The Weather 39 



the immediate air envelope of the earth are charged with 

 electricity. By the use of observational balloons and other 

 means of investigation, it has been discovered that there is a 

 regular and rapid increase in the amount of electricity until a 

 height of about four miles is attained. Beyond this limit the 

 quantity of energy seems to remain constant. During fair, 

 dry weather the electric energy is equally distributed. It is 

 prevented from passing to the earth by the dry air near the 

 earth surface ; for dry air is a very poor conductor of electricity. 

 In cloudy weather the differences in electrification are much 

 greater at different altitudes. It is only in cloudy weather 

 that lightning occurs. But atmospheric changes, such as the 

 winds, or the ascent to the upper regions of water vapor pro- 

 duced by evaporation, and especially the condensation of 

 moisture to form cloud and rain, bring about marked disturb- 

 ances with more or less concentration of electrical energy within 

 limited areas. 



The electrical discharge. In condensation, electricity exists 

 only on the surface of water particles that make up a cloud. 

 With continued condensation the water particles combine again 

 and again to form larger drops or globules. In this coalescence 

 of globules the surface area is rapidly decreased, which means 

 a reduced capacity for holding the electricity. Every globule 

 has about all the electric charge that it is capable of holding. 

 When two globules combine, the new surface area formed is 

 only about one fourth larger than the surface area of either of 

 the globules, while the electric charge is about doubled. The 

 inevitable result is that a cloud in which condensation continues 

 is having its capacity for electricity rapidly reduced and its 

 electrical charge made more concentrated and powerful. The 

 electric charge finally becomes so powerful that the cloud can 

 no longer retain it or the intervening mass of air block its 

 transfer; then it forces its way to an oppositely electrified 

 cloud or to the earth. 



Such an electrical discharge is known as lightning. It at 



