The spacing of the isobars is called the _barometric gradient_. One of the conventional ways of expressing a gradient numerically is to state the horizontal difference of pressure, in hundredths of an inch, for an interval of fifteen nautical miles; but meteorologists as a rule merely describe gradients as “steep,” “gentle,” “moderate,” etc., without indicating their numerical values. If the great difference in temperature between the equatorial and polar regions were the only factor in the control of atmospheric circulation, there would be a strong barometric gradient between these regions and there would result a simple circulation of winds, blowing poleward from the equator aloft and equatorward from the poles at the earth’s surface. The former tendency of writers on meteorology and physical geography was to regard such a circulation as a substantial fact, though modified by the effects of the earth’s rotation and various local causes. Thus the idea has prevailed of a wholesale, direct exchange of air between the poles and the equator. Nowadays we can hardly maintain this idea, because we see that, on account of the great deflections they undergo, the main drifts of air are approximately along parallels of latitude and not along meridians of longitude. Within the tropics the general drift of the lower air is from the east (and near the equator this drift prevails up to a great height); in middle latitudes it is from the west; and in the circumpolar regions it is again from the east. Air from the equator presumably does find its way to high latitudes, and _vice versa_, but neither rapidly nor directly. Perhaps the dominant feature of the whole circulation is the banking up of the air in so-called high-pressure belts at about latitude 30° North and South. From these “belts”--which are really broken up into separate areas of high pressure, and which shift north and south to a certain extent with the seasons--blow the northeast and southeast _trade winds_, in the full development of which, found only over the Atlantic and the eastern Pacific, we have the most remarkable “permanent winds” of the globe. Between the trade wind belts lies the equatorial region of low pressure, known as the “doldrums.” This is, in general, a region of light and variable winds, heavy rains, and thunderstorms.