PART II.

CHAPTER XII.

THE DISTRIBUTION OF TEMPERATURE.

IN Chapter II. it has already been explained that the sun is practically our sole source of heat; but in order to set forth the facts of the distribution of temperature in a clear light, it will be well to repeat some of the previous statements.

The sun's apparent motion round the earth in a day gives rise to a diurnal period of temperature. The sun has also an apparent motion in the ecliptic in the year, passing from one tropic to the other and back again, and this causes a corresponding annual period of temperature.

The effect produced by the solar rays depends in great measure on the angle at which they strike the surface of the earth. In the daily period the sun is highest, or most nearly vertical, at noon; and in the yearly period, outside the tropics, he is highest at the summer solstice,-June for the northern, December for the southern, hemisphere.

In the first place, it is evident that, if we consider the case of a cylindrical beam, its heating power will be concentrated on a smaller area the more nearly vertical its path is.

Secondly, a nearly vertical beam has to pass through a less thickness of the dense lower strata of the atmosphere than an inclined one, and we have already learnt from Chapter IV. that a large proportion of heat is absorbed by those strata.

The action of such heat as succeeds in passing through the atmosphere is not, in the first instance, to warm the air, but to raise the temperature of the surface of the earth, and the amount of rise of temperature produced by it depends in great measure on the nature of the surface on which the beams happen to fall, being greatest over bare earth-such as rocks or a desert, less over vegetation-such as forests, and least of all Dver the sea, for the reasons already explained in Chapter III., p. 46.

As soon as the heat from the sun has reached the earth, its warming effect on the air begins to be available; for it has now changed its character and has become 'dark' heat, that is, heat coming from à source of comparatively low temperature. Such heat is absorbed by the atmosphere with much greater facility than heat coming from the sun, as already explained in Chapter IV., p. 53.

When the earth is heated by the solar rays it begins to radiate to the atmosphere resting on it, and the amount of heat passing from the earth in this way is greatly affected by the condition of the sky, the radiation being much more active in the absence of clouds. As we have seen in Chapter VII., the presence of moisture in the air presents great obstacles to the escape of the heat radiated from the earth-a cloud covering reflecting it back to the earth almost entirely. The reader will remember that it has been explained that

dew or hoarfrost cannot occur on cloudy nights, as the heat cannot escape from the earth into space.

The first action of the heat, when it has warmed the surface of the earth, is to warm the layers of the atmosphere in immediate contact with it, and as heat is very slowly communicated from stratum to stratum of the air, we find the lowest layers to be the warmest.

The reader need hardly be reminded that on mountain tops the temperature is, on the whole, much lower than on the plains below. This difference is far more marked in the daytime than at night, in summer than in winter. In fact, in the case of severe frosts the conditions are ordinarily reversed and the temperature rises with height instead of falling. This gives the well-known phenomenon of the 'up-bank thaw,' when it thaws on the hills while the frost is unbroken in the valleys below. I shall return to this subject in Chapter XVII., when speaking of climate.

Supposing that the air were perfectly dry, the rate at which the temperature would fall in an ideal vertical column standing upon the earth is 1° F. for every 180 feet; but as the atmosphere contains moisture, which is condensed by cold, giving out its latent heat, and the air itself is imperfectly diathermanous, the heat so rendered available will diminish the rate of cooling, and, as a very general rule, we may take about 1° F. for every 300 feet, the value given many years ago by Herschel, as the regular rate of decrease.

This consideration shows us that if we wish to lay down on a map the temperatures all over the globe, from the observations furnished by a number of stations situated at very various elevations above the sea, we

must reduce the results to their equivalents at sea level, if we wish to attain anything like accordant figures, although the representation these afford has no real existence in nature.

Let us now return to the subject of the direct action of solar heat, and consider how it affects temperature, producing the daily and yearly march of that element.

In Chapter III. we have treated in general terms of the movable equilibrium of heat, and have stated the reasons, based upon that principle, why the hottest portion of the day is not at noon, or of the year, in our latitudes, at midsummer, but we must now consider the subject more closely.

That the diurnal range of temperature is directly connected with the sun's rays is apparent from the fact