Page:Popular Science Monthly Volume 86.djvu/144

140 From a study of individual storms based upon the records of many stations 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 restoration 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 convection 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 insufficient 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 lightning discharge are not disturbed by such a discharge there is great probability that they may attract the lightning a second time. In general, any good electrical conductor projecting above material which offers resistance to the passage of electricity will tend to attract lightning. If this projecting conductor is insulated from surrounding material 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