182 GENERAL SCIENCE 



formed in the ordinary way ; then as other ascending 

 currents of warm air strike the lower part of the cloud 

 they are deflected along its surface and cooled so that 

 there is usually a heavy downpour of rain. The duration 

 of the storm is often but a few minutes, though sometimes 

 it continues for several hours. 



When water is condensed rapidly in the air, electricity 



is generated, and each 

 particle of water be- 

 comes charged with 

 electricity. Lightning 

 is due to the passage 

 of a large electric 

 spark from one cloud 

 to another or from 

 the cloud to the 

 ground (Figure 152). 

 Thunder which fol- 

 lows the flash of light- 

 ning is due to the vibrations in the air along the path of 

 the discharge. It is possible to estimate the distance of 

 a flash of lightning by observing the time in seconds 

 that elapses between a flash and its thunder and multiply- 

 ing by the speed of sound in air. The velocity of light is 

 so great that the time consumed by the flash in travel- 

 ing from its source to the observer may be neglected. 

 Heat lightning is lightning at such a great distance that 

 the thunder accompanying it cannot be heard. The 

 flash shows merely as light on the surrounding clouds. 



At the heat equator heavy rains usually accompanied 

 by thunder and lightning occur almost daily. These 

 storms give the name " rainy season " to the time of year 

 in which they occur. 



