130 
tially neutral snowflakes. However, of the flakes meas- 
ured, they found free positive charges on twice as many 
flakes as carried negative charges. They also found that 
snow drifting over the ground was strongly negative. 
Chalmers and Pasquill [4] reported that snow is pre- 
dominantly negative. It seems apparent from the lit- 
erature and from other measurements that the sign 
and magnitude of the charge on fallmg snow depends 
critically upon its crystalline structure and mode of 
formation. As an illustration of this fact, this author 
found that snowflakes fallmg quietly with an average 
velocity of 48 em sec“ carried average positive charges 
of 0.00067 esu, whereas simultaneously falling negative 
flakes of average charge 0.0010 esu fell with a velocity 
of 80 em sec. The difference in sign was definitely 
correlated with the rate of fall. It is probable that the 
rate of fall is determined by the structure and density 
of the flake, which, in turn, is determined by its mode 
of formation. It is a fair inference from the data, there- 
fore, that the opposite electrical charges result from 
erossly different developmental histories. 
Average values of free charge carried by both posi- 
tive and negative individual droplets of various kinds 
of precipitation, as measured by Gschwend [11], 
Banerji and Lele [3], Chalmers and Pasquill [4], and 
Gunn [12, 14, 15] are summarized in Table I. 
Tasue I. AVERAGE FREE ELECTRICAL CHARGE ON 
InpivipuaL Dropiets (esu X 10’) 
4 6s) ; Electri- 3 a 
Observer Peel te rasta reac ercean 83 3 
(S) rain & BS 
Gschwend surface| + | 0.24 1.75} 8.11) 0.09) 5.64 
(1921) — | 0.53 5.43) 5.88) 0.06) 4.78 
Banerji and surface | + 6.4 6.9 
Lele (1932) = 6.7 18) 
Chalmers and | surface| + | 2.2 1.3 Bolte 10.5 
Pasquill — | 3.0 22 9 .2* Hd 
(1988) 
Gunn (1947) 4,000 | + = 
— 24 
12,000 | + Al 
_ 100 
20,000 ) + 63 
Gunn (1949) surface] -+ 15 0.67 
— 19 1.0 
Gunn (1950) 5,000 | + 81 
= 63 
10,000 | + 148 
= 112 
15,000 | + 123 
= 76 
20,000 | + 52 
a 62 
* Actual lightning activity doubtful. 
Cloud Elements. If raindrops are formed by the as- 
sociation of cloud elements, it is obvious that the free 
electrical charge collected on cloud particles is of fun- 
damental importance. A number of measurements have 
been made on the charges carried by clouds and fog, 
notably by Wigand [27]. He found that in a dry fog the 
ATMOSPHERIC ELECTRICITY 
cloud elements carried a positive electrical charge, but 
sometimes negative charges were measured. The mag- 
nitude of the charge varied from a few to a few hun- 
dred elementary charges. Using a specially instru- 
mented aircraft [18], the author made a number of 
measurements of the charges on cloud particles in small 
swelling cumulus clouds and found that the charges 
were usually negative. The average charge on each 
element was estimated to approximate 22 elementary 
charges. Scrase [24] found that cloud elements in heavy 
wet fogs frequently carried a negative charge approxi- 
mating 35 elementary units. Accumulation of informa- 
tion on the electrical charges carried by cloud droplets 
under various meteorological conditions is urgently 
needed. 
Processes Responsible for the Electrification of Pre- 
cipitation 
Because all large-scale atmospheric electrifications 
derive their energy originally from expenditure of me- 
chanical or gravitational work and because this work 
can be converted only when a free electrical charge is 
attached to some physical entity like a raindrop, it is 
of utmost importance to understand in detail the physi- 
cal processes whereby free charge, of either sign, can 
be systematically deposited on droplets. One of the 
outstanding characteristics of precipitation elements in 
the atmosphere is the enormous surface area exposed 
to the atmospheric ions and to the chemical activity of 
the air. It has been noted frequently that precipitation 
elements in the air share many of the remarkable prop- 
erties of a colloidal suspension. 
Droplet charging processes may be divided into two 
major categories: first, charging processes which are of 
a basic nature and dependent upon the physical and 
chemical properties of water and air; and second, charg- | 
ing processes which are critically dependent upon spe- 
cial environmental conditions. 
Basic Processes. As a common example of electrifi- 
cation by a basic process, one may mention the separa- 
tion of electricity produced by friction. The rubbing 
together of materials having contrasting physical prop- 
erties usually results in the selective transfer of elec- 
trons in the outer orbits from one material to the other. 
Dry ice crystals sliding along the metallic wing of an 
aircraft communicate large amounts of negative elec- 
tricity to the aircraft and positive electricity to the ice 
crystal. It is well known that snow blowing along the 
ground acquires a strong negative charge which is very 
likely of similar frictional origin. 
One of the important basic processes that produce 
electrical effects in the atmosphere results from the 
chemical adsorption of ions at the surfaces of precipi- 
tation particles. Systematic polarization and orienta- 
tion of surface molecules frequently result. This orien- 
tation produces electrical double layers that are 
responsible for electrophoresis and a number of allied 
surface phenomena [9]. In pure water the polarization 
of the surface molecules is such that the outer surface 
is made up of negative charges, while some 10° cm 
below this negative surface a positive distribution of 
