THE EARTH'S WINDBELTS AS FACTORS OF CLIMATE.

Abstract

SYNOPSIS The purpose is to show, with the aid of particular illustrations, that the study of causative climatic factor, No. 5, above, really resolves itself into a discussion of all the temporary phases of wind and pressure which make up the characteristic wind belts as seen on the mean annual and seasonal charts of the globe. These mean wind belts are to some extent fictitious in that they but rarely represent any close approach to actual momentary phases in the general circulation of the atmosphere, so that it is necessary to study them in the light of the day-to-day variations of which they are the resultant effect. In other words, it is necessary to regard climatic factor No. 5 as constituted of more or less accentuated departures, often amounting to complete subversions of type, from the steady flow as depicted on the mean charts. It is to these departures that we really owe the changes of daily weather with their immense consequences in the economy of the earth's surface. The ?trades? and ?westerlies? are studied in this light. It is shown, for example, that the theory of the ?polar front? of Bjerknes constitutes a splendid system of reference in studying the day-to-day conditions of the westerly wind belts of both Hemispheres. It is to this theory we must look to explain one of the most impressive climatic features of the North Atlantic, namely, the midwinter concentration of cyclonic storms. Emphasis is laid on the effect of the ?monsoonal? or seasonal factor of the mean surface circulation which in the Northern, far more than in the Southern, Hemisphere distorts the simple parallelism of wind and pressure belts which should exist if the surface of the earth were all land or all water. Some puzzling anomalies are explained on the principle of the regional competition for air supplier. Thus the obstinate summer drought of the Mediterranean Basin with its intense insolation and excessive evaporation can only be understood in the light of a drought-providing wind and pressure régime which has to fit in dynamically with different régimes coexisting in other regions.