Heat wave

“ Before this equality of temperatures, however, is reached, the hotter body will radiate more heat waves than it absorbs, while the colder body will absorb more heat waves than it emits. All bodies, whatever their temperature, are incessantly radiating heat waves. This may be proved experimentally with proper apparatus, as for example with an instrument known as the thermopile. When, however, the total heat waves radiated out by a body are less than it absorbs, the body gets gradually colder, and the temperature decreases. ”

“ On a cold day in winter, when the sun is shining brightly, a room with spacious windows may become as warm as though heated by a furnace, simply by the capacity of the glass in the windows to transmit [56] the heat waves of the sun without considerable absorption, and at the same time prevent the escape of the longer heat waves that are radiated from the interior walls of the room. This capacity of matter to transmit heat waves without absorption is called diathermancy. The clear atmosphere is an exceedingly good transmitter, and rock salt is one of the best of all solids. ”

“ 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. ”

“ 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. ”

“ Our argument, however, with regard to the increase of heat as we look back is the very opposite of this. It is the essence of a cooling body to lose heat more rapidly in proportion as its temperature is greater. Thus though the one-thousandth of a degree may be all the fall of temperature that our earth now experiences in a twelvemonth, yet in those glowing days when the surface was heated to incandescence, the loss of heat per annum must have been immensely greater than it is now. ”

“ In this way does the capacity of air for water vapor increase. Thus it is seen that the relative humidity of the air may increase during the cooling of nighttime without the addition of any vapor of water, and, in fact, with a decrease. The increase of relative humidity after nightfall is greater in the country than in the city, where the presence of pavements and brick buildings retards the loss of heat. ”

“ And then, as previously stated, on such nights there is what is called temperature inversion, and the temperature actually rises with the first few feet of ascent, and the heated air soon reaches air of its own temperature, when no further ascent occurs. When the air forty feet from the ground is ten degrees warmer than it is around and in contact with vegetation, as often occurs on frosty nights, the heat from the [97] fires is nearly all expended in raising the temperature of the air within this forty feet. ”

“ The heat of vaporization of water is important in natural processes also. Many living creatures, including man, depend on the cooling effect of water evaporating from the skin or lungs to regulate their internal body temperature. The evaporation of water into vapor and the condensation of the vapor back into water in the form of clouds or precipitation has a major effect on our weather and climate. Because of the large volumes of water and vapor involved, enormous amounts of thermal energy can be released or stored. ”

“ The burning of coal and wood produces heat, and the heat thus given out warms the air. Hence it is by the giving off or radiation of the heat from some burning substance that the air of our houses is made warmer than the air outside.
Now, it is really by radiation from a heated body that the air outside gets its heat. In summer, this air is sometimes far hotter than is usual in dwelling-houses in winter. All this heat comes from the sun, which is an enormous hot mass, continually sending out heat in all directions. ”

“ The action of the absorbed heat in reference to cloud-formation is directly opposite to that of the transmitted solar heat, as this reaching the surface of the earth evaporates the superficial water, and thereby produces the material of clouds. On the other hand, the heat which is absorbed by the air increases its vapor-holding capacity, and thus prevents the formation of clouds, or even effects the dissolution of clouds already formed. ”

“ 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. ”

“ 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. ”

“ The meteorological power of latent heat.—This explanation of the calm places and of the movements of the low austral barometer shows, comparatively speaking, how much the latent heat of vapour, and how little the direct heat of the sun has to do in causing the air to rise up and flow off from these calm places, and consequently, how little the direct action of the solar ray has to do either with the trades or the counter trades. It regulates and controls them; it can scarcely be said to create them. ”

“ 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 ”

“ Rise of temperature, expansion, liquefaction, evaporation. The first indication of the presence of heat is discovered by an elevation in temperature. Though man is not a reliable thermometer, he would be able, ordinarily, even if blind, to chronicle the progress of the sun, from horizon to horizon, by the increasing and decreasing warmth. The little thermometer placed beneath the tongue of the invalid gives reliable report of the combustion going on within his system. We see a thousand illustrations of the expansive effects of heat, many of which are familiar to all. ”

