The Weather 41 



masts of ships. Occasionally there is a crackling noise accom- 

 panying the brush-like flames, similar to that of the flow of 

 electricity from points on condensers. This kind of discharge 

 is called St. Elmo's fire. Discharges between clouds occurring 

 generally in brush-like forms may be what is called sheet 

 lightning. A very powerful discharge encountering great 

 resistance and passing over a long distance produces branches 

 and is known as forked lightning. 



Lightning rods or conductors have been extensively em- 

 ployed in many localities to avert the dangers from electrical 

 discharges. When lightning conductors are of suitable metal, 

 such as copper, of sufficient number, are thoroughly insulated 

 from the building and connected with the earth through water 

 or damp soil, they may be effective. They are intended to 

 prevent violent, destructive discharges by securing the silent 

 transfer of electrical energy from the upper air to the earth 

 and thus equalizing the electrical difference between them. 



SUMMARY 



Two types of local storms are the tornado and the thunderstorm. 



The tornado belongs to the group of rotary storms, which includes 

 simooms of the Sahara desert and waterspouts. 



Tornadoes develop southeast of the center of lows and travel to- 

 ward the northeast. They occur usually in the summer months in 

 the Mississippi valley. 



Thunderstorms are local disturbances in cyclonic areas. They 

 are due to down drafts of cold air and up drafts of warm, moist air. 

 The normal electrification of the atmosphere is disturbed, and light- 

 ning produced. 



REVIEW QUESTIONS 



1. What disturbances similar to tornadoes occur over the sea? 

 Over the Sahara desert? 2. Compare a tornado with a cyclone (low- 

 pressure area) as to diameter, velocity of wind, distance traveled, 

 time of year in which they occur, time of day, and effects. 3. On what 

 sort of day are tornadoes most likely to occur? 4. In what part of 

 the year are tornadoes most frequent? 5. Describe the development 

 of a tornado, its position with reference to a low, and other favoring 



