Many other instances of the importance of a knowledge of morphological structure might be adduced, but these will suffice to show that in order to understand the present it is frequently necessary to appeal to the past.
The chief factors which control the climate of any region are its latitude, its altitude, and its position with regard to the various land and water masses of the globe. The effect of each of these may best be observed by beginning with an extremely hypothetical case, and by gradually introducing the various modifications necessary to bring it into accordance with actual facts.
If the sun were constantly above the equator, if the surface of the earth consisted entirely of land, and if there were no atmosphere, it is obvious that the temperature of the globe would be greatest at the equator where the sun would be directly overhead each day throughout the year ; it would gradually become less in higher latitudes, owing to the greater obliquity of the solar rays ; and it would be at a minimum at the poles where the sun would constantly appear on the horizon. In these circumstances the temperature of any place, and therefore its climate, would depend upon its latitude and upon that only. As the sun does not remain constantly above the equator, however, but alternately " moves " each year to about 23-i° on either side of it, a disturbing element is introduced, since the increasing length of day caused thereby towards each pole in turn more than compensates, in higher lati tudes, for the decreased amount of insolation caused by the sun's rays making an oblique angle with the surface of the earth. To such an extent would this be the case, indeed, that at the summer solstice the north pole would receive in twenty-four hours not only a greater amount of insolation than the equator would receive at that time, but a greater amount than the equator could receive in twenty-four hours at any season of the year. Under these cir cumstances the distribution of temperature over the globe would be more complex, but it would still be broadly true to assume that the amount of insolation received by the earth in the course of a year would diminish from a maximum at the equator to a minimum at the poles, though that minimum would no longer be zero as in the previous case.
The introduction of the atmosphere affects the problem in several ways. In the first place the decrease in temperature from the
equator towards the poles becomes more marked. Much of the radiant energy of the solar rays is absorbed by the atmosphere and by the water vapour and dust which it contains, and this absorption necessarily increases with latitude on account of the greater obliquity of the sun's rays and the longer path which they have consequently to traverse through the earth's atmosphere.
The circulation of the atmosphere must next be considered. In the accompanying diagram let A B represent a portion of the earth, and C D a horizontal plane some distance above it. The pressure of the atmosphere, as indicated by the barometer at any point in A B, is x, and at any point in C D, x,' and, as pressure decreases with altitude, x' is obviously less than x. Let S be the region of greatest insolation.
The air over S becomes heated to a greater extent than elsewhere, partly by the direct action of the sun's rays, and partly by the radiation from the earth of the heat obtained from the sun, and as a result it expands. For example, the air which formerly occupied the column a b c d now occupies the column a b c' d' and the pressure at S" now equals the former pressure at S' v:z., x'. But the pressure towards C' and D' in the horizontal plane C' D' is less than x', which is the pressure on the horizontal plane C D towards C and D where conditions have not altered. Therefore the pressure at S" is greater than it is towards C' and D'. But, if in a fluid acted upon by an external force, such as gravity, the pressure is not the same in all parts of the same horizontal plane, a move ment takes place from the area of high pressure to that of low pressure and continues until equilibrium is restored. Hence the air flows outwards from the region round S" towards C' and D', thus reducing the pressure on the surface of the ground around S where it now becomes less than x, and increasing it towards A and B, where it becomes greater than x. Accordingly, the air moves inward along the surface of the earth from the high-pressure regions around A and B towards the low pressure region about S. Thus a regular system of convection currents is established, as is indicated by the arrows in the diagram.