On the mechanism of electricity generation in thunderstorms (o mekhanizme generirovaniya grozovogo elektrichestva)

National Aeronautics and Space Administration

Goddard Space Flight Center

Contract No. NAS-5-3760

ST - AM - 10 307

*" ■ IMABA CR OR TUX OR AD NUMBfeH*

NASA TT F-9656

CTHlllJ

i

(CATEGORY)

ON THE MECHANISM OF ELECTRICITY GENERATION IN
THUNDERSTORMS

[REVIEW PAPER]

by
V. D. Reshetov

[USSR]

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2 2 MARCH 1965

NASATT F-9656
ST— AM— 10 307

ON THE MECHANISM OF ELECTRICITY GENERATION IN
THUNDERS TORMS •

[ REVIEW PAPER ]
Central Aerosol Observatory** by V, D. Reshetov

NATURE OF AEROSOL PARTICLE CHARGING

Works on atmosphere electricity, carried out during IGY, includ-
ing the time periods before and after it, allow to derive a series of
conclusions as to the nature of thunderstorm electricity.

In the work [1] the author has shown that electric charges, whose
sign depends on the acid or alcaline properties (pH) of the atomised
solution, are formed on particles of stationary or slowly evaporating
aerosol* If the pH of the moisture, of which aerosol particles are formed,
is greater than 5$ positive charges are formed on particles, and when
pH < 5f these charges are negative. This phenomenon is explained by
water dissociation into H + and 0H~ ions and by selective sorption of
these ions by aerosol moisture. The charge, then forming on aerosol par*
tides, constitutes several tens of elementary charges. The electric
potential of particles relative to air reaches 20 — 30 MW.

These results allowed the author to derive a theory of origin
of the Earth's normal electric field [2]* The latter is explained by gra-
vitational settling of positively charged aerosol in negatively charged
gas that surrounds the terrestrial glove. The negative charge of the
Earth is formed by transfer to Earth of air particles with negative charge
due to its conductance. This is a further development of the idea of
colloid electricity application to atmospheric electricity events, first
postulated by Frenkel 1 C53*

* MEKHANIZME GENERIROVANIYA GR0ZOV0GO ELEKTRICHSSTVA.

** The periodical, where the present paper was published could not be
ascertained, but, it is evidently in the cateogry of IGT series, not
published by the USSR Academy of Sciences, but by some regional agency
or institute, in the years posterior to 1962.

/

Imyanitov and Jhubarina [31 have shown that, judging from the
results of measurements during IGT, the v/idely spread theory of electric
field of the Earth, known as theory of spherical condenser , failed to
find corroboration. They reached the conclusion that it is more appropriate
to pass to the model of a charged sphere, surrounded by a space charge,

VJhile considering the possibility of cloud drop charging owing to
selective sorption of ions of various salts, Nikandrov indicated [6] that
the excess of ions of one sign in the superficial layer should lead to
their evaporation alongside with water molecules.

Muleisen PO has shown that the sign of the drop charge in arti-
ficial and natural aerosols depends upon, whether or not, condensation
and growth of particles or their evaporation take place : in the first case
a negative charge forms on particles, in the second case— a positive one
is generated* Muleisen brings forth the assumption, that the cause of this
resides in the liberation of negative ions from the surface of the water
during evaporation* These ions are captured by condensation nuclei and
form a negative charge? at fog evaporation the negative ions are liberated
again and carried by the electric field.

The mechanism of aerosol particle charging was the object of a
conclusion in the work [1], derived on the basis of the works by Huleisen
[Jf] and Nikandrova [6]. It may be made more precise in the following manner.

Starting from the principle of selective adsorption of ions H and
OH" by water, and assuming that the concentration of hydrogen ions, being
more adsorbent, increases in depth of the droplet (whereas at periphery
mostly OH" ions are concentrated), we may write after the Boltzmann law [1] :

W
[H+|. = [H+| B e* r . (1)

where (H + | B and [H + j p are respectively the concentrations of hydrogen ions
inside and on the surface of the droplet.

The processes of condensation, evaporation or stationary state
may be characterized by the number of water vapor molecules, outgoing into
the air fron the surface of the drop n u , and the number of vapor molecules
^ , settling on it out of the moist air.

