Atmospheric circulation

“ The distribution of cloud over the globe is intimately connected with the general atmospheric circulation, being greater where there are rising currents and less where there are downward currents. The reason, naturally, is that as descending air becomes warmer and therefore relatively drier, the clouds in it evaporate and disappear. ”

“ The atmosphere is a system of balances never at rest, unless in equilibrium, and never in equilibrium except at rare intervals, and then in limited and favored spots. This state of inequilibrium causes constant motion, currents, storms, winds and precipitation, whether of rain or snow, depending on temperature and position. Now the motor power of the movement in all this atmospheric mass is difference of temperature, the hot air rising and flowing to the poles, and the cold air of the poles descending and flowing to the equator. ”

“ Thus the heated current moves more easily and rapidly than the colder; and the latter, consequently, turns back a portion of what was going towards the poles, and adjusts the equilibrium of the atmosphere. I have already shown you the great importance of the circulation of the air in the economy of nature; and how, among the many offices of the atmosphere, it distributes moisture over the surface of the earth, making the barren places fruitful, and tempering the climates of different latitudes, fitting them as the abode of civilised man. ”

“ If we conceive the entire globe spotted with high and low areas, we may picture the air surging upward in the lows, flowing outward under the isothermal layer, descending 374in the highs, then flowing outward along the earth’s surface toward the lows in a continuous cycle. Thus, chiefly is maintained the vast and multifold circulation of the atmosphere over the entire world. ”

“ It is a reservoir of dynamical force for the winds—a regulator in the grand meteorological machinery of the earth. The heat which is transported by the vapours with which that sea loads its superincumbent air is the chief source of the motive power which gives to the winds of the southern hemisphere, as they move through their channels of circulation, their high speed, great regularity, and consistency of volume. ”

“ Thus the heated air makes its way from the equator to the poles in the upper regions of the atmosphere, while the cold air makes its way from the poles to the equator along the lower regions. Very often, too, aqueous vapour as well as air is carried up by means of the sun’s heat to the upper and colder atmospheric regions, and there deposited in the shape of rain, or hail, or snow, which ultimately finds its way back again to the earth, often displaying in its passage immense mechanical energy. ”

“ He will find that the centers of the Lows will follow closely the direction indicated by the isotherms that lead eastward out of their centers, and that they move across the country from the west in quite regular succession, and that the frequent changes from sunshine to clouds and from warm to cold are the result of the mixing of the air by these atmospheric eddies. ”

“ Winds are also only currents of air produced by unequal temperature in different places. The heated air ascends, and the colder fluid rushes in sometimes with great velocity to fill the space. “Land” and “sea” breezes are constant; the cool air blows in from the sea during the day, and as the land cools more rapidly at night, the breeze passes out again. When we touch upon Meteorology, we will have more to say respecting Air Currents and the various Atmospheric Phenomena. ”

“ On the other hand, the local contraction by cold during night is greatest over land, where the air is clear, dry, and calm, or nearly so, and least over the ocean, where the air is clouded, and loaded with moisture. As familiar illustrations of atmospheric movements result ing from local expansions by heat and contractions by cold, we may refer to the land and sea breezes, and what de pend upon exactly the same principle, the dry and rainy monsoons in different parts of the globe. ”

“ In these movements and their causes we have the general outline of our subject, within which we must now sketch the weather. The causes of atmospheric movement, which we have thus far considered, are the unequal distribution of the sun's heat, the absorption and precipitation of moisture, the direct and the inductive action of the earth's rotation and friction. If to these we should add the tidal action of the sun's and moon's attractions, we should perhaps complete the list of vera causae which are certainly known to exert a more or less general influence upon the atmosphere. ”

“ It must occur to everyone to adopt the intermediate view that, on the one hand, the local vertical circulation, with its clouds, rain, hail and snow, and evolution of latent heat, and, on the other hand the waves and whirls in the general circulation, mutually contribute toward our storms and fair weather. It only remains to allot to each its proper importance in any special case.
Undoubtedly aerial billows, and the clouds that must frequently accompany them, exist everywhere in the earth’s atmosphere. ”

“ These ceaseless motions, it must be clear, contribute greatly to the cooling of our bodies; but the effect is most marked in the open air, when we are exposed to the action of the winds. In our climate, the average velocity of the atmospheric currents is about ten feet a second, or seven miles an hour. Supposing that the extent of the surface of the body exposed to the currents is one square metre, there pass over a man walking out for an hour about eleven thousand cubic metres of fresh air. ”

“ The importance of atmospherical circulation.—If the ordinances of nature require a constant circulation and continual mixing up of the water in the sea, that it become not stagnant, and that it may be kept in a wholesome state for its inhabitants, and subserve properly the various offices required of it in the terrestrial economy, how much more imperative must they not be with the air? It is more liable to corruption than water ”

“ But as water warms so much more slowly than air, its circulation would be comparatively insensible were it not for the effects produced by the air currents upon the surface of the sea, combined, as in the case of the winds, with the effects of the rotation of the earth. ”

“ He considers also that the ascending winds produce descending ones, and that the rain produced in the higher regions brings air and vapor with it in its descent, and thus constitutes the lower atmospheric currents ”

“ When the upward diminution of temperature is less than the adiabatic rate, in the forced circulation of air crossing a mountain ridge, there occurs the dynamic heating which is observed in the case of the foehn winds. ”

“ These currents, both hot and cold, are affected by the diurnal rotation of the earth, the action being essentially the same as that for the winds. They exert so great an influence as conveyers of heat that they disturb the ordinary climate relation depending on the sun's position. ”

