Convection current

“ Since air has weight, it compresses the lower layers near the earth, and hence as we ascend from the earth’s surface the air becomes continually lighter. Convection currents must, therefore, adjust themselves by the air expanding as it rises. But expansion of a gas always results in its cooling, as every one must have observed who has placed his finger in the air current which escapes from the open valve of a warm rubber tire. ”

“ 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 ”

“ 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. ”

“ Some materials will conduct the heat much quicker than others, but if we have a standard, such as the calorie, then the amount of heat transmitted and also the amount lost on the way may be measured.
Convection applies to the transmission of heat through liquids and gases. If heat is applied to the top or surface of a liquid, the lower part will not be affected by it. If the heat is applied below, then a movement of the gas or liquid begins to take place, the heated part moving to the top, and the cooler portions going down and thus setting up what are called convection currents. ”

“ Both in liquids and gases the effects of convection currents are so much greater than those of diffusion or conduction that the latter are very difficult to measure, and, except in special cases, comparatively unimportant as affecting the transference of heat. Owing to the difficulty of eliminating the effects of radiation and convection, the results obtained for the conductivities of liquids are somewhat discordant, and there is in most cases great uncertainty whether the conductivity increases or diminishes with rise of temperature. ”

“ Gigantic convection-currents throughout the sun's liquid mass are continually maintained by fluid, slightly cooled by radiation, falling down from the surface, and hotter fluid rushing up to take its place. ”

“ Indeed, a dense mass of gas presents to the conduction of heat, which represents momentum, a wall far more impenetrable than the surrounding glass, and the diffusion of heat is almost wholly brought about by convection-currents which rise from the heated surfaces. It will thus be seen that the great non-conducting power of air comes into play to prevent not only the transfer of momentum from the vanes to the glass, but also, almost entirely, any direct transfer to the surrounding mass of gas. ”

“ At higher pressures the effect of conduction was masked by convection currents. The question of the variation of conductivity with temperature is more difficult. If the effects depended merely on the velocity of translation of the molecules, both conductivity and viscosity should increase directly as the square root of the absolute temperature ”

“ Convection currents between upper and lower air strata, it is suggested by Koppen, explain diurnal variations in wind velocity and direction. At low stations the maximum velocity occurs at the time of the highest temperature, while at high stations the reverse obtains. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Conduction of Heat is the passage of heat from one particle to another, whether it be of a gas or solid. It is an extremely slow process when air is concerned, and may be practically ignored.
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Convection of Heat is the process by which a gas or liquid actually carries the heat in itself from one part to another. The heated particles are relatively lighter, and ascend to the higher parts of a room, while colder and heavier particles descend, and are subjected to the same process. Heat can be carried by convection only by gases and liquids. ”

“ Water is clearly, therefore, the proper substance to employ for the dissipation of the heat generated, although it is strictly speaking almost devoid of heat-conducting power; for not only is the specific heat of water much greater than that of any other material present, but it possesses in a high degree the faculty of absorbing heat throughout its mass, by virtue of the action known as convection, provided that heat is communicated to it at or near the bottom, and not too near its upper surface. ”

“ 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. ”

“ The forenoon maximum is conceived to be due to the rapidly increasing temperature, and the rapid evaporation owing to the great dryness of the air at this time of the day, and to the increased elasticity of the lowermost stratum of air which results therefrom, until a steady ascending current has set in. As the day advances, the vapour becomes more equally diffused upwards through the air, an ascending current, more or less strong and steady, is set in motion, a diminution of elasticity follows, and the pressure falls to the afternoon minimum. ”

“ Q. What is meant by “convection of hot and cold currents?”
A. The air (which is heated by the surface of the earth) ascends, warming the air through which it passes. Other air (being warmed in a similar way) also ascends, carrying heat; till all the air is made hot.
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Q. Is the air made cold in a similar way?
A. Yes. The air resting on the earth is made cold by contact: this cold air makes the air above it cold; and cold currents or winds shake the whole together, till all becomes of one temperature. ”

“ Communication of Heat.—Heat has a constant tendency to an equilibrium. If therefore any warm substance be in the neighborhood of one which has less heat, a flow of heat from the former toward the latter takes place. Now this communication of heat occurs in three different ways, called Convection, Conduction, and Radiation. I will speak of each of these separately.
296. Convection.—This mode of diffusion of heat is in operation in those substances whose particles are movable among each other—viz., liquids and aeriform substances. ”

“ Another principle of heat is Convection, by which water, air, and gases are warmed. This is, literally, the process of conveying heat from one portion of a fluid body to another by currents resulting from changes of temperature. It is secured by bringing one portion of a liquid or gas into contact with a heated surface, whereby it becomes lighter and expanded in volume. In consequence, the cooler and heavier particles above pressing downward, the lighter ones rise upward, when the former, being heated, rise in their turn, and give place to others again descending from above. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ What is this mode of carrying heat called?
It is called Convection.
What is needed so as to heat a substance by convection?
Its particles must be able to move about freely.
Why is the air easily heated by convection?
Because the particles of the air move about freely.
Why is a liquid easily heated by convection?
Because its particles move about freely.
Why cannot a solid be heated by convection?
Because its particles cannot move.
When water near the bottom of a vessel is heated, what becomes of it?
It rises to the surface. ”

“ If currents of air are continually removing the saturated atmosphere from above the surface of a liquid, evaporation takes place most rapidly, since new portions of air, capable of absorbing moisture, are presented to it. ”

“ A current of cold air meeting a current of warm, moist air in its course may condense a considerable portion of the moisture into clouds and rain, and this condensation will go on as long as the currents continue to meet. ”

“ And since the weight of the vapors which ascend must continually equal that of the products of condensation which are falling, it is further evident that the upward currents, rushing through contracted channels, must move with enormous velocity, and therefore, of course, that the pressure and temperature must rapidly increase from the free surface downward. ”

“ But we have pointed out that if the rate of cooling as we ascend in the still air is less than the rate at which an ascending current will be dynamically cooled, such a rising current will come to rest. If, on the other hand, the rate of cooling in the ascending current be less than in the still air, the equilibrium will be unstable, and a violent uprush will result. ”

“ A few words will indicate the magnitude of the forces which are called into silent and comparatively unobserved operation in the atmosphere by the sun's heat in the production and recondensation of aqueous vapor. It has, as I noticed, been estimated that on the average five feet of water falls annually as rain over the whole earth. Supposing that condensation takes place at an average height of 3,000 feet above the surface, the force of evaporation must be equivalent to a power capable of lifting five feet of water, over the whole surface of the globe, 3,000 feet during the year. ”

“ 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. ”

“ When we consider that in converting a given volume—say, one cubic metre—of water into vapour, enough heat is consumed to lower about 1,650,000 cubic metres of air by 1° C. in temperature, and that the same amount of heat is liberated when the vapour so produced returns to the liquid state, we perceive how powerfully the ordinary processes of evaporation and condensation must affect the temperature of the air. ”