The air itself absorbs some of the sun’s heat, and the greater the thickness of air through which that heat has to make its way, the more heat will be absorbed. Besides this, the more the rays of heat are slanted the weaker do they become. At noon, for example, the sun stands high in the sky. Its rays are then nearest to the vertical, and have also the least thickness of air to pass through before they reach us. As it descends in the afternoon, its rays get more and more slanted, and must also make their way through a constantly increasing thickness of air. Hence the middle of the day is much warmer than morning or evening.
At night, when the sun no longer shines, its heat does not directly warm the part of the earth in shadow. That part not only receives no heat from it, but even radiates its heat out into the cold sky. Hence night is much colder than day.
Then, again, in summer the sun at noon shines much higher in the sky with us, or more directly overhead, than in winter. Its heat comes down less obliquely and has less depth of air to pass through, and hence is much more felt than in winter, when, as you know, the sun in our part of the world never rises high even at mid-day.
If we were dependent for our warmth upon the direct heat of the sun alone, we should be warm only when the sun shines. A cloudy day would be an extremely cold one, and every night as intensely frosty as it ever is in winter. Yet such is not the case. Cloudy days are often quite warm; while we are all aware that the nights are by no means always very cold. There must be some way in which the sun’s heat is stored up, so that it can be felt even when he is not shining.
In summer the ground gets warmed; in some parts, indeed, becoming even so hot at times that we can hardly keep the hand upon it. In hot countries this is felt much more than in this country. Soil and stones absorb heat steadily, that is to say, soon get heated, and they soon cool again. When they have been warmed by the sun, the air gets warmed by contact with them, and keeps its heat longer than they do; so that even when at night the soil and stones have become ice-cold, the air a little above is not so chilly. On the other hand, when the surface of the ground is cold, it cools the air next it. The ground parts easily with its heat, and a vast amount of heat is in this way radiated at night from the earth outward into the cold starry space. Much more heat, however, would be lost from this cause did not the abundant aqueous vapor of the atmosphere absorb part of it, and act as a kind of screen to retard the radiation. This is the reason why in hot climates, where the air is very dry—that is, contains a small proportion of the vapor of water—the nights are relatively colder than they are in other countries where the air is moister. In like manner, clouds serve to keep heat from escaping; and hence it is that cloudy nights are not so cold as those which are clear and starry.
The atmosphere, then, is heated or cooled according as it lies upon a warm or cold part of the earth’s surface; and, by means of its aqueous vapor, it serves to store up and distribute this heat, keeping the earth from such extremes of climate as would otherwise prevail.
The air lying next to a hot surface is heated; the air touching a cold surface is cooled. And upon such differences of temperature in the air the formation of winds depends.
Hot or warm air is lighter than cold air. You have learned how heat expands bodies. It is this expansion of air, or the separation of its particles further from each other which makes it less dense or heavy than cold air, where the particles lie more closely together. As a consequence of this difference of density, the light warm air rises, and the heavy cold air sinks. You can easily satisfy yourselves of this by experiment. Take a poker, and heat the end of it in the fire until it is red-hot. Withdraw it, and gently bring some small bits of very light paper, or some other light substance, a few inches above the heated surface. The bits of paper will be at once carried up into the air. This happens because the air, heated by the poker, immediately rises, and its place is taken by colder air, which, on getting warmed, likewise ascends. The upward currents of air grow feebler as the iron cools, until, when it is of the same temperature as the air around, they cease.
This is the principle on which our fire-places are constructed. The fire is not kindled on the hearth, for, in that case, it would not get a large enough draft of air underneath, and would be apt to go out. It is placed some way above the floor, and a chimney is put over it. As soon as the fire is lighted, the air next it gets warmed, and begins to mount, and the air in the room is drawn in from below to take the place of that which rises. All the air which lies above the burning coal gets warmer and lighter; it therefore flows up the chimney, carrying with it the smoke and gases. You will understand that though a bright blazing fire is a pleasant sight in winter, we do not get all the heat which it gives out. On the contrary, a great deal of the heat goes up the chimney; and, except in so far as it warms the walls, passes away and warms the outer air.
What happens in a small way in our houses takes place on a far grander scale in nature. As already pointed out, the sun is the great source of heat which warms and lightens our globe. While the heat of the sun is passing through the air, it does very little in the way of warming it. The heat goes through the air, and warms the surface of the earth. You know that in summer the direct rays of the sun are hot enough to burn your face, and yet, if you put even a thin sheet of paper over your head, enough to cut off these rays, the sensation of burning heat at once goes off, although the same air is playing about you all the time.