THE THUNDERSTORM— EVANS AND McEACHRON Ig'J 



SHIFTING OF DISCHARGE CENTERS 



From the foregoing one might be left with the conception of a 

 thunderstorm proceeding over the country with one or more discharge 

 centers always maintained at the same locations in the cloud. This 

 conception must be modified because, as the storm travels, moisture- 

 laden air is not constantly supplied to the same part of the cloud. 

 Air is naturally stratified, and nonhomogeneous, so that conditions are 

 not constantly favorable in front of the centers for a supply of moist 

 air. Topographical conditions, the presence of rivers, etc., also affect 

 the position and amount of moisture-laden air supplied to a given part 

 of the cloud. As a consequence, the regions of greatest convection 

 shift about in the cloud as it travels. Old centers discharge their 

 electrical energy and, since the convection currents have decreased 

 greatly, fresh accumulations of charge occur more slowly. Dis- 

 charges therefore occur less often or cease. Meanwhile, a nearby 

 region in the cloud may be more favorably supplied with moisture. 

 A convection system of sufiicient intensity to electrify the rain drops 

 and separate the oppositely charged particles then develops in that 

 region. Thus a new center is born, and lightning discharges occur 

 from it. 



FAMILIES OF THUNDERSTORMS 



Wlien conditions over a wide area are favorable for the formation of 

 thunderstorms, a number of such storms may be formed and several 

 may pass over the same region within an hour or so of each other. If 

 these occur at night and people are not in a position to watch the 

 storm movement accurately, they frequently get the impression that 

 the same storm "hangs around all night", paying them repeated visits. 

 Actually, the general air movement at the cloud level has carried 

 them a succession of storms. In the daytime several members of a 

 family of such thunderstorms can frequently be seen at one time from 

 good observation points. 



CAUSES OF THUNDERSTORM FORMATION 



The necessary conditions for the formation of a thundercloud are: 

 (1) the presence of sufficient moisture in the atmosphere; (2) the 

 presence of meteorological and (or) topographical conditions favorable 

 to the movement of moisture-laden air up to the condensation level; 

 and (3) conditions favorable to the formation of sustained strong 

 upward convection systems. 



There are five general types of thunderstorms, the type depending 

 on the nature of the meteorological and topographical conditions. 

 These are the heat, the mountain, the cold-front, the overrunning- 

 cold-front, and the warm-front thunderstorms. 



