Convection

“ 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. ”

“ 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. ”

“ The cooler air from the land is carried over the water by convection currents, flows in and under the warm air, is heated by the water and rises, to be replaced in turn by the further flow of cold air from the land. ”

“ Water is often heated in a reservoir at a distance from the source of heat by the circulation produced in pipes leading to the source of heat and back. The winds and the great currents of the ocean are convection currents. ”

“ 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. ”

“ The ocean and the Great Lakes are the source from which the air gets its water. It rises into the air in the form of vapor, that is, vapor rising from the surface of the water, and the wind distributes it over the land. Condensation turns it into clouds, and when it is over-saturated, or rather, when the temperature drops and the air is unable to retain any more water, then it forms into drops of water and falls as rain. When the clouds get into the air, below the freezing point of the water, the drops of water are changed into ice crystals or snow flakes. ”

“ It is not enough that we have the evaporation of moisture that will ascend into the higher regions of the air and there be condensed into cloud and possibly rain, but we must have the means for distributing these conditions over a large area, and for this purpose we have the phenomenon of wind. Why the winds blow can be accounted for to a certain extent,—we might say to a large extent,—but there yet remain many unsolved problems relating to wind and weather. Of the phenomena of wind we will speak more fully in a future chapter. ”

“ Misty clouds, forming or hanging on heights, show wind and rain coming, if they remain, increase, or descend; if they rise or disperse, the weather will improve or become fine.
RAIN. The atmosphere at a given temperature is capable of retaining only a given quantity of aqueous vapour, invisibly diffused through it, at which temperature it is said to be saturated. Should the temperature from any cause be lowered, the aqueous vapour at once becomes visible in the form of either cloud, dew, rain, snow, or hail. ”

“ The air from B—"
"Do we begin all over again now?"
"More or less, yes."
"So that at this place, A, it's always raining or just going to rain?"
"Yes, if it starts by being hot there, as it did just now, I suppose it is."
"What happens if it starts by being cold?"
"It rains. I've explained that. The cold air can't contain so much moisture—"
"Don't begin that again. What about B? Is it any good going there? We had frightfully high pressure there at one time." ”

“ It follows, therefore, that wherever water exists in considerable masses, and has a temperature nearly equal to that of the land, or only a few degrees below it, and above 45° at sunset, its surface during the night, in calm and clear weather, will be warmer than that of the contiguous land; and the air above the land will necessarily be colder than that above the water; and when they both contain their due proportion of aqueous vapour, and the situation of the ground is such as to permit the cold air from the land to mix with the warmer air above the water, mist or fog will be the result ”

“ 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 third circumstance, shewing that heat is the principal cause of evaporation, is, that some of the steam becomes again condensed when any part of the heat is withdrawn. Thus when warmer south-west winds replete with moisture succeed the colder north-east winds all bodies that are dense and substantial, as stone walls, brick floors, &c. absorb some of the heat from the passing air, and its moisture becomes precipitated on them, while the north-east winds become warmer on their arrival in this latitude, and are thence disposed to take up more moisture, and are termed drying winds. ”

“ The water in all these rivers, lakes, and oceans is constantly rising into the air in what is called moisture or vapor. We can not see this moisture, neither can we see the air.
If the air is cold, moisture does not rise rapidly; but, as the air becomes heated, it takes up more moisture, so that the more heat there is in the air, the more moisture rises. ”

“ The thirteen inches of water are of course supplied by evaporation from the sea, and are carried back to the land through the atmosphere. Vapour is perpetually rising from the ocean, and is condensed in the hills and high lands, and through their pores and crevices descends, till it is deflected, collected, and conducted out to the bay, by some stratum or channel which is watertight. The condensation which takes place in the higher parts of the country, may easily be recognised in the mists and rains which are the frequent occupants of such regions. ”

“ The warmer the atmosphere, the greater its capacity to hold moisture. The heated, thirsty air of the tropics drinks up the water of the ocean, and bears it away to the colder regions, where, through condensation by cold, it becomes visible as a cloud ”

“ The average precipitation equals the evaporation. The heat that is absorbed and evolved in the process of lifting up and letting down such a body of water has a powerful influence upon climates as well as upon winds; it is the chief source of that motive power which gives to the winds their force, to the storm its violence. Six hundred and twenty pounds of aqueous vapour occupy in the open air the space which it takes one thousand pounds of dry air at the same temperature to fill. ”

“ This condensation is dew. It always forms when the conditions are right, summer and winter. In cold weather we call it frost. It has been stated in a former chapter on evaporation that the capacity of the air for holding moisture in a transparent form depends upon its temperature. If the temperature is at the freezing point it will contain the 160th part of the atmosphere's own weight as aqueous vapor. If it is 60 degrees Fahrenheit the air will retain six grains of transparent moisture to the square foot of air, while at 80 degrees it will contain nearly eleven grains. ”

