PRECIPITATION ELECTRICITY 
exactly the same amount of electricity exists. Inside 
this double layer, a more or less random distribution of 
both positive and negative free charges is re-estab- 
lished. The net result of the double layer on the surface 
of a spherical drop is that the potential inside the drop 
may be greater than that outside by a fraction of a 
volt. This does not mean that free electrification is pro- 
duced, since it must be remembered that exactly equal 
amounts of positive and negative electricity are en- 
compassed by the double layer. However, if the double 
‘layer is subsequently broken and mechanical work is 
done on it or if ions are selectively captured, measurable 
amounts of free electrification may result. 
One familiar charge-producing mechanism, inti- 
mately related to this double-layer process, is the so- 
called waterfall effect first studied by Lenard [19]. 
Subsequent experimentation has shown that when a 
‘droplet of pure water is broken up by mechanical 
means, the residual droplets carry positive charges 
while the adjacent air acquires both positive and nega- 
tive ions. Electrifications produced by breakup, atomi- 
zation, splashing, or bubbling are all intimately related 
to the double-layer characteristics; this subject has 
received much study [2, 5]. Although one widely quoted 
theory invokes such mechanisms to describe thunder- 
storm electricity, quantitative agreement with obser- 
vation is quite unsatisfactory. 
One aspect of double-layer electrification may be of 
importance in the atmosphere when hail is produced. 
Dinger and Gunn [6] discovered that when water freezes 
in the atmosphere a large amount of air is entrapped in 
the ice in the form of tiny bubbles. Upon melting, 
these bubbles are released, and upon breaking the sur- 
face they transfer to the adjacent air a negative charge 
of 1.25 esu gram while the melted water droplet re- 
tains an-equal and opposite positive charge. The free 
charge thus produced is appropriately distributed near 
the freezing level and is sufficiently large to explain the 
presence of active cloud electrification. These experi- 
menters also discovered that the freezing of relatively 
pure water is accompanied by transient changes in the 
contact electromotive force amounting to 6 or 10 vy. 
Because the charge distribution at the surface, respons- 
ible for the contact electromotive force, is in the nature 
of a double layer, they did not believe it contributed to 
a net electrification of a freezing droplet. Using special 
dilute solutions, Workman and Reynolds [29] have re- 
examined this latter process and report that potential 
differences exceeding even 100 v are produced under 
special circumstances. 
It is important to note that the magnitudes of elec- 
trical effects in all double-layer phenomena depend 
erttically upon the purity of the water. Banerji [2] has 
remarked that impurities commonly existing in precipi- 
tation in the atmosphere are usually sufficient to reduce 
the expected electrical effects to small values. 
Environmental Processes. It is an observed fact that 
the atmosphere is pervaded by relatively large numbers 
of ions of both signs. The small negative ions move 
10-40 per cent faster than the positive ions when acted 
on by the same force. Therefore, the negative ions usu- 
131 
ally determine the charge captured by an initially un- 
charged and insulated body. 
Tt has long been known that an msulated conductor 
supported in an ion stream becomes charged due to 
the selective capture of ions. Pauthenier and Moreau- 
Hanot [21] have formulated this process in a quantita- 
tive theory that appears to agree well with experiment. 
Wilson [28] has also pointed out that a droplet fall- 
ing in an electric field is polarized, and as it falls it 
selectively captures, because of its motion, the more 
mobile ions in the volume it sweeps out. Experimental 
results confirm the reality of this process [10]. Since 
this important charge-separating effect depends upon 
the existence of an initial electric field, its application 
to the description of the electrical properties of the 
atmosphere is obscure. The mechanism is useful in 
describing changes in the electrical state subsequent to 
the establishment of an electric field by some more 
fundamental mechanism. 
Electrification occurring when rime is deposited on 
a conducting surface or on graupel is considered im- 
portant by Findeisen [8], who has based a theory upon 
it. The effect is real, but its quantitative relation to 
thunderstorms has not yet been completely worked 
out. The application of this mechanism is attractive 
because it correlates the observed high electrification 
occurring near the freezing level with theory. 
An environmental process first investigated by Gunn 
[13] depends on the differential migration of atmos- 
pheric positive and negative ions under the influence 
of a systematic transfer of water molecules. He showed 
that the transfer of water vapor towards a condensing 
droplet results in a transfer of momentum to both the 
positive and negative ions in the vicinity and the es- 
tablishment of a greater concentration of the most 
mobile ion adjacent to and upon the condensing drop- 
let. The charge capable of being transferred to such a 
droplet is related to the vapor stream velocity and to 
the thermal kinetic energy of the molecules, and hence 
in the atmosphere is something less than 0.1 v. It 
should be clear that reversing the direction of the water- 
vapor stream will reverse the sign of the charge on the 
evaporating or condensing droplet. 
Association Processes. In attempting to understand 
the basic mechanisms responsible for the surprisingly 
large electrical charge sometimes carried by precipita- 
tion, one process should be emphasized. Without dis- 
cussing the details of association, it seems certain that 
rain produced below the freezing level results from the 
association of an extremely large number of cloud 
particles. In typical cases, the number of cloud particles 
associated to produce a single raindrop is surprisingly 
large, and if any process systematically transfers even 
small charges of a given sign to the cloud particles, then 
the total charge may be large. Gunn [13] worked out a 
complete “association theory” of electrical storm activ- 
ity based on this idea. He remarked that a number of 
physical and chemical forces could be expected to trans- 
fer small charges to the cloud particles. To illustrate 
the theory, he adopted the notion that each cloud 
particle was an electrical concentration cell and that 
