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Page:Popular Science Monthly Volume 32.djvu/671

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651
WEATHER-PROGNOSTICS.

scale rain falls as a squall drives by. The motion of these areas was found to follow certain laws, so that forecasting weather-changes in advance became a possibility.

6. That in the temperate zones sometimes, and habitually in the tropics, rain fell without any appreciable change in the isobars, though the wind conformed more regularly to the general law of these lines. This class of rainfall will be called "non-isobaric rain."

In Fig. 1 we give in a diagrammatic form the broad features only

PSM V32 D667 The seven fundamental shapes of isobars.jpg
Fig. 1—The Seven Fundamental Shapes of Isobars.

of the distribution of pressure over the North Atlantic, Europe, and the eastern portions of the United States on February 27, 1865. Coast-lines are omitted, so as not to confuse the eye, so also are lines of latitude and longitude; but the foot-note at the bottom of the figure represents the equator, and the top of the diagram would be on the Arctic Circle. All pressures of and under 29·9 inches (760 mm.) are shown with dotted lines, so that the eye sees at a glance the broad distribution of high or low pressure. The whole seven fundamental shapes of isobars will be found there.

Looking at the top of the diagram, we see two nearly circular areas of low pressure, round which the isobars are rather closely packed. Such areas, or rather the configurations of isobars which inclose them, are called "cyclones," from a Greek word meaning a circle, because they are nearly circular, and, as we shall see presently, the wind blows nearly in a circle round their center. Just south of one of the cyclones, the isobar of 29·9 inches (760 mm.) forms a small sort of nearly circular loop, inclosing lower pressure; this is called a "secondary cyclone," because it is usually secondary or subsidiary to the primary cyclones above described. Farther to the left the same isobar of 29·9 inches bends itself into the shape of the letter V, also inclosing low pressure; this is called a "V-shaped depression," or, shortly, a "V." Between the two cyclones the isobar of 29*9 inches projects upward, like a wedge or an inverted letter V, but this time incloses high pressure; this shape of lines is called a "wedge." Below all these we see an oblong area