When the principles in the foregoing paragraph are considered with regard to the general distribution of temperature prevailing over the earth as a whole, it is seen that there is a belt of low pressure at the equator, where the air, being heated, expands and flows outwards towards the poles. The flow is, however, not due north and south as might be expected. On account of the rotation and shape of the earth, every free moving body tends in the northern hemisphere to turn to the right of its direction of motion, and in the southern hemisphere to the left. Thus the air which flows out aloft, from above the equatorial low pressure area, moves towards the north-east in the northern hemisphere and towards the south-east in the southern, and becomes heaped up between latitudes 30° and 35° on either side of the equator. Two belts of high pressure thus tend to be formed round the globe, from which winds blow along the surface of the earth towards the equator on the one hand, and towards, but not to, the poles on the other. The winds which blow from these high pressure belts towards the equatorial low-pressure belt blow from the north east in the northern hemisphere, and from the south-east in the southern, and on account of their steadiness are known as the trade winds. On the other hand, those winds, which blow from the north and south high-pressure belts to the regions of lower pressure lying to the north and south of them respectively, blow from the south-west and west in the northern hemisphere, and from the north-west and west in the southern. Into these regions of lower pressure, also, winds appear to blow from the polar ice-caps which are areas of great cold, and therefore of high pressure. Thus, the ideal distribution of pressure and winds over the face of the globe is as follows : at the equator there is a belt of low pressure, where, on account of the ascending air, calm and variable winds prevail. To the north and south of this blow the steady trade winds, beyond which are the high-pressure belts where, as the air is descending from higher altitudes, calm and variable winds are again found. In higher latitudes are the westerly and south-westerly winds of the northern hemisphere, and the westerly and north-westerly of the southern. From the polar areas of high pressure, winds blow from the north-east in the northern hemisphere, and from the south-east in the southern.
That this distribution of pressure and winds prevails over the earth not in its ideal but in a much modified form, is due to the fact that the surface of the globe consists not of land only, but of land and water, unequally distributed. This further modification has now to be introduced into the hypothetical case with which the consideration of climate was begun. For several reasons the temperature of the ocean rises more slowly in warm weather than that of the land. The specific heat of water is greater than that of the land. The sun's rays penetrate it to a greater extent, and there fore warm it less on the surface ; much heat is spent in the work of evaporation, and, as will be seen later, much is carried off by the warm currents which flow to colder regions. For somewhat similar reasons the ocean cools more slowly than the land. The water gives up its heat less rapidly, and, as that lying on the surface becomes cold and sinks, warmer water from below rises to take its place. Except in low latitudes, therefore, where the land tem perature is high throughout the year, the surface of the ocean is generally colder than the land during the summer and warmer during the winter, and the range of temperature between summer and winter is greater over the land than it is over the sea. These facts
have an important bearing upon the distribution of pressure and winds. During the summer months the air over the great land masses becomes heated, expands, and flows outwards over the ocean, thus decreasing the pressure over the land and increasing it over the sea. To restore equilibrium, winds blow inward over the surface of the earth from the sea to the heated interior of the land mass. These winds being deflected to the right of their direction of motion in the northern hemisphere, in which all the great land masses of the globe are situated, tend to bring about the development of cyclonic movements in which the air circulates in a counter-clock wise direction. During the winter months the conditions are reversed. The air over the land is rapidly cooled, contracts, and sinks to lower altitudes, so that in the upper horizontal layers of the atmosphere pressure is greater over the sea than it is over the land, and there is an inward movement of the air which causes an increase in pressure over the surface of the land and a decrease over the surface of the sea. Accordingly, there is an outward movement of the winds from the land to the sea, and these, being deflected to the right, as in the previous case, have a tendency to circulate round the region of high pressure in a clockwise direction in the northern hemisphere. The actual distribution of pressure and winds over the face of the globe is therefore very different from the ideal distribution as sketched above. In equatorial regions the belt of low pressure moves northwards and southwards with the sun. On either side of it lie the high-pressure belts which change in form and extent during the course of the year, as a result of the unequal heating of land and water. During the northern summer, the north high-pressure belt is broken up over the land and extended over the sea, while in winter it is generally extended over the land and contracted over the sea. To the north of this high-pressure belt there are, in the Atlantic and Pacific, both in summer and winter, but much further south in winter than in summer, areas of low pressure into which winds both from the high-pressure belt and the polar area of high pressure are con tinually blowing. In the southern hemisphere the normal dis tribution of the winds is much less affected owing to the much smaller land area which exists there in temperate latitudes. These variations in pressure modify, to a great extent, the planetary distribution of winds already discussed. In some cases the winds are strengthened, in others weakened, and in still others are entirely reversed. For example, the westerly and south-westerly winds which blow at all seasons of the year from the tropic high-pressure belt towards the west coasts of Europe are strengthened during the winter months by the presence in the North Atlantic of the area of low pressure already mentioned. In corresponding latitudes on the east coast of North America, however, the prevailing winds during the winter months are those which blow from the high pressure area over the continent towards the low-pressure area over the Atlantic, and, being deflected to the right, appear as northerly and north-westerly winds. During the summer months, again, the low-pressure area over the Asiatic land mass sucks in the air from over the Indian Ocean to such an extent that the north-east trade winds disappear and are replaced by the south-west monsoon.