PRECIPITATION ELECTRICITY 
By ROSS GUNN 
Physical Research Division, U. S. Weather Bureau 
Introduction 
Although atmospheric electricity does not play an 
important part in the control of world-wide weather 
phenomena, it does have considerable bearing on spe- 
cial weather problems. Because of the basic electrical 
nature of all matter, most mechanical and thermody- 
namical energy transformations are accompanied by 
some type of electrical phenomenon. It is not surprising, 
therefore, that the production of precipitation in the 
atmosphere frequently gives rise to interesting and im- 
portant electrical effects. These phenomena share many 
of the peculiarities of weather because of the extreme 
complexity of charge production and transport proc- 
esses in the atmosphere. One serious difficulty in the 
proper evaluation of precipitation electric phenomena 
is that the data thus far collected have been limited to 
a relatively few geographical regions and with few 
exceptions have covered only short periods of time. 
Many of the data are contradictory. 
In view of the complexity of the electrical phenomena 
accompanying precipitation, it might appear that the 
most rapid progress on basic problems would be made 
if laboratory investigations were undertaken. However, 
it is apparent that this would be extraordinarily diffi- 
cult for the same reason that investigations of weather 
in the laboratory have met with considerable difficul- 
ties. Electrification of the atmosphere and of precipi- 
tation is related to the characteristics and development 
of cloud structures, and these have not yet been suc- 
cessfully reproduced under controlled conditions. The 
scientist is forced, therefore, to collect data wherever 
and whenever available and to attempt to deduce from 
them the main characteristics and important processes 
of nature. 
The principal practical problems of precipitation 
electricity that require intensive work are (1) to de- 
seribe the detailed electrical processes responsible for 
the production of lightning, (2) to describe the mech- 
anisms responsible for the maintenance of the observed 
negative free charge on the surface of the earth, and 
(8) to describe those processes that transfer free elec- 
trical charge to aircraft flying through natural precipi- 
tation and to devise a method for counteracting such 
processes. Solution of these practical problems requires 
a detailed understanding of other still more basic ques- 
tions: (1) How is a free electrical charge placed on 
precipitation? (2) Why does charge of a selected sign 
appear principally upon the larger precipitation ele- 
ments? (3) What are the mechanisms responsible for 
the separation of positive and negative charges? and 
(4) How large are the electric fields so produced? Quan- 
titative understanding of these fundamental problems 
128 
will provide a suitable foundation for the solution of 
the more practical problems. 
Earth electrification processes that become manifest 
through easily obtainable measurements are intimately 
related to a dual basic process consisting first of the 
deposition of free charge on precipitation particles and 
then of the subsequent mechanical separation of charges 
haying opposite signs. Mechanisms responsible for plac- 
ing a free electrical charge of selected sign on the pre- 
cipitation particles are not well understood, but they 
are basically related to atomic forces that are both 
physical and chemical in nature. 
The occurrence of electrification and of coexisting 
available electrical energy implies the expenditure of 
mechanical work to establish the electrified state. In 
the earth’s atmosphere this systematic mechanical work 
comes principally from gravitational forces and acceler- 
ations due to turbulent atmospheric motion, acting on 
precipitation particles. Large-scale separation of elec- 
trical charges will occur only when the acting forces 
operate on particles of one sign in a way quite different 
from the way they operate on particles carrying the 
opposite electrical charge. Because precipitation par- 
ticles normally fall im the earth’s atmosphere, the 
observed separation of charges always implies that the 
aerodynamic characteristics of carriers of the positive 
charge are notably different from the characteristics of 
carriers of the negative charge. This aerodynamic con- 
trast between the particles carrying opposite charges 
is of importance in the description of all large-scale 
atmospheric electrifications. 
Observed Free Electrical Charge on Precipitation 
Early investigators were of the opinion that the free 
electrical charge carried to the earth by rain was ade- 
quate to replenish the normal discharge current ob- 
served in fair-weather areas throughout the earth. 
Later work has recognized that other processes are also 
important, but the original ideas stimulated the first 
measurements. The literature on the subject is contra- 
dictory in many cases, and the actual values of free 
electrical charge carried down by precipitation particles 
differ so much from place to place and with different 
meteorological situations that average values are of 
questionable significance. Better agreement between 
various observers is secured if the electrical character- 
istics of precipitation are classified in accordance with 
three distinct types of rainfall: (1) continuous or quiet 
rain (Landregen), (2) shower or squall rain (Béenregen), 
and (8) electrical storm rain (Gewitter). This useful classifi- 
cation was adopted by Gschwend in his measurements 