“ A large part of the increase of temperature would no doubt be used up in melting ice and evaporating water, so that there would be a much less increase of sensible heat; while only a portion of the theoretical lowering of temperature in winter would be actually produced owing to equalising effect of winds and currents, and the storing up of heat by the earth and ocean. ”

“ The heat appears to increase to a certain point in the different latitudes so as to necessitate a change, by some law similar to that which regulates the intense cold in other countries. After several days of progressive heat here, on the hottest of which the thermometer probably reaches 103 degrees in the shade, a break occurs in the weather, and a thunderstorm cools the air for a time. ”

“ In hot climates we seek the shade, not only because the air under it is fresher, but also because it has more motion, in consequence of the differences in density arising from the unequal heating. Notwithstanding all the devices that have been contrived for the reduction of temperatures, it is evident that civilization is possessed of more varied and efficacious means of contending against the cold than of mitigating the effects of the heat. ”

“ 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. ”

“ Variation in radiative heating controls weather The angle at which the sun's radiation strikes the earth's surface varies with the time of year and with latitude. Heating is greatest along a band at the equator, where the incoming radiation is most nearly perpendicular, and least at the poles, where the radiation strikes the earth at a low angle. At any latitude, land masses are heated more than are the oceans. This variation with latitude and between land and water masses creates widespread temperature differences in the atmosphere, thus generating large-scale air circulation patterns. ”

“ If we now incline the axis of our imaginary earth 231⁄2°, we shall introduce seasons whose only change, the one from the other, will be in the duration of sunlight, as there is no water vapor to absorb and utilize the sun’s rays in the initiation of motion and the creation of storms. We are assuming that there would be enough heat absorbed to prevent the atmosphere from liquefying, which it would do at any temperature lower than 312° below zero. ”

“ How unphilosophical, too, to talk of latent heat in connection with evaporation, at the lowest temperature known. Meteorologists must revise their opinions on the subject of condensation. This latent heat has never been actually met with; on the contrary, the most sudden and complete condensations of the vapor of the atmosphere are attended by as sudden and extraordinary productions [Pg 318] of cold, and consequent hail, and the connection between condensation and electricity is shown by too many facts to permit the old theory to stand.
Fog never forms with the thermometer below 32°. ”

“ 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. ”

“ The inevitable result is, from the known laws of heat, that its radiation into the celestial spaces is constantly going on, and consequently the earth’s temperature is being constantly lowered. Who can tell us now when this process of refrigeration commenced? If no one, then there must have been a time when the heat was great enough to fuse the whole globe. ”

“ To those whose constitution has been hereditarily adapted to a temperate or cold climate, the enfeebling effect of hot countries depends much more on the constant continuance of a high temperature than on its amount. A place with a mean temperature of 80° Fahr., which varies little above or below that point, is far more injurious to a European than one where intervals of great heat alternate with periods of cooler weather. Still more important, perhaps, is the effect of a hot climate in places where the air is habitually nearly saturated with aqueous vapour. ”

“ Water vapor has a great capacity for heat. It is the most effective of the various gases present in the atmosphere in obstructing radiation of heat from the earth, as well as in absorbing incoming radiation from the sun. The night temperature, therefore, falls more slowly when the relative humidity is high than when it is low, that is to say, when the air is nearer saturation, or nearer its dew point. Drops of water that collect on the outside of a pitcher of ice water on a warm day are formed through the chilling of the air in contact with the pitcher ”

“ 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. ”

“ The north and south trend of our mountain systems, quite different from the systems of Europe and Asia, facilitates the coming together of conflicting winds of widely different temperatures in the lower reaches of the atmosphere where there is an abundance of water vapor; no tornadic whirls probably can occur without an abundance of water vapor and the energizing effect of the heat liberated in the whirling cloud as this vapor is suddenly carried aloft and liberated by condensation right in the center of the disturbance. ”

“ The condensed vapour or cloud particles are here supposed to be carried up with the cooling air, but the temperature of freezing or zero degrees centigrade is soon attained—as the diagram shows—when the pressure is about 472 millimetres. At this point the special evolution of latent heat of freezing comes into play; and although the air rises higher and more moisture is condensed, the temperature does not fall because the water already converted into vapour and now becoming ice is giving out latent heat sufficient to counteract the cooling due to expansion. ”