Atmospheric convection

“ The gentlest summer breezes and the fiercest blasts of winter are produced by the unequal heating of air. We have seen that the air nearest to a stove or hot object becomes hotter than the adjacent air, that it tends to expand and is replaced and pushed upward and outward by colder, heavier air falling downward. We have learned also that the moving liquid or gas carries with it heat which it gradually gives out to surrounding bodies.
When a liquid or a gas moves away from a hot object, carrying heat with it, the process is called convection. ”

“ Sea breezes are also the result of convection. The land, under the heat of the day's sunshine, becomes of a high temperature, and the expanded air on its surface flies away towards the ocean. As the sun goes down, the earth cools again, and the air flies back to find its equilibrium. ”

“ This diversity of heat distribution is partly overcome by the agency of the winds and waters, particularly the latter, since the great ocean currents carry vast supplies of heat from the torrid zone toward the poles, and drive far backward the boundaries of the realm of frost. The agency of the air in this heat convection is of less importance. The anti-trade winds move through the upper atmosphere, and lose their heat before descending to the earth; but surface winds from the tropics convey a considerable share of the torrid heats to the colder zones. ”

“ The combination of many conditions, probably, is necessary, such as a peculiar distribution of heat and moisture and atmospheric movements; though the immediate cause of the fall of rain is doubtless the rising, and consequent expansion and cooling, of the saturated air.
The winds that blow hither and thither, vainly striving to restore equilibrium to the atmosphere, burden themselves with the moisture they absorb from the seas; and this moisture absorbs their heat, retards their motion, and slowly modifies the forces which impel them. ”

“ Conditions are then favourable for inflowing damp winds from the adjacent oceans; there is the best opportunity for convection; thunder-showers readily develop on the hot afternoons; the capacity of the air for water vapour is greatest. ”

“ With the same degree of humidity, cold air is denser and heavier than warm air, and by its contraction lowering the "column" of atmosphere—the temperature of which is reduced—a flow toward the upper part of the column increases the actual mass of air pressing upon a particular place. Elevation of a locality above the general level of the earth reduces atmospheric pressure, sensibly as well as measurably. ”

“ It may be defined as being in convective equilibrium while in such a condition. Now, if a body of air at the top be cooled a few degrees, it contracts and becomes relatively heavier. Because it is heavier, it begins to drop. ”

“ 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 ”

“ But the illustra tion of the principle on the broadest scale is the system of atmospheric circulation known as the equatorial and polar currents of the atmosphere, which originate in the unequal heating by the sun of the equatorial, temperate, and polar regions. The other principal motive force in atmospheric circula tion depends on the aqueous vapour. ”

“ The heated air from the equator flows at first as an upper current towards the poles, but as it gradually loses its high temperature, it becomes mixed with the currents setting from the poles towards the equator, causing the atmospheric disturbances and variable winds characteristic of the cooler temperate zones. As a rule, bodies of air of different temperatures do not very quickly mix, but tend to arrange themselves in layers or strata in which masses of unequal temperature are superposed. ”

“ When any portion of the earth’s surface is periodically more heated above its normal temperature, or average for the year, than the neighboring region, the resulting abnormal temperature gradient causes a periodic surface wind tending toward the excessively heated place, and a counter wind above. That is, the cooler and heavier column of air sinking and uplifting the lighter, results in a lowering of the common center of gravity of the two 386columns of air, and thus furnishes the driving power of the wind. ”

“ With a persistent inversion of temperature there is sometimes an increasing absolute humidity.
SUMMARY The problem is many sided and we must consider the motion of the air vertically as well as horizontally. Air gains and loses heat chiefly by convection, and any gain or loss by conduction may be neglected. The plant gains heat by convection, radiation and perhaps by conduction of an internal rather than surface character. The ground gains and loses heat chiefly by radiation. ”

“ This heat is relative, and though it be imparted to air far below the freezing-point, raising its temperature only a few degrees, its expansive power for that change is as great when those few degrees are low down as it is when they are high up on the scale. If such condensation of vapour do take place, then liberation of heat and expansion of air must follow, and consequently the oblateness of the atmospherical covering of our planet will be altered ”

“ When energetic motions raise the ascending air to such a height that the temperature, cooled dynamically in ascending, goes below the dew-point, then the great store of latent heat thereby set free becomes, it is assumed, the main source of energy in maintaining the upward convectional movement. The subsidiary causes are attributed to the diminution of pressure on the collapse of the vapor, and also to the direct absorption of the sun's heat at the upper cloud surface. ”

“ The proximity of large masses of snow has not the influence I should have expected in lowering the temperature of the surrounding atmosphere, partly no doubt because of the more rapid condensation of vapours which it effects, and partly because of the free circulation of the currents around it. ”

“ There are many causes to determine the condensation of the vapour, which the atmosphere always holds in larger or smaller quantities, but cold is the chief agent, for when a current of air passes over seas or rivers, or the damp surface of the earth, it becomes loaded with moisture, and being capable of holding only a certain quantity (less when cold than hot) , it follows that warm air when saturated with moisture must let some of it fall when it becomes cooled. ”

