Currents of air having their origin in the different pressures which exist in various regions of the atmosphere. Since the sun is shining more powerfully upon the equatorial than upon the more northern regions of the globe, the heated and therefore rarefied air ascends, while a current of colder air flows in from the poles on both sides to take the place of the ascending air. The ascending column of air flows over from the equator towards both poles, so that the general atmospheric circulation consists of an under current from the north and south poles towards the equator, and of an upper current from the equator towards both poles. This general circulation proceeds in spite of other circulations on a smaller scale or of a local character which may be going on at the same time. North-east and south-east trades occur in the northern and in the southern hemisphere. (See Weather and Trade Winds.)