THE THUNDERSTORM— EVANS AND McEACHRON 



179 



largest drops finally approach a limiting size, which according to 

 Lenard ^ is about half a centimeter in diameter. Drops larger than 

 this are unstable and soon break up into many small droplets surround- 

 ing a rather large center drop. 



In the breaking of rain drops lies the electrification process of 

 thunderstorms according to Simpson, for he believes (and has ad- 

 vanced laboratory experiments supporting his belief) that, when the 

 drops break, negative ions are released into the air while the water 

 drops become positively charged. The negative ions joining with 

 minute cloud particles are carried upward at the velocity of the air 

 currents and are thus quickly removed from the vicinity of the 

 positively charged rain drops. The rain drops are also carried 

 upward but at a slower rate until by recombination they again 



Royal Society of London. 



Figure 1.— Distribution of electrical charge in a heat thunderstorm. (After Simpson.) 



become large enough to fall. In falling they continue to grow until 

 they break up again. This liberates more negative ions into the 

 au" and leaves a higher positive charge on the rain drops. 



So the electrification process goes on. More and more negative 

 ions are liberated and transported to the upper and rear regions of 

 the thunderstorm. The rain drops acquire a higher and higher 

 positive charge and accumulate in the lower part of the cloud in a 

 restricted region. The location of this region is determined by the 

 velocity of the upward air currents; for, since the largest drop which 

 can exist can be supported by an air column moving upward at a 

 velocity of 8 meters per second, no rain can fall through regions 

 where the air movement is equal to or greater than that velocity. 



Thus Simpson arrives at the thunderstorm model reproduced in 

 figure 1. In this diagram, positively charged water drops accumulate 

 in region B above the region where air velocities are 8 meters per 

 second or more (indicated in the illustration by an ellipse at the 

 base of region B). The negative charge is distributed as shown 



• Lenard, P., Ueber Kegen, Meteorol. Zeitschr., vol. 21, p. 249, 1904. 



