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Page:Popular Science Monthly Volume 32.djvu/672

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652
THE POPULAR SCIENCE MONTHLY.

of high pressure, round which the isobars are very far apart; this is called an "anticyclone," because it is the opposite to a cyclone in everything—wind, weather, pressure, etc. Between every two anti-cyclones we find a furrow, neck, or "col" of low pressure analogous to the col which forms a pass between two adjacent mountain-peaks. Lastly, as marked in the lower edge of the diagram, isobars sometimes run straight, so that they do not include any kind of area, but represent a barometric slope analogous to the sloping sides of a long hill. The cyclones, secondaries, V's, and wedges are usually moving toward the east at the rate of about twenty miles an hour; but the anticyclones, on the contrary, are usually stationary for days and sometimes for months together. We should also note that, though the general principles of prognostics and the broad features of the weather in each of these shapes of isobars are the same all over the world, the minute details which we intend to give now apply to Great Britain and the temperate zones only.

We will now take the cyclone separately, and detail the kind of wind and weather which is experienced in different parts of it. In Fig. 2 we give a diagram on which we have written in words the

PSM V32 D668 Cyclone prognostics.jpg
Fig. 2.—Cyclone Prognostics.

kind of weather which would be found in every portion of a typical cyclone; arrows also show the direction of the wind relative to the isobars and to the center. First let us look at the isobars. We find that they are oval, and that they are not quite concentric, but the center of the inner one we will call the center of the cyclone. Now observe the numbers attached to the isobars; the outer one is 30·0 inches (762 mm.), the inner one 29·0 inches (737 mm.). But suppose the outer one was the same, but the inner one was 29·5 (755 mm.).