CHAPTER VIII

PREVAILING WINDS OF THE GLOBE

128. HAVING in the previous chapter dealt briefly with the theory of the subject of atmospheric currents, it will now be necessary to investigate some of the effects which the varied and complex distribution of land and water surfaces has upon the winds of the globe. It would be out of place in the present work to give any but a rough and brief consideration of such a vast subject, about which numerous works have been written by various authors. But so intimate is the connection between clouds and the winds which cause or are caused by them, that a description of the one would not be complete without some notice of the other.

The laws regulating the distribution of atmospheric pressures and currents already described must everywhere hold good. Over extensive oceanic surfaces the belt of equatorial low pressure round which the isothermal and isobaric surfaces are most elevated will resemble in the main the equatorial belt of our hypothetical sphere. Within this belt irregular squalls, due to local inversion currents, are interrupted by intervals of dead calm, forming a most undesirable

region for sailing vessels. It is this belt which controls the general atmospheric pressures and currents of the globe, and it is for this reason that we must pay a closer attention to it.

129. The mean annual position of the equatorial calm-belt does not, as it would in a sphere covered with water, lie over the equator itself, but some degrees to the north of it in the two great oceans. This fact is well marked in the Atlantic, where it is only in the spring months of the northern hemisphere that the southern limit of the belt passes to the south of the equator. The cause of this phenomenon lies principally in the great frictional resistance experienced by the north-east Trades as compared with that encountered by the south-east Trades. Over these two oceans, therefore, the former never cross the equator, becoming north-westerly currents, as supposed in our hypothetical sphere. The annual oscillations of the belt are accurately known. It is most sharply defined over the greater part of the Atlantic and throughout the greater part of the year on its northern limit.

130. It will be convenient here briefly to describe the characteristics of the north-east and south-east Trades, which, over the oceans above mentioned, cover so extensive a surface. These currents, as we have already implied, tend to leave the surface of the sea when they approach the equatorial calm-belt; so that we frequently observe them carrying the exterior clouds of the belt over an atmosphere which is calm or irregularly disturbed on the ocean's surface. They also move somewhat more from the east at a slight

elevation than they do on the actual surface, a fact which is principally due to their tendency to rise, inasmuch as their upper portions, having travelled from the greatest distance, suffer the greatest deflection towards the west. The atmosphere throughout the Trade Winds has a very low relative humidity as compared with that of the equatorial belt, and in these, the great evaporating currents of the globe, it is felt as an invigorating and refreshing air. Over both the Atlantic and Pacific Oceans, but especially over the former, the south-east Trade blows with greater strength and persistence than the north-east, owing to the relatively small resistance of land surfaces.

131. It will now become most fitting to examine the principal effect upon the equatorial belt and upon the Trade Wind systems over the Atlantic and the eastern portions of the Pacific Ocean by extensive land-surfaces, before we proceed to consider the far vaster effect of the whole Asiatic continent upon the wind systems of the Indian Ocean and China Seas, and of the southern portion of that continent itself.

The reader who has studied the previous chapter will be prepared to find great tracts, in which atmospheric pressure is below that experienced in the oceanic equatorial belt, in the interior of the South African and South American continents. Round these areas the winds tend to blow clockwise wherever this motion is not checked by the interposition of lofty and continuous mountain chains. Such chains do not exist near the coasts of the African continent, and a glance at Chart I. shows that an area of pressure below 29.8