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.