The drop will have a zero potential relative to air only when

^[H+J^nJH+1,. (2)

Assume, that at significantly remote distant from the drop

IH^-IOH-U = !(»-»• (3)

This means, that there exists near the surface of the drop an
increased concentration of ions (0H~), described by the correlation

W
[OH-] a = [0H-l 0O ** r . (4)

Factually, this correlation is disrupted by the turbulent diffu-
sion of ions OH"" from the boundary layer to air, free from the action of
superficial forces of aerosol particles. Bearing in mind that lgtH + ]« — pH,
we obtain from (l), (2) and (3) an equation describing the condition of
neutrality of the drop or of the aerosol particle

Therefore, the sign of the charge of a drop depends not only on acid
or ale aline properties of the solution forming it, but also on whether or
not there take place on the drop either condensation or evaporation.

In Fig. 1 we reproduced the curve of the electrokinetic potential
of the drop and its chcjrge (counting on drop's radius of 5 mk) dependence
pH, forming the solution's drop, obtained by the author by empirical method
in stable or slowly-evaporating aerosol (curve a)# Plotted in the same figure
is the curve f for the case of intense condensation and the curve 6 for
the case of violent evaporation. The curves f and 4 were obtained by shift-
ing the experimental curve accordinn to equation (3)» At the same time, we
took for the example of condensation lg-j L -" B 2 t and for evaporation lg^ — 1.
It may be seen in the figure that the drops of solutions of same acidity,
for example with pH = 5 to 6, will have a different potential relative to
air at condensation and evaporation processes. In the stationary state the
charge of the drop at such pH will be positive or near zero; at condensation

k.

such a drop will have a negative potential and charge relative to air;
at evaporation they will be positive.

Fig. 1. - Dependence of the electrokinetic potential
at the surfr^e of aerosol particles (of the cloud)
on acid or alcaline properties of moisture, conden-
sation arid evaporation processes, forming thenu

a— state of equilibrium or stage of slow evaporation;
5 — intensive condensation; fl— intensive evaporation.

Let us recall, that the moisture of falling out precipitations
has an average pE of 5— 6 [2]. Assuming for a moist air the prevalence
of ions OH" over ions H + , which is regular since X_>X + [19 f 20], instead
of [OH-loo~10- 7 we may take, for example, [OH-]oo~ m-10-7, where m>l. 'Then,
in the ri^ht hand part of the equation (3)j the first term will be some-
what greater than 7 (for example, at m= 2 it will be equal to 7#2).
This does not virtually introduce any changes in Fig. 1.

A series of works appeared during IGY, which corroborate one way
or another the above-discussed mechanism of charge formation on cloud
drops and aerosol particles. Thus, Blanchard £21] ascertained, while
investigating the charges of drops forming during rupture of air bubbles
passing through sea water, that the tiniest spray is charged positively.
It is well known, that sea water has an alcaline reaction with pH near 9*
that is, ions OH" are in excess in it# The primary ion 0H~ desorption
from the evaporating sea wnve spray leads to positive charging of the
then formed tiniest droplets and particles of aerosol. Therefore, the ocean
is apparently the main supplier of positively charged atmospheric aerosol.
ihe negative ions OH*, liberated from the surface of such an aerosol,
increase the negative charge of the air» Part of such a charge is yielded
to terrestrial surface • Thus forms the negative charge of the Earth. In
the unitary wave of the daily course of atmosphere^ electric field the
maximum occurs near 15 00 — 20 00 hours GMT, when the greatest part of the
universal ocean is lit by the £un [2].

Kuettner and Lavoil [233 f while studying the mechanism of aircraft
electrization, had set up the experiment, whereby the sampler, in the form
of icy grating was exposed in a screening cylinder beyond the window of
the mountain observatory tower. In clear weather a positive charge occurred
in 100?£ of cases on the surface of the sampler. During mist with drifting
snow, that is when the observed conditions were favorable for sublimation
(condensation), a negative charge appeared on the sampler.

These experiments corroborate also the above-described mechanism
of electrokinetic potential emergence and charge subdivision at water-air
boundary on account of spontaneous water dissociation and selective sorp-
tion and desorption of the then emerging ions H + and OH*, different in
conditions of evaporation or condensation.

According to data on electric field measurement under clouds and
above them on the slopes of Elburs, Krasnogorskaya C93 has established
that in their initial development stage clouds have a negative space
charge. According to Pudovkina, the same clouds acquire a positive charge
when passing from generation to stationary state and also in the state
of breakdown.

