THE THUNDERSTORM— EVANS AND McEACHRON ^35 



regions attracts them. Similarly, the small positively charged drops 

 being carried up by the air currents are repelled by the charge at the 

 top of the cloud and attracted by that in the lower part of the cloud. 

 This opposition, of course, increases in proportion to the increase in 

 field as the centers of charge are replenished. The secpnd reason 

 advanced by Wilson for the lower rate of field increase as the charge 

 builds up is the greater dissipation of energy due to local ionizations 

 in the intense fields about charged regions as their voltages are 

 increased. 



Summarizing, electrical field measurements graphically show the 

 separation of charge and its accumulation in limited regions in the 

 cloud ; show how this charge is removed or neutralized by a lightning 

 stroke; and show how after a stroke the electrification process builds 

 up the charge again to the value necessary to cause another discharge. 



CHARACTERISTICS OF THUNDERSTORMS 



SINGLE THUNDERSTORMS 



Upon the approach of a thunderstorm, strong gusts of 'svind are 

 frequently experienced blowing from the storm toward the point of 

 observation. In the distance, heavy rain can be seen falling mth 

 frequent lightning discharges occurring near the front of the rain area, 

 as in plate 2, figure 1. On the nearer approach of the storm, a scat- 

 tered fall of large rain drops or possibly a light rain occurs. This is 

 soon followed by a heavy downpour usually accompanied or imme- 

 diately preceded by the heaviest lightning of the storm, as the front 

 of the rain area and the Ughtning discharge center arrive overhead. 

 As the storm cloud is carried along with the general air movement, 

 lightning soon starts striking beyond the point of observation, rain 

 settles down to a steady downpour, and the observer realizes that the 

 worst of the lightning for that section of the storm is past. 



Thunderstorms viewed from a distance frequently show several 

 prominent thunderheads resulting from the formation of several 

 convection systems within them. Each of these systems may produce 

 one or more lightning discharge centers. Therefore a succession of 

 conditions and events as described for a single center is often expe- 

 rienced during the passage of a thunderstorm. 



It is apparent that in many storms the discharge centers are of 

 relatively limited dimensions. By knowing the velocity of travel of 

 the cloud, it would be possible to form a fair estimate of the length 

 of a center in the direction of storm movement. As an illustration, 

 assume 20 miles per hour for the velocity of a particular thunderstorm 

 and 1 minute as the time during which discharges were occurring 



