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Page:Popular Science Monthly Volume 32.djvu/673

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653
WEATHER-PROGNOSTICS.

We should then have two cyclones, differing in nothing but depth; that is, in the closeness of the isobars, or the steepness of the barometric slope. Observation has shown that under these circumstances the general character of the weather and the direction of the wind everywhere would be the same; the only difference would be that the wind would blow a hard gale in the first and only a moderate breeze in the second case; and that what was a sharp squall in the one would be a quiet shower in the other. This is one of the fundamental principles of synoptic meteorology—that the character of the weather and direction of the wind depend entirely on the shape of the isobars, while the force of the wind and intensity of the character of the weather depend only on the closeness of the isobars.

The difference in the details of the weather in a cyclone, or any other isobaric shape which are due to difference in the steepness of the isobars, is called a difference in the intensity of the weather. Hence, when we speak of a cyclone as being intense, we mean that it has steep isobars somewhere. When we come to talk about the general sequence of weather from day to day, we shall find that there is no difference between the cyclones which cause storms and those which cause ordinary weather except intensity. This is another of the fundamental principles of meteorology.

Returning now to our cyclone, the whole of the portion in front of the center facing the direction toward which it moves is called its front, and the whole of this portion may obviously be divided into a right and left front. The other side of the center is, of course, the rear of the cyclone. Then, as the whole cyclone moves along its course, it is evident that the barometer will be falling more or less at every portion of the front, and rising more or less everywhere in the rear, so that there must be a line of places somewhere across the cyclone where the barometer has touched its lowest point and is just going to rise. This line is called the "trough" of the cyclone, because if we look at the barometer-trace at any one place, the "ups" and "downs" suggest the analogy of waves, so that the lowest part of a trace may be called a "trough." Or we may look at the cyclone as a circular eddy, moving in a given direction, and so far presenting some analogy to a wave.

So far for the shape and names of the different portions of the cyclone. Now for the wind. A glance at the arrows will show that, broadly speaking, the wind rotates round the center in a direction opposite to the motion of the hands of a watch. That is to say, that in the extreme front, following the outer isobar, the wind is from the southeast; farther round, it is from the east-northeast; still farther, from the north-northwest; then from about west; and, finally, from the southwest. Then we note that in front the wind is slightly incurved toward the center, and therefore blows somewhat across the isobars, while in rear it has little or no incurvature, and blows nearly