“ But it is otherwise in tropical regions where the body and the air bathing it differ much less in temperature, and where, indeed, the air is often higher in temperature than the body. Here the rate of evaporation depends almost wholly on the quantity of surrounding vapor. If the air is hot and moist, the escape of water through the skin and lungs is greatly hindered; while it is greatly facilitated if the air is hot and dry. ”

“ Evaporation may now be considered, and is distinguished from Ebullition by the production of vapour on the surface of liquids, the latter term signifying the formation of vapour in the body of the liquid. Evaporation takes place at all temperatures, and from every liquid surface exposed to the air. We know what we call a “drying wind.” The air in fresh layers continually passing over the wet ground, takes up the moisture; like the east wind, for instance, which has great capabilities of that nature. ”

“ The amount, taken by itself, of vapour, moisture, wet, rain, or snow, in the wind, or current of air (direction and strength remaining the same) seems to cause a change amounting, in an extreme case, to about half an inch.
3rd. The strength or force alone of wind, from any quarter (moisture and direction being unchanged) is preceded, and foretold, or accompanied by a fall or rise, according as the strength will be greater or less, ranging, in an extreme case, to more than 2 inches. ”

“ The conditions which surround the liquefaction of ice and snow, the evaporation of water, and the condensation of the water vapor of the air are fundamental factors in the science of weather. The distribution of precipitation—that is, rain, snow, hail, and the floating forms of fog and cloud—affect the habitability of the earth and human activities to a very great degree. ”

“ Felling of forests.—The felling of forests has been attended, in many countries, by a diminution of rain, as in Barbadoes and Jamaica.998 For in tropical countries, where the quantity of aqueous vapor in the atmosphere is great, but where, on the other hand, the direct rays of the sun are most powerful, any impediment to the free circulation of air, or any screen which shades the earth from the solar rays, becomes a source of humidity; and wherever dampness and cold have begun to be generated by such causes, the condensation of vapor continues. ”

“ The ordinary account given of their structure is, I believe, that the moisture raised from the earth by the sun’s heat becomes visible by condensation at a certain height in the colder air, that the level of the condensing point is that of the cloud’s base, and that above it, the heaps are pushed up higher and higher as more vapor accumulates, till, towards evening, the supply beneath ceases; and at sunset, the fall of dew enables the surrounding 123 atmosphere to absorb and melt them away. ”

“ Our westerly and south-westerly winds, passing over sun-bathed seas, collect in their courses the vapour of warmer climates, and when this vapour, coming into contact with the cooler air of more northerly latitudes, is again condensed into water, a certain amount of the heat thus collected is set free, and raises the temperature of the air, of the rain itself, and of the land on which it falls. ”

“ Cold air, as well as cold ground, condenses the vapor of warmer air. If you watch what goes on along the course of a river, you will often see examples of this kind of condensation. The ground on either side of the river parts with its heat after sundown sooner than the river itself does, and consequently cools the air above it more than the air above the river is cooled. ”

“ Perhaps the most important work of the atmosphere as a dynamic agent lies in its function as the medium for the circulation and distribution51 of water. Atmospheric temperature is the primary factor governing evaporation, an important factor in the circulation of the vapor after it is formed, and controls its condensation and precipitation. ”

“ It is undoubtedly the case that mingled with these little bubbles of moisture there are fine particles of solid water hanging on and carried along with them. Undoubtedly this is true; at least just before the final act of condensation takes place; and when the little hollow spherules collapse they are gathered together in drops of water larger or smaller according to the rapidity of condensation. [Pg 70] There is probably another power at work to prevent the too ready precipitation of moisture when condensed, and that is the wind. ”

“ After sunset, when the sky is clear, you know that the grass gets wet with dew. In the morning you may see mists hanging over woods, and streams, and hills, and gradually melting away as the sun mounts in the sky. At all times of the year you may watch how clouds form and dissolve, and form again, ever changing their size and shape as they move through the air. Now these are all examples of the condensation of vapor. ”

“ 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. ”

“ Again, the most copious condensation both of dew and rain is greatly influenced by vegetation and especially by forests, and also by the presence of hills and mountains, and is therefore greater on land than on the ocean; while evaporation is much greater on the ocean, both on account of the less amount of cloudy weather and because the air is more constantly in motion. ”

“ Evaporometer.—The moisture in the atmosphere comes from the surface of the earth or ocean by evaporation, a process which goes on continually, replacing the moisture that is precipitated as rain, hail, snow and dew, and maintaining the total quantity of the moisture in the atmosphere at a very uniform figure. ”

“ Thus the water, which by parting with its heat of liquefaction, has expended its meteorological energy in giving dynamical force to the air, is like the exhausted steam of the engine; it has exerted its power and become inert. ”

“ The precipitation of the water from the air is primarily due to the cooling to which it is subjected as it rises in the atmosphere. Over the sea the ascent is accomplished by the simple diffusion of the vapour or by the uprise through the aërial shaft, such as that near the equator or over the centres of the whirling storms. ”