Krasnogorskaya assumes [93, that the adsorption mechanism of
charged particles during the initial stage of cloud development plays a
specific role in the formation of negative space charge; this adsorption
mechanism being based upon the property of water drops to adsorb mostly
negative ions*

Surging up the above data, one may note that during condensation
processes, aerosol particles or the forming cloud droplets acquire a nega-
tive charge f but at evaporation or in the stationary state they acquire a
positive charge, provided their reaction is not too acid (pH>5)» The
selective desorption (or adsorption) of ions OH** and H* appears to be the
cause of this phenomenon. By the strength of selective sorption of these
ions, mostly ions OH are concentrated on the surface of aerosol particles*
At evaporation they break away alongside with vapor molecules and outgo
in the air. This leads precisely to positive charging of aerosol particles
(droplets) in the stationary or evaporation stage.

Jhen violent condensation onsets, water vapor settles on condensation
nuclei (aerosol particles), which has mostly negative ions OH". At the same
time there are formed embryonic negatively charged droplets. Apparently,
the cause of preferential settling of vapor molecule complexes containing
ions OH on the surface of embryonic droplets or aerosol particles at con-
densation, consists not only in their relative excess in the air, but also
in that these particles have, by the strength of great mass, a great micellar
force potential, contrary to ions containing H + . Their "condensation point"
is thus lower than that of the ion complexes containing H + # Therefore, the
energy of charge division between water and gas-like aerosol medium and, in
the last resort, the enerry of formation of localized charges in clouds
are drawn out from adsorption and condensation heat. Taking the value of
condensation heat equal to 600 cal/g for a 5 g/** cloud water content, we

shall obtain the heat energy of condensation, liberated (or stored) in lknr f

9 10

equal to 3 # 10 k cal or 1#25 # 10 k. joules. The energy density of electric

fields in thunderclouds for their usual field intensity near 100 v/cm is

*U5 # 10 k joule/km , and at high intensity of 10 000 v/cm it reaches

^•5 • 10 k joule /km # Therefore the transformation of a small part, no more

7.

than 0.0004 — O.O^f % of thermal energy of condensation and selective ad-
sorption into energy of electric charge separation at boundary water - air,
is sufficient for the explanation of atmosphere's electric field energy*

GEHmTION O F CEABGES IN A THUHDERCLO TO

Contemporary observations corroborate the distribution of charges
in a thundercloud given in the Simpson and associate ref, Cl03 scheme.

Thus, Tamura [24] established on the basis of synchronous measure-
ments of the electric field in thunderclouds from 8 points, that in thunder-
storms the centers of negative charges, inducing discharges from cloud to
ground, are at heights from 6 to 8 km, while the centers of positive char-
ges are by 5 to 6 km higher, that is at about 12 km height level*

Fitzgerald and Byers [253 established, while conducting oonveotive
cloud observations from aircraft, that the electric structure of cumulus
clouds corresponds to ascending current (updraft) structure and reminds
one of "charged columns 11 . At the same time it was revealed that in regions
with great content of droplet moisture and strong updrafts, excess nega-
tive charges are observed in the stages of active development of a power-
ful cumulus cloud. The solid precipitations, including the lines of fall
from anvil-shaped thunderclouds have, immediately upon formation, a posi-
tive charge. It v/as noted, that the field intensity in clouds increases
with the latter* s water content.

Chapman [26] made more precise the methods of computations and
the data of Simpson et al on measurement of electric fields in thunderclouds.
The improved values of mean field intensity in thunderclouds were found to
be near 10 v/cm.

Eatakeyama [27], while admitting that the positive charge is at
10 km height and the negative one at 6 km, and measuring the rapid fluctu-
ations of the electric field at lightning discharges, has established that,
as an average, a charge of one sign in a thundercloud is equal to 90 k
and reaches up to *f00 k.

According to Norinder data [10], the predominating number of
lightning discharges transfer charges of nearly 2 k, but there occur dis-
charges, transferring up to 35 k. At the same time discharges to ground

8.

from negatively charged part of the cloud constitute 93% of all lightning
strokes on ground. The usual duration of a discharge is within the 100 —
200 fisec range. The strength of the current in vertical lightning is pre-
dominantly below 20 ka, but there occur discharges with currents to l*fO ka.
The time of current accretion to half the optimum value is about 6 f± sec.
Calculation by Norinder show, that if the disposition of the space charge
in the cloud is represented in the form of an ellipsoid,, a field strength
near 25 kv/cm will be necessary to initiate a lightning stroke toward
Earth at the lower end of the ellipsoid, situated at the height of 2 km
above ground. Lt the same time, a field intensity near 3»5 kv/cm is ob-
served*

