230 WEATHER AND CLIMATE 



charges take place which cause lightning flashes. These 

 discharges occur along the lines of least resistance and are 

 often very irregular and forked. As tall objects are likely 

 to offer good paths for the discharge, it is safest to keep away 

 from trees and walls during a thunder-storm. 



The air becomes greatly agitated by the lightning dis- 

 charges and makes us aware of this by the noise of the 

 thunder, just as the agitation of the air caused by the dis- 

 charge of a gun is made apparent to us by what we call 

 the noise of the report. The flash of lightning reaches the 

 eye almost instantly after the electrical discharge; but 

 since sound travels at the rate of about a mile in five seconds, 

 there is often a noticeable lapse of time between the ap- 

 pearance of the flash and the sound of the thunder. The 

 noise from different parts of the discharge will reach us at 

 different times, and to this and the echoing from clouds or 

 hills is due the roll of the thunder. To tell in miles the 

 approximate distance of the flash, one has only to divide by 

 five the number of seconds that elapse between the appear- 

 ance of the flash and the noise of the thunder. 



Frequently in the evening flashes called heat lightning 

 are seen near the horizon. These are due to the reflection 

 on clouds of flashes of lightning in a storm which is below 

 the horizon. Thunder-storms occur sometimes in winter. 

 They are very prevalent in the tropics. 



Tornadoes and Waterspouts. Sometimes causes like 

 those which produce a thunder-storm are so strongly de- 

 veloped that the indraft is exceedingly violent and a furious 

 whirling motion is produced. Such storms are called 

 tornadoes. The warm, moist air rises rapidly and spreads 

 out into a funnel-shaped cloud with the vertex hanging 



