286 



PHYSIOGRAPHY 



As the air moves in toward the center of a cyclone, it also moves 

 spirally up. This upward movement is of great consequence in its 

 effect on precipitation, as we shall see. The upward and outward 

 course of the air in a cyclone is shown in Fig. 242, which represents 

 a vertical section of a cyclone. It shows that the outflow above is 

 chiefly to the eastward, the direction in which prevailing winds blow. 



3. Cloudiness, precipitation, etc. The open circle on the shaft 

 of an arrow (Fig. 239) indicates clear skies, the half-blackened 



Fig. 240. Diagram showing the circulation of air about a low, L, and a high, 

 H, in the northern hemisphere. 



Fig. 241. Diagram showing the circulation of air about a low and a high in 

 the southern hemisphere. 



circle (as in Wyoming) shows that the sky is partly cloudy, while 

 the black circle (as in Illinois and Indiana) indicates general cloud- 

 ness. An R on the shaft of an arrow indicates rain, and an $ in the 

 circle on an arrow shows that snow is falling. These symbols do 

 not appear in Fig. 239. 



4. Temperature. The broken lines of the weather map are 

 isotherms. The isotherm of 50 (Fig. 239) crosses the Gulf States, 

 and south of it the temperature is above 50, but not so high as 

 60, within the area of this map. The isotherm of 40 is very 

 crooked, extending from Georgia to Nebraska, and thence to 

 Arizona. All points between this isotherm and that of 50 have 



