Atmospheric convection current

“ Wind, as we know, is air in motion, and we feel colder on a windy than on a calm day, although the thermometer may register the same degree. Nevertheless, air is a bad conductor of heat: compare it, for instance, with water, which scalds at 150° Fahr.; whereas a man in a Turkish bath can without danger enter a room, the air of which is heated to 200°. Both air and liquids have heat imparted to them by a process called convection, in which heated particles, as they become lighter by expansion, rise, forming an upward current, while colder particles, being heavier, sink ”

“ In a paper on the atmospheric changes that produce rain and wind, by Thomas Hopkins, Esq., in the Geographical Journal, it is shown that, when vapour is condensed and falls in rain, a partial vacuum is formed, and that heavier air presses in as a current of wind. Thus the vacuum arising from the great precipitation at the tropics causes the polar winds to descend from the upper regions of the atmosphere and blow along the surface to the equator as trade winds to supply the place of the hot currents that are continually raising them into the higher regions. ”

“ In dealing with common cumulus, it has been pointed out that the cessation of the uprising convection currents which determines the maximum height to which the cloud will grow is due to the rate of cooling within the ascending column being greater than the rate of cooling outside it. It follows that when the ascending current has reached a certain height it will, as a whole, be just as heavy as an equal column outside. Ascent must then cease. ”

“ The movements of the waters of the ocean, it will be remembered, are largely 178 under the control of the winds, so that the essential factor in the transfer of heat from place to place is atmospheric circulation. The primary causes of movements in the air, as is a matter of current knowledge, are the differences that arise in temperature at various localities. In regions where the air becomes more highly heated than over adjacent areas it expands, and in consequence becomes lighter, volume for volume, than the air over neighbouring areas, and is forced upward and overflows aloft. ”

“ 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. ”

“ 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. ”

“ Thus during the hot period a moist wind blows landward; during the cold period a dry wind blows seaward. If the land has vast and lofty slopes the uprush of air during the hot period and the downrush during the cool period may be very powerful. The currents so produced by the aggregate of local agencies, including the deviation caused by the earth’s rotation, combine with the general circulation of the atmosphere to form the actual wind of the place. Thus the periodic current may conspire with the general circulation, or oppose it ”

“ The interactions occur so often that the light energy takes thousands of years to escape from the Sun.
CONVECTIVE ZONE
The convective layer ranges from 500,000 km to almost its visible surface. Here, energy does not travel by radiation, but by convection. The cooling-down process happens differently from the previous layer because the Sun’s material moves as heat “bubbles” (convective cells) —which comes from the Tachocline and expand, making them “float” or moving upwards — carrying this innermost part heat to the outermost ones. ”

“ 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. ”

“ Also of a low barometer.—But, be that as it may, the air which these vapour-bearing winds—vapour-bearing because they blow over such an immense tract of ocean—pour into this stopping-place has to ascend and flow off as an upper current, to make room for that which is continually flowing in below. In ascending it expands and grows cool, and, as it grows cool, condensation of its vapour commences; with this, vast quantities of latent heat, which converted the water out at sea into vapour for these winds, are set free in the upper air. ”

“ Because of the vertical rays of the sun in equatorial regions, the air is not only warmed to a much higher temperature, but it is also warmed more quickly than in higher latitudes. Being expanded by the greater warmth, it becomes specifically lighter and is pushed upward by the denser cold air which flows in to take its place. The updraught of air flows poleward in upper currents, until it is chilled and descends to the surface again. A part of the descending current continues poleward but a considerable part flows back to tropical regions as a surface wind. ”

“ Now, in a climate such as our own, where the lower regions of the air contain large quantities of water vapour, any considerable rise brings about more or less condensation, and that condensation is attended by a liberation of very large quantities of heat, which retard cooling in the ascending current, and so facilitate the production of instability. ”

“ 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 formation of a cloud tells us not only that vapor is being condensed in the air, but that warm and cool currents have encountered each other.
In the horizontal movement, the cold and warm currents overlap each other, the vapor-laden warm air from the equator rising over the colder current. A consequence is, the warm air ascends until its waves reach an elevation where condensation takes place along their crests, producing flecks and bars of cloud-caps of the aerial waves. ”

