Meteorology: The Science of the Atmosphere

by Charles Fitzhugh Talman

At the earth’s surface, if the atmospheric pressure is measured simultaneously at various places by means of barometers, we can get a clear picture of the horizontal distribution of pressure by drawing on a map lines, called isobars, through places at which the pressure is identical. The isobars reveal the presence of extensive areas over which the pressure is above the average and of others over which it is below the average. If, at the same time, we chart the flow of air by indicating the direction of the winds at various points, we shall notice that the air shows a strong tendency to travel around these areas; and if we could observe its course a thousand feet or so above the earth we should find the tendency even more pronounced at that level.

Another important law, springing in part from Ferrel’s Law and describing the movements of the air around areas of high and low pressure, is called Buys Ballot’s Law. One way of stating this law is as follows:

“If you stand with your back to the wind, in the northern hemisphere, the barometer will be lower on your left than on your right. The reverse is true in the southern hemisphere.”

The reader, whether he lives in the United States or any other civilized country, will have no difficulty in securing documentary evidence of the correctness of Buys Ballot’s Law in the shape of the daily weather maps issued by the various meteorological services. On a weather map showing conditions anywhere in the northern hemisphere it will be found that the winds (which are indicated by little arrows), though subject to a good many local variations, have a general tendency to blow in the direction followed by the hands of a clock (“clockwise”) around an area of high pressure, and in the opposite direction (“counterclockwise”) around an area of low pressure. It will likewise be noticed that, in general, the winds, instead of blowing along the isobars, are strongly inclined inward in the case of a low-pressure area and outward in the case of a high-pressure area. Lastly, if the map indicates the force of the winds at different places, it will be seen that winds are strongest where the isobars are close together and weakest where they are far apart.

It is a matter of much interest to aeronauts that the force of the wind generally increases with altitude, and that, in the lower flying levels, the winds are little, if any, inclined to the isobars.

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.

In the temperate zones of both hemispheres, on the poleward side of the high-pressure belts above mentioned, the general drift of the atmosphere near the earth’s surface is from west to east. In the south temperate zone there is a very strong preponderance of west winds, especially over the vast oceanic tract of the “roaring forties,” where blow the boisterous “brave west winds,” well known to sailors. The corresponding belt of the northern hemisphere, which includes all but the southernmost part of the United States, is described as a region of “prevailing westerly winds,” but it is also a region of storm tracks, and hence, as local episodes in the general movement of the atmosphere from west to east, there are constant shifts of the wind to all points of the compass, for reasons that will presently be explained.