“ An immense quantity of sun-heat is also used up in evaporating water, and the vapour thus produced is conveyed by the aerial currents to distant countries, where, on being condensed into rain, it gives up much of this heat to the earth and atmosphere. ”

“ Even when there is no sensible breeze, the unequal heating of the surface causes ascending and descending currents, which lose or gain heat by expansion or contraction. More commonly winds are experienced which, by impinging on the inclined surfaces, force bodies of air to higher elevations, and thereby directly cause a fall of temperature. ”

“ In both cases, certain atmospheric gases—notably water vapor—absorb a great deal more heat than others.
124 The first step in the production of a wind is a difference in temperature between two parts of the earth’s surface, and hence of the overlying air. Such contrasts of temperature always exist, both locally and on a large scale. The high sun of the equatorial regions heats the earth much more strongly than the low sun of high latitudes; a water surface has a more equable temperature than an adjacent land surface ”

“ As already explained, when a column of air ascends it cools by expansion, and tends to precipitate its water content as cloud or rain; and conversely, when the air sinks it heats by compression, thus acquiring greater moisture capacity and tending to clarify. As a consequence, the areas of low pressure and a rising atmosphere are usually marked by clouds and rainfall, while the areas of high pressure and falling atmosphere are marked by clear, or clearing weather. In the low, damp areas, then, the air feels heavy while it is really light ”

“ Compounding, then, the northward motion of the vapour with the earth's axial rotation, we track our fugitive through the higher atmospheric regions, obliquely across the Atlantic Ocean to Western Europe, and on to our familiar Alps. Here another wonderful metamorphosis occurs. Floating on the cold calm air, and in presence of the cold firmament, the vapour condenses, not only to particles of water, but to particles of crystalline water. These coalesce to stars of snow, which fall upon the mountains in forms so exquisite that, when first seen, they never fail to excite rapture. ”

“ 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. ”

“ The variations of this density with pressure, temperature, moisture and gravity are given in the Smithsonian meteorological tables, and give rise to all the movements of the atmosphere; they are, therefore, of fundamental importance to dynamic meteorology.
Expansion.—The air expands with heat, and the expansion of aqueous vapour is so nearly the same as that of dry air that the same coefficient may be used for the complex atmosphere itself. ”

“ In essence, heat creates the different densities that lead to currents and countercurrents. Evaporation, which is nil in the High Arctic regions and very active in equatorial zones, brings about a constant interchange of tropical and polar waters. What’s more, I’ve detected those falling and rising currents that make up the ocean’s true breathing. I’ve seen a molecule of salt water heat up at the surface, sink into the depths, reach maximum density at -2 degrees centigrade, then cool off, grow lighter, and rise again. ”

“ If a dry atmosphere is of uniform temperature and pressure at various levels, but has a vertical temperature gradient a little greater than the normal cooling of an ascending gas, a portion of air started upward in any casual way becomes warmer than its lateral environment, and hence continues to rise until the unstable condition due to abnormal temperature gradient ceases. ”

“ 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. ”

“ Differences in temperature, changing the specific gravity of the air, are the cause of the general circulation of the atmosphere about the earth, modified by the rotation of the earth; likewise the local circulation between land and water is caused by the different quantities of heat radiated by the two widely differing forms of matter, each attaining to a different temperature under the influence of the same solar radiation; and the inflow of winds to the cyclone and the outflow from the anti-cyclone are due to the same forces that cause the general and the local circulations. ”

“ Thus one genius, observing the deposit of water on the outside of an ice pitcher in a warm room, proposed to set up a barrier packed with ice in the path of moisture-bearing winds. Another plan occasionally suggested is to sprinkle the atmosphere aloft with liquid air or liquid carbon dioxide, in order to lower the temperature, by the rapid evaporation of these substances, below the point of condensation. A third proposal is to create a local updraft of air by means of powerful fans or blowers, thus imitating the convectional process by which clouds and rain are formed in nature. ”

“ The uses of the atmosphere are many. It is the medium for regulating the dispersion of watery vapours over the earth. If there were no atmosphere, and that, as now, the equatorial climes were hot and the poles cold, evaporation would be continually going on at the equator, and condensation in the colder regions. The sky of the tropical climes would be perpetually cloudless, whilst in the temperate and arctic zones we should have constant rain and snow. ”

“ According to the theory, nothing is required for the production of rain besides the mixture of portions of the atmosphere with humidity, and of mixing the parts that are in different degrees of heat. But we have seen the causes of saturating every portion of the atmosphere with humidity and of mixing the parts which are in different degrees of heat. Consequently, over all the surface of the globe there should happen occasionally rain and evaporation, more or less ”

“ Both Espy and Hann have abundantly shown that the formation and downfall of rain do not produce any low barometric pressure unless they produce a whirling action of the wind—that, in fact, the latent heat evolved by the condensation of vapour into rain may so warm up the cloud as to produce a temporary rise in pressure even at the surface of the ground, due to the outward push produced by the sudden expansion of the cloud. ”