Smith [28] found, while investigating electric fields in Florida
thunderstorms, that most of the observed discharges took place inside the
clouds* At the same time in 95 # of cases 5 the positive charge was situated
above the negative one* The frequency of discharges at the center of thun-
derstorm activity constituted as an average one stroke every two minutes *
He noted that the restoration of the field at downpours took place very
rapidly, and assumed that here the effect of droplet sprinkling was manifest,

Vonnegut and iloore [291 « while describing the electric storms
taking place in tornado regions, pointed out that the gigantic thundercloud
of a tornado spreads in height to 20 km, the updraft velocities attaining
100 m/sec. The lightning at the center of the funnel hits continually,
while at periphery of the tornado, sounds are observed, reminding one
of d powerful humming, which apparently is the consequence of glow-discharge,

When studying the position of the lower seat of the positive dis-
charge in a thunderstorm, Williams [303 found that the seat is mainly
situated ahead of the rainy zone and has 1 km in height and width. For a
series of storms of width near 10 km, he established that the discharges
to ground took place in a narrow zone of most intense precipitations, of
about 2 km in width, moving together with the cloud.

Very interesting results were obtained by Atlas [313 during radar
observations of lightning discharges. To that effect he utilized a powerful

9.

radar with a 10.7 cm wavelength with a sounding pulse duration of 2 M-sec
and 5 Mr power. Because of the concealing reflecting signal from precipita-
tions he could mainly observe lightnings above 5 km. He established that
lightning reflections begin, as a rule, above reflections from precipitations
and namely from a height of l'f — 15 km. The discharges in the upper part of
the cloud have a shape of vertical streamers, widening downward. Observed
also are reflections from large volumes, being apparently regions of conti-
nuous ionization in the upper parte of clouds or above them. Discharges of
great vertical extension are seldom observed, while those of small extension
are very frequent.

Chuvayev and Imyanitov [113 have established that the intensity of
of the electric field of a powerful cumulus cloud rises sharply at the moment
of time when a bright spot appears on the radar screen from the region of
large-size particles emerging in the cloud* It is possible that the transfer
of water from liquid to solid state is attended by a certain variation of
the electrokinetic potential at the boundary ice-air, which would be an
explanation of the authors' opinion that the rise of electric field intensi-
ty is caused by the appearance in the supercooled summits of the clouds of
ice particles. But the question of the possibility of generating significant
charges at transition of water from liquid to solid state has not, so far,
been fully studied.

Moreover, well known are the facts of large thunderstorm fields,
of discharges in the form of summer lightnings and lightnings, even in
powerful cumulus clouds devoid of iced summits and precipitation zone,
for example of the so called dry thunderstorms.

Thus, Moore, Vonnegut and Botka [32] point to the appearance of
significant electric fields in powerful cumulus clouds prior to detection
of radar reflections from precipitations. At the same time, the intensity
of the field in the cloud is found to be greater than under it. In the lower
p?rt of the cloud, which is completely free from the icy phase, great nega-
tive charges are formed, xhe authors note that this negative charge increases
with the rise of cloud's height. At lowering of the "tower 1 , 1 the charge dis-
appeared and the field intensity under the cloud acquired normal values.

10.

Therefore, the mechanism of generation and separation of charges
begins to act from the time of thunder cloud development and still prior
to its summit's icing, but reaches its maximum only in a period of distinct
thunderstorm activity. This leads us to think that the electrization of
cloud elements does not take place on account of the causes assumed in
the Simpson and associate theory , but is due to the action of other factors.
One of such factors can be the effect of cloud drop electrization under the
action of cloud moisture partial dissociation on ions and of selective
adsorption (desorption) of these ions at the surface of cloud particles.

STAGES OF A T HUHDEBSTOKM

Pre thunderstorm Stage o f the C loud , As follows from the data
brought up in Fig. 1, there emerges at the boundary water droplet — air
at moisture's pH near 5 — 6* usual for atmospheric aerosol, an electro-
kinetic potential of several millivolts, apparently not higher than 10 mv,
which conditions the appearance on droplets of *** 5 n*k radius, up to 10-20
elementary charges. At usual content of some 100— 300 droplets per en?
of cloud volume this leads to the appearance of a space charge near
1 • io^ — 6 • 10* elementary charges in Icnr . Such order of space charge den-
sity was also obtained by Huleisen during his experiments [*f] in the con-
densation processes in laboratory conditions as well as at field experi-
ments with fo^s. During violent condensation in an ascending jet of a power-
ful cumulus cloud the number of particles probably exceeds 1—2 thousand
per 1 cm3. At the same time, the space charge too may constitute in thin-
droplet part of the cloud near 5 • 10 elementary charges in 1cm . At the
same time, in a volume of 10 km , which is characteristic for a thunder-
cloud, there can be a space charge to 80 k.

