Page:Popular Science Monthly Volume 60.djvu/509

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509
THE FORMATION OF CLOUDS.

Newton. To lay out the paths of motion in the atmosphere is just as important a work as that of finding the orbits of planets. There is more difficulty in doing it accurately because the motions of the air are much less steady and symmetrical than those of single masses like planets, comets and meteors, but it can be accomplished by patience and well-directed work. Unfortunately for lack of this sort of data much of our common meteorology is incorrect, and must be laid aside as of only an historical value. The subject is itself very complex, and it is unsuitable for a popular exposition, but an idea can be given of its scope and tendency by reference to the accompanying charts.

The Chart 4, marked 'Storm in the Lake Region' 'Winter Cyclone or Low Area,' is a composite map of the motions of the air around a winter storm central near Lake Superior. The upper or cirrus cloud movements are shown by the dotted arrows, the lower or cumulus, by PSM V60 D509 Wind direction and velocity at various heights.pngChart 5. Direction and velocity of the motions of the air in the cirrus (6 miles high), the cumulus (1 mile high) and on the surface (wind). those drawn with a broken line, and the surface wind by those with one unbroken line. The region affected by this storm extends from the Gulf of Mexico to the Lakes, and from the Atlantic Ocean to the Rocky Mountains. It is noted that the arrows drawn with an unbroken line and those drawn with a broken line generally blow nearly side by side, but that they differ from the dotted arrows in many cases, especially near the center of the storm and eastward to the ocean. This shows that the real storm circulation is confined to the lower strata as if it were quite independent of them. There is seen to be a slight deflection of the dotted arrows southward around the low; a corresponding chart of a high area would show a similar deflection to the northward of the center. This feature is brought out in Chart 5. Direction and velocity of the motions of the air in the cirrus, cumulus and on the surface. The arrows are not very smoothly laid down with reference to one another, but this is due to the fact that there were not enough observations taken in order to smooth out the local irregularities. Still, it is easy to trace a sort of wave motion, a crest over the high and a hollow under the low. As we come down from the cirrus, six miles high, towards the ground the sinuous motion becomes more pronounced, till in the strato-cumulus level the rotary component is dis-