LOCAL STORMS 239 



1. Isolated or independent clouds not connected with another 

 shower. 



2. Clouds that develop near a shower already in operation. 

 This group includes: 



(a) Clouds that develop ahead of another shower, and begin 

 to rain before the older shower arrives (Fig. 169, A). 



(6) Clouds that are behind another shower, and begin to 

 rain at "z" after the older shower had passed and its 

 rain had ended at "z" for a short time. This new shower 

 (Fig. 169, B) sometimes gives the impression that the 

 departed shower has returned. 



(c) Clouds that develop at the side of an older shower (Fig. 

 169, C). When this occurs it widens the rain area of the 

 old shower and often makes it appear as though the old 

 shower was advancing obliquely instead of in the direction 

 its clouds are moving. For example : in Fig. 169 (7, a shower 

 in the northwest is moving due east across the northern 

 sky. A small cloud "a" grows rapidly and begins to rain 

 at the broken line. Meantime cloud "6," still farther 

 south, is growing, and begins to rain at the broken line. 

 Likewise the other clouds in succession, all moving due 

 east, begin to rain at the broken line. The original shower 

 has thus appeared to move southeastward over all the 

 region east of the broken line. What happened was the 

 southeastward growth or widening of the older shower. 

 The same sort of extension may occur on either side of 

 any shower in any part of the sky. Showers frequently 

 spread in that way and this development needs careful 

 watching to foresee your own weather correctly. Some- 

 times a shower continues to increase on one side and to 

 exhaust itself on the other side. 



The growth and movement of these clouds can often be seen. 

 If the important clouds are hidden by a sheet of lower clouds, 

 occasional breaks in the lower layer sometimes permit see- 

 ing the clouds and movements above (Fig. 164). Some 

 experience in observing clouds is necessary, for showers develop 

 somewhat differently in different sections of the country and 

 in different seasons. Moreover, the various types and 

 movements of clouds are not always easy for the beginner 

 to recognize. 