In the upper part of such cloud, in the region of "fused" summit,
when conditions prevail for the evaporation of a certain part of droplets
under the action of compensating currents [12], a comparatively small posi-
tive charge may be forming. Because of small vertical power and water
content of such a cloud, it still does not manifest clear thunderstorm
activity, but the electric fields may attain in it increased values.

11*

First Sta ge of a Thu nderstorm (stage of stunner lightnings
Fig. 2a). - In this stage the cloud develops to a great height. The
ascending currents in its lower half become still more intense (becau-
se of liberation of the latent condensation heat). The above-considered
process of charge formation will continue. Because of great vertical
dimensions ajid great wr.ter content, the negative charge on the droplets
forming and growing at condensation in the lower half or two-thirds of

-60*

Fig. 2. - Development of a Thunderstorm

a— first stage (stage of summer lightnings; & - stape of
upper discharges (stage of observable radar shadow); * —
stage of maximum intensity of discharges to earth (stage
of thundershowers) ; V — stage of storm attenuation.
1— summer lightning; 2 — suspended cluster of coarse par-
ticles in the ascending current (updraft) f carrying a great
negative charge; 3 — lightning between the negative charge of
coarse particle accumulation and the positive charge of the
upper part, of the cloud; k — region of coarse particles 9 fall;
5— region of the thundery rain; 6 — region of upper dis-
charges; 7 — region of intense negative discharges to ground;
8 —region of horizontal discharges in the cloud.- 9 -region
of a showery rain ; 10 — region of positive charges in downward
currents of the collapsing part of the cloud; 11 — accumula-
tion of coarse particles in the preserved jet of an ascending
current- The magnitude and the direction of the arrows beyond
clouds conditionally designate the magnitude and the direction
of the vector of electric field intensity.

the cloud, will increase. In the upper part of the cloud, having reached
great heights, the updraft velocity is comparatively small. Droplets

12-

reaching here from below are either in the stationary state or in the
state of slow evaporation. The ions 1 selective sorption process leads
then mostly to preferential ion OH - " yield and the droplets , alongside
with the whole upper part of the cloud acouire a substantial positive
charge. The electric fields may attain very high values in the upper
part of the cloud. Here very mild (quiet) discharges, or discharges in
the form of lightnings noted by Atlas may begin to take place C3l3.
We noted on the basis of prole ^ed visual observations, that high and
powerful cumulus clouds are often attended in nighttime by summer-type
lightning without sound effect and with no precipitations* In daylight
these go unnoticed, but they create radioifiterferences,

Sta^e of Upper Discharges (corresponding to that of the observable
radar shadow, fig. 2^). The formation of coarse precipitation particles
suspended in the cloud and inducing the appearance of a radar spot, may
apparently set in at formation of the "anvil", i« e. the appearance of
the crystalline icy phase in the upper part of the cloud, or without
summit icing of the cumulus-nimbus cloud. The author managed to effect
a few flights on free aerostats during powerful cumulus cloudiness. In
one puch flight the aerostat was carried by an updraft under a powerful
cumulus, from w:ich there was no precipitation* Inspite of pilo's efforts
to release ::-as , the aerortrt reached 4 — 5 km heights and vas ejected
through tie powerful cumulus cloud's summit. During flight in the upper
hrlf of the cloud, strong' turbulent motions could be observed. Wind gusts
threw now end then solid ice precipitations of "sleet" (amorphous grain)
type, and noise of its hitting the aerostat could be perfectly heard
fro:.: the nacelle. Yet, when the aerostat passed the cloud summit, no
indications of any icing were noted.

The precipitations falling from the supercooled strato-cumulus
clouds in the form of rain or snow also begin often inside clouds, whose
lower boundary provides no indications of crystalline structure Cl] .

Moore, Vonnegut and Botka [32] point out, that the most intense
bright spot of radar reflection occurs shout 20 minutes before the cloud
summit acquires that characteristic filament shape of the icy phase.

13.

The comparatively email precipitation particles, felling from
the cloud, are found to be somehow suspended in the updraft at 4 — 6 km
height, where their fall velocity becomes equal to that of the ascending
current (updraft). The latter carries into that region from down below
newer and newer masses of supercooled cloud drops, carrying negative
charges that emerge in ti e violent condensation zone.

This mass of tiny supercooled and negatively-charged drops
settles in the suspended accumulation of coarse particles, which then
growