion of the massy spheroid in its annual revolution round the sun, and rotating with it upon its axis, it has independent movements which present very complex phenomena, however clear the causes which put them in operation. The particles of air are constantly suffering displacement, and it is easy to conceive of various circumstances disturbing the dilatable and elastic fluid in which we live. A body in movement will communicate its motion to the adjoining particles, which may be sensibly propagated by them to a considerable distance; but this cause operates so slightly in the production of atmospheric currents that it might be entirely overlooked. It will be sufficient to state that some of the vast oceanic streams are supposed to produce a corresponding flow in the air. The varying attractions of the sun, moon, and planets on the atmosphere, will occasion tides in it analogous to those of the ocean, or an alteration in the heights of vertical columns of air, winds and currents arising from the resulting inequalities of horizontal pressure; but La Place has proved the action of this cause to be scarcely appreciable. The atmospheric agitations of which we are sensible, both the more violent and gentle, appear to proceed either from a change in the temperature of a portion of the air, or from a change in the quantity of water which it holds in a state of vapor. In both these cases a temporary destruction of the equilibrium subsisting between different parts of the atmosphere is produced, and its particles are set in motion to restore the balance. The effect of heat upon a volume of air is to rarefy and expand, to increase its bulk and diminish its density. When any portion, therefore, of the earth’s surface is more heated than the surrounding districts, the air there ascends and flows over the adjoining cooler and denser strata, causing an upper outward current, while the colder and denser fluid rushes toward the spot where the balance has been lost by expansion, and a lower inward current is produced. An easy experiment will illustrate this interchange. In a room warmed by a good fire, if a candle be held at the crevice between the door and the floor, an inward current will be observed from the exterior colder air, but near the ceiling, by the same means, an outward flow will be detected. In the other condition an addition of vapor to the atmosphere gives rise to a wind blowing on all sides away from the district of evaporation, while an abstraction of it by showers creates a partial vacuum, toward which the air rushes from all points of the compass. The diversity of the winds in power is principally owing to the different degrees of vigor with which these causes act.

The currents of the atmosphere display an endless variety in their velocity and force, from the zephyr, which scarcely stirs the leaves of the forest, to the gale under which its mightiest branches bend, and the hurricane which tears up its trees by the roots, and destroys the habitations of mankind. It has been observed that in the temperate zone the most violent winds occur, when neither the heat nor the cold common to such localities are at its maximum—that they generally extend over a considerable tract of country—and are accompanied by sudden and great falls in the mercury of the barometer. The latter circumstance attends the storms of the tropics, but they are often confined within narrower limits than the extra-tropical hurricanes. It was noticed by the superstitious as a coincidence, not without meaning, that at the time of Cromwell’s death the enchained winds were liberated, and went forth raving and howling through the land, uprooting the largest trees, and whirling them about like straws, and toppling down chimneys and turrets; but the same tempest, at the self-same hour, dashed the vessels of the Baltic sea even upon the strand, and buried Venetian argosies in the Adriatic, shivered the pines of Norway, and swept before it the cypresses of the Bosphorus—a similar war of the elements attending the termination of the earthly career of Cardinal Wolsey, Bonaparte, and George IV. Sometimes the upper regions of the atmosphere have been remarkably agitated, while the lower stratum of the air has been quite calm. Lunardi, on one occasion, traveled at the rate of seventy miles an hour in his balloon, while at Edinburgh; when he ascended, the air was quite tranquil, and continued so throughout his expedition. To ascertain the velocity and force of winds, a variety of experiments have been made with instruments constructed for the purpose. The following table contains some results obtained by Smeaton, inserted in a volume of the Philosophical Transactions:—

VELOCITY OF THE WIND.

Miles per Hour.Feet per Second.Perpendicular Force on one Square Foot, in Avoirdupois Pounds and Parts.Characteristics.
11·47.005Hardly perceptible.
22·93·020 }Just perceptible.
34·4·044 }
45·87·079 }Gentle, pleasant wind.
57·33·123 }
1014·67·492 }Brisk wind.
15221·107 }
2029·341·968 }Very brisk wind.
2536·373·075 }
3044·014·429 }High wind.
3551·346·027 }
4058·687·873 }Very high wind.
4566·019·963 }
5073·3512·300Storm.
6088·0217·715Great storm.
80117·3631·490Hurricane.
100147·749·200Hurricane carrying trees and buildings before it.

The currents of the atmosphere far surpass in velocity those of the rivers and the ocean, a gentle pleasant wind blowing at a rate equal to that of the mighty Father of Waters when in flood, but a hurricane will outstrip the swiftest locomotive in its speed. In speaking of the direction of currents of air and water, the indicating terms are employed in an inverse sense, an easterly wind signifying a breeze coming from that quarter, an easterly stream a flow of water toward it. Winds may be divided into three classes or genera, the Permanent, the Periodical, and the Variable; of which, the first excepted, there are many different species. We shall prefer, however, to consider them under their local recognized titles.