“ All atmospheric movements, in fact, however local or general their character may be, are either movements of direct assimilation by which the atmosphere is seeking, so to speak, to bring all its areas into like temperature and pressure with each other, or are disturbances involved and arising indirectly out of such acts of assimilation. ”

“ In general, the wind may be considered to be the result of a change of pressure and temperature in the atmosphere at the same level. The air of a warmer region, being lighter, ascends, and gives place to a current of wind from a colder region. These two currents—the higher and the lower—will continue to blow until there is equilibrium. ”

“ But the air thus warmed by the earth is in continual motion owing to changes of density. It rises up and flows off, owing to the greater weight of the cooler air which forces it up and takes its place; and thus heat can never accumulate in the atmosphere beyond a very moderate degree, the excessive sun-heat of the tropics being much of it carried away to the upper atmosphere and radiated into space. ”

“ To cross the Atlantic—three thousand miles—in forty-eight hours, would be attended with no other difficulty than the cost of the gas, and that of the return carriage of the empty balloon.
176 It is our exceptional meteorological position that has generated the popular expression “as uncertain as the wind.” We are in the very centre of the region of meteorological uncertainties, and cannot go far, either northward or southward, without entering a zone of greater atmospheric regularity, where the direction of the wind at a given season may be predicted with more reliability than at home. ”

“ At night the direction of the wind is reversed, that is, it blows from the land to the sea. Why? Because the land during the day retains its heat, while the water diffuses it. What is the result? The air on the land expands, becomes light. The air over the water being cool, it does not expand, and the result is that it presses toward the land. At night the land loses its heat more rapidly than the water, so that it is not long before the land is cooler than the water, and when this happens, the air over the land, which has become cooler, presses seaward. ”

“ If the concentration of heat and stagnation of the air are necessary to their formation, it would follow that they should not abound in those places where the air circulates freely, which is not less contrary to fact than the former supposition. ”

“ There is a general circulation in the aërial envelope covering the earth, caused by unequal heating of different parts of the surface.374 Heated air rises in the equatorial zone, and its place is filled by currents from the temperate and subtropical zones. ”

“ The atmosphere flows in a grand harmonious system of currents and counter-currents, with their corresponding eddies, just like the ocean; and the grand final results of its varied action are to equalise in some degree the temperatures of the world, to carry off and distribute moisture where it is required, to sweep away noxious vapours, and generally to ventilate the Earth and gladden the heart of man.
The primary cause of all wind is the combined action of heat and cold. ”

“ The whole atmosphere must have increased in quantity, because it appears by the barometer that there exists one-fifteenth part more air over us for many weeks together, which could not be thus accumulated by difference of temperature in respect to heat, or by any aerostatic laws at present known, or by any lunar influence. ”

“ For example, an increase of heat from the sun may produce increased evaporation from certain water areas of the earth's surface, and the moisture thus drawn up being carried away by the movements of the atmosphere, may be deposited elsewhere in the form of snow or rain. It is almost impossible without a much more intimate knowledge of the laws governing the movements in our atmosphere to say in advance what the effect of a slow and periodic change in the sun's heat will be. ”

“ Of the periodical variations of atmospheric pressure, the most marked is the daily variation, which in tropical and sub-tropical regions is one of the most regular of recurring phenomena. In higher latitudes the diurnal oscillation is masked by the frequent fluctuations to which the pressure is subjected. If, however, hourly observations be regularly made for some time, the hourly oscillation will become apparent. ”

“ The earth becomes warmed by the sun, and radiates the heat thus acquired back upon the air, which, expanding and becoming lighter, ascends to higher regions, while colder and denser currents rush in to occupy the vacated space. Two points are to be noted in connection with this rush of air which we call wind, viz., its direction and velocity or force. ”

“ It may be remarked, that the periodical winds are more regular at sea than on the land; and the reason of this is, that the land reflects into the atmosphere a much greater quantity of the sun’s rays than the water, therefore that part of the atmosphere which is over the land is more heated and rarefied than that which is over the sea. ”

“ B. Since so large a portion of the atmosphere, as is over the torrid zone, is in continued agitation, these agitations in an elastic fluid, which yields to the slightest impression, must extend every way, to a great distance; the air, therefore, in all climates, will suffer more or less perturbation, according to the situation of the country, the position of mountains, valleys, and a variety of other causes: hence it is easy to conceive, that almost every climate, must be liable to variable winds ”

“ Now, besides the mutual Affinity that Water has to almost all Kinds of Air, and to Floguiston; added to its Power of Absorption; and as the sea, particularly in Summer, also rivers and damp meadows are generally cooler than the Lands and Countries bordering on them; Currents of damp cool Air press forwards to supply the Defect or Vacancy caused by Heat, Rarefaction and Elevation of dry warm Air, which is necessarily, and almost constantly rising into the Atmosphere, from heated Lands, Plains, and gentle Eminences long shone on by the Sun. ”

“ Cold air comes from the side to take the place of hot air that rises.
When the rays of the sun heat an area of the earth, the air over such a place expands and becomes lighter, naturally rising, and the result of this is that the winds are produced by cool air moving in to take the place of the heated air. This cool air moves in from all directions. When such a thing happens at any point on the earth’s surface, it is known as a storm center, an area of low pressure. ”

“ Its vertical movements—how produced.—As a physical necessity, the vertical circulation of the air seems to be no less important than its horizontal movements, which we call wind. One begets the other. The wind, when it blows across parallels of latitude—as it always must, except at the equator, for it blows in arcs of great circles, and not in small ones [8] —creates a vertical circulation either by dragging down air from the upper regions (§ 224) , or by sloughing it off and forcing it up from the lower (§ 228) , according as the wind is approaching the pole or equator. ”