LOCAL STORMS 



241 



blows outward from the north and extends several miles south 

 of the rain. When a thunder shower passes near on the 

 south, a similar wind often blows out from the south, reaching 

 several miles north of the storm (Fig. 170). These outflowing 

 winds on all sides of the storm have no connection with its 

 forward movement, and do not show the direction the storm 

 is traveling. They are evidently caused by a downward rush 

 or settling of air within the 

 storm. This air must spread 

 outward on approaching the 

 ground and thus causes these 

 winds. This downward mov- 

 ing air may be (1) colder air 

 from above, (2) air cooled^ 

 within the storm itself, or (3) 

 air brushed downward by the 

 falling rain. The air within a 

 thunderstorm could be cooled 

 by (a) cool raindrops falling 

 through the heated lower air; 

 (6) evaporation from the fall- 

 ing rain. Probably all these 

 processes share in causing the 

 downward air movement and 

 the outflowing winds. The 

 outflowing wind is usually 

 strongest in front, because there its velocity is added to the 

 forward movement of the storm. The backward flowing 

 wind in the rear is sometimes absent, because the forward 

 movement of the storm may be faster than the rearward flow 

 of air caused within the storm. The tumbling and rolling of 

 the squall cloud in front is caused by its position between the 

 rising air slanting upward into the storm front, and the down- 

 outward flow of the squall wind (Fig. 167). 



268. The Thunderstorm; Its Lightning and Thunder. 

 LIGHTNING is an electric flash or discharge, and seems to be 



16 



FIG. 170. Outflowing winds at the 

 ground in a summer thunderstorm. 

 Storms differ; not all these winds are 

 present in every storm. The area of 

 storm may sometimes be either nar- 

 rower or wider than shown in the 

 figure. 



