I40 TEE POPULAR SCIENCE MONTHLY 



From a study of individual storms based upon the records of many sta- 

 tions it has been found that thunderstorms are most frequently formed 

 in the southern half of a cyclone, where warm and light southerly winds 

 are superimposed by cold and heavy northwesterly winds. In the resto- 

 ration of equilibrium between these horizontal air masses there is violent 

 vertical convection, accompanied by lightning, thunder, heavy rain, and 

 occasionally hail. Though called local storms they usually advance 

 along well defined convex wave-fronts, which measure from 50 to 200 

 miles in length, moving broadside in an easterly direction across the 

 country, at about the rate of a fast express train. The horizontal 

 breadth of this line varies from 10 to 30 miles, while the vertical con- 

 vection extends to heights 5 miles or more above the ground. When one 

 considers the vastness of the mass of air in violent agitation in one of 

 these storms it is apparent that the topography of the ground can have 

 no appreciable effect in determining the course of the storm. Certain 

 it is that throughout the central and eastern parts of the United States, 

 where thunderstorms are a characteristic feature of summer weather, 

 the ground relief is insuflScient to influence the courses. Nor is there 

 any foundation for the belief that the storm has a center of extreme 

 violence, which is usually stated to have passed a point either north of 

 or south of the observer. When the storm line is passing an observer 

 from west to east, perspective causes the cloud to appear darker to the 

 north and the south, rather than in the front or the rear of the storm, 

 or even overhead. 



Tradition has it that "lightning never strikes twice in the same 

 place." The idea is not only without scientific basis, but the opposite 

 may be nearer the truth, for if the conditions which attracted a light- 

 ning discharge are not disturbed by such a discharge there is great 

 probability that they may attract the lightning a second time. In gen- 

 eral, any good electrical conductor projecting above material which 

 offers resistance to the passage of electricity will tend to attract light- 

 ning. If this projecting conductor is insulated from surrounding ma- 

 terial and is anchored deep in the soil, down in the level of permanent 

 moisture, the conductor will protect the surrounding objects. This is 

 the theory of the lightning-rod, which, when properly installed, is a 

 good protector. Though it does actually attract the lightning it may be 

 struck any number of times without damage to things nearby. The 

 Eiffel Tower, in Paris, France, a steel tower 1,000 feet high, has often 

 been struck, six times during the passage of one particularly severe 

 storm. As ample provision is made to conduct the electricity to the 

 earth no serious destruction has resulted. The tradition "lightning 

 never strikes twice in the same place " is therefore more nearly correct 

 when the word never is omitted. 



That freezing temperatures are necessary for the formation of hail 



