28 Introduction to the Study of Science 



when it faces the northwest. Test these points by the 

 maps. 



19. Rainfall. The area over which rainfall is distributed 

 in connection with a " low " is determined by the direction 

 from which the supply of moisture-laden air comes. The 

 rainfall is usually on the eastern and southern side and near 

 the center of a " low." There is, however, little regularity in 

 the position and area of rainfall, as is shown on maps having 

 precipitation indicated. 



20. Weather features of cyclone area. The character- 

 istic features of a low or cyclonic area are represented graphically 

 in Fig. 11. The first approach of the low area is usually in- 

 dicated by the appearance of the dainty, feathery clouds called 

 cirrus. They are often described as mare's tail clouds. They 

 are between five and seven miles high, and are more or less 

 irregular in form, sometimes like badly frayed plumes or streaks 

 of haze. They move in advance of the low center by many 

 hundred or even a thousand miles. The cirrus clouds, which 

 are made up of minute ice particles or spicules, merge gradually 

 into a cobweb-like sheet or layer cloud called the cirro-stratus. 

 This extends over more or less of the heavens and is often made 

 conspicuous by the halo formed by the sun or moon. Such 

 cloud formation is due to the condensation of the moisture of 

 the warm air, which, forced upward, overflows far and wide 

 the denser masses of air. 



The barometer begins to fall with the approach of the cirro- 

 stratus ; the rate of its falling indicates the speed at which the 

 storm area is moving; and the amount of its fall indicates the 

 intensity of the storm. 



To the east and south of the cyclone the winds blow from 

 the east or southeast, and the temperature rises. To the north 

 and west of the center the winds blow from the northeast 

 quarter, and the temperature falls. 



Clouds denser and lower than the cirro-stratus may appear 

 and move directly inward toward the low center. These, from 



