240 THE WEATHER 



265. The Older Thunderstorm ; Its Advance Cloud. Local 

 storms often grow to cover large areas. After a thunder 

 shower has become well developed, the cirro-stratus overflow 

 at its top may reach 80 miles or more in advance, and is often 

 the first indication of the storm to be seen in regions ahead. 

 This high cloud sheet may seem to advance broadside, cover- 

 ing the whole sky as it comes. For a narrower storm, it 

 may approach endwise, covering only a narrow band or strip 

 of the sky. It may be more or less hidden by lower clouds 

 not connected with the storm (Fig. 167, M, N, R). 



266. The Thunderstorm ; Its Approach and Passing. As 

 the storm draws near, there appears along the horizon the 

 black or greenish-black mass of the main storm cloud (Figs. 

 167, T, and 175, A). A little later can be seen the dense cur- 

 tain of rain pouring from its base and hiding all behind (Figs. 

 175, B, and 160, B' and C"). Along the front of the rain is often 

 a roll or bank of low, ragged, gray, or dark cloud (Fig. 167, X), 

 tumbled and tossed by the wind. With this " squall cloud " 

 usually comes the first heavy dash of rain. The rainfall may 

 last from a few minutes to an hour or two, and varies in 

 intensity. Toward the last, it usually slackens gradually until 

 it ceases. Not long after the rain ends, and sometimes before, 

 the clouds break and the sky clears in part; and later, in the 

 east, may often be seen the brilliant tops of the departing 

 nimbus. 



267. Winds of a Thunderstorm. As the SQUALL CLOUD of 

 the storm front arrives, the wind, which till then had blown 

 from south or easterly toward the storm, shifts quickly and 

 blows with greater force directly outward from the storm front 

 (Figs. 167 and 170). Sometimes this wind is almost absent; 

 occasionally it destroys light buildings. Usually it blows 

 during the first of the rain, then dies away; often near the 

 end of the shower it has changed to nearly the opposite di- 

 rection and blows back from the departing storm (Figs. 167, 

 7, and 170, R). When a well-developed thunder shower 

 passes near by on the north, a moderate or brisk breeze usually 