“ The capacity of space to hold moisture increases with its temperature, and hence any lowering of this temperature will reduce the capacity. If lowered sufficiently, the point of complete saturation will be reached and further cooling must result in precipitation. Hence, anything which forces an air current to rise into more rarefied zones above, will lower the pressure upon it and so bring about a cooling effect in which no heat is abstracted. ”

“ 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. ”

“ It is produced by the obliquely upward movement of the tropical current over the polar current, and by its rising more or less vertically in the vicinity of contact, after its horizontal progress northward has been checked by encountering the polar current. This oblique and upward movement of the tropical current diminishes the atmospheric pressure there, as shown by the barometer, and produces that depressing calm which is always felt by persons in any locality where this meeting of currents takes place, or over which its area moves or oscillates during the continuance of a storm. ”

“ 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. ”

“ A similar change is supposed to take place when two currents of air having different temperatures, but both saturated with vapour, are mingled together; an excess of vapour is set free, which forms a cloud or falls down as rain. If the currents continue to mingle uniformly, “the clouds soon spread in all directions, so as to occupy the whole horizon; while the additional moisture, incessantly brought by the warmer current, keeps up a constant supply for condensation, and produces a great and continued deposition of moisture in the form of rain. ”

“ 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. ”

“ It depends on the rapidity with which convection currents can supply heat from the interior to replace that radiated, and on a number of other nicely balanced circumstances which cannot well be calculated. ”

“ As already pointed out, the combined effect will be nearly proportional to the excess of temperature in any given case provided that the excess is small, but it is not necessarily proportional to the extent of surface exposed except in the case of pure radiation. The rate of loss by convection and conduction varies greatly with the form of the surface, and, unless the enclosure is very large compared with the cooling body, the effect depends also on the size and form of the enclosure. Heat is necessarily communicated from the cooling body to the layer of gas in contact with it by conduction. ”

“ I believe that, in reflecting on the general laws of the equilibrium of the gaseous masses constituting our atmosphere, we may find, in the interruption of the current that blows from an homonymous pole, in the want of the renewal of air in the torrid zone, and in the continued action of an ascending humid current, a very simple cause of the coincidence of these phenomena. ”

“ As the atmosphere, however, becomes less dense, the diffusion of heat by convection diminishes, and that by molecular motion (conduction) increases until the last greatly predominates. When, now, the exhaustion reaches so great a degree that the heat, or momentum, is rapidly transferred from the heater to the cooler by an exaggeration, or, possibly, a modification of the mode of action we call conduction, then we have the reaction on which the motion of the radiometer-wheel depends. ”

“ 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. ”

“ When in summer a cool current is moving southward, it encounters the warm equatorial or tropical current, which again glides upward and over it, and forms horizontal bands of stratus clouds along the upper line of contact, as in winter storms; but, in addition, the denser cold air from the north, moving with more momentum, will lift up the warm and saturated air from the tropics, and its moisture will be condensed into masses of cumulus clouds banked up against the top of the cold current, and arranged over the horizontal stratus clouds. ”

“ The surface strata are radiating heat into surrounding space. To maintain the supply, it is imperative that convection currents should carry the cooled masses down into the interior, and bring corresponding hot masses up to the surface. These currents make the sun a very tempestuous body. Further, the outrushing materials must acquire the higher rotational speeds of the surface strata, and the inrushing must lose their tangential momentum; and these can scarcely be ineffective factors in the sun's circulatory system. ”

“ Why such greatly “undercooled” drops should sometimes occur in the atmosphere when at other times, with higher temperatures, atmospheric moisture takes the form of ice is not at all clear.
There are several ways in which the free air may be cooled to the point at which condensation occurs. The commonest is dynamic cooling, due to the rise of a mass of moist air and its expansion under the reduced pressure that prevails at the higher levels. This process is beautifully illustrated in the formation of the roundish masses of fleecy cloud known as cumulus, on a warm summer day. ”