PRECIPITATION ELECTRICITY 
of charge and mass of individual falling raindrops re- 
ported in 1920 [11]. 
Continuous or Qwet Rain. Quiet rain, usually asso- 
ciated with smaller droplets, carries relatively smaller 
charges per drop than storm rain and is more likely to 
be predominantly of one sign. The majority of experi- 
menters have found that quiet rain is usually positive, 
although examples of contmuous negative rain are 
known. The ratio of positive to negative charges re- 
ported generally varied from 1.1 to 1.5 when adequate 
samples were taken. In spite of the excess observed 
positive charge, it was found that the negative charge 
per droplet im most cases was greater than the positive 
charge per droplet. This implies, and measurements 
show, that usually a larger number of positively 
charged droplets fall. A dependence upon the rate of 
rainfall has usually been observed. A long series of 
measurements by Chalmers and Pasquill [4] in Eng- 
land showed that the number of particles carrying posi- 
tive charges was some 70 per cent more than the num- 
ber carrying negative charges, and that the total 
positive free charge delivered at the earth was some 30 
per cent greater than the negative. It is interesting to 
note in this connection that, if data are collected over 
a sufficiently long period, the net free transported 
charge may be a small fraction of the total. For exam- 
ple, Scrase [23] measured the convected charge con- 
tinuously for a two-year period. His results showed that 
positive charges predominated one year and negative 
the other, but for the entire interval the positive charge 
exceeded the negative by 10 per cent. 
Shower or Squall Rain, Squall-type showers share 
the electrical characteristics both of quiet rain and of 
rain falling in typical electrical storms. Wide excursions 
in the electrical characteristics are normally observed. 
The analysis of rain falling from postthunderstorm 
showers and from those that have not quite proceeded 
to the poimt of active charge separation will be of the 
utmost value in determining the basic precipitation 
charging processes. 
Electrical Storm Rain. The rain falling in a typical 
electrical storm is usually characterized by large droplets 
and large free charges that approximate a few hun- 
dredths of an electrostatic unit per drop. Gschwend 
[11] pointed out the remarkable fact that such rain 
frequently changes its sign. It has been found in active 
storms that after a very few consecutive droplets of 
one sign had been measured the chance of capturing a 
droplet having an opposite sign was very large. The free 
charge brought down by individual droplets falling 
from active thunderclouds has been measured on the 
ground by Gschwend [11], Banerji and Lele [3], and 
Gunn [12]. Gschwend found that the largest charges 
were associated with positive droplets, while the other 
experimenters found the largest charges associated with 
negative ones. In very active electrical storms, Gunn 
found a general trend connecting the charge on a drop- 
let and its radius. The electrification of these droplets 
increased on the average until an electric field that 
approximated 2.5 esu cm was established on the sur- 
face of the droplets. Negatively charged droplets were 
129 
nearly twice as massive as the positively charged ones, 
and each carried about 25 per cent more charge. 
The sign and magnitude of the integrated free charge 
transported to the earth by precipitation in electrical 
storms is uncertain. A number of measurements in 
thunderstorms have shown that negative charge is usu- 
ally transferred to the earth by the acting mechanisms. 
For example, Banerji [1] has estimated that the excess 
negative charge convected to the ground by rain falling 
from an active thunderstorm in India was 2 x 103 
coulombs. 
The electrical state m an active storm is so compli- 
cated and confused that there is doubt as to the value 
of precipitation data as a guide to the interpretation of 
basic electrical processes. This uncertainty in interpre- 
tation has become serious as a result of a recent paper 
by Simpson [25]. He has presented evidence suggesting 
that the charge on falling rain is deposited by conduc- 
tion or corona currents discharged near the surface of 
the earth by the electric fields usually present. Al- 
though a number of earlier experimenters looked in 
vain for such a correlation, Simpson now reports a good 
correlation between the sign of the free charge and the 
direction of the electric field. A careful check of the 
facts im this matter by independent observers is badly 
needed.! 
Measurements taken in an aireraft at various alti- 
tudes up to 26,000 ft have been reported by Gunn [14]. 
These measurements were made mm regions far above 
surface corona discharges and may be the only ones 
that give a clear-cut indication of the charge-producing 
mechanisms in the earth’s atmosphere. In a weak cold 
front exhibiting no thunderstorm activity, positive 
charges averaging 0.033 esu per drop were observed 
from 10,000 to 26,000 ft. Negative charges averaging 
0.040 esu per drop were measured between 4000 and 
20,000 ft. Positive particles were not observed below 
10,000 ft and negative ones were not detected above 
20,000 ft. The freezing level durmg these measure- 
ments was at 11,000 ft. Direct measurement in the 
vicinity of the plane showed that the electric field did 
not exceed 25 vy em, and therefore thunderstorm ac- 
tivity was negligible. A coherent set of similar data [15] 
taken in an active thunderstorm gave notably greater 
free charges on the precipitation and suggests that the 
interpretation of such collected data will be extraordi- 
narily difficult. 
Snow. Gschwend made a number of measurements 
of the charge carried by individual snowflakes. Positive 
charges, in general, exceeded negative charges, and it is 
important to note that the charge on newly formed 
snow is nearly one hundred times larger than the charge 
on quietly falling, and presumably aged, snowflakes. 
Nakaya and Terada [20] found that snow carried a 
preponderately negative charge unless the flakes had 
frozen water droplets attached. Recently, Pearce and 
Currie [22] remarked on the large number of essen- 
1. (Note added in proof.) W. C. A. Hutchinson and J. A. 
Chalmers have just published a paper, “The Electrical Charges 
and Masses of Single Raindrops,” Quart. J. R. meteor. Soc.; 
77:85-95 (1951), that provides needed data on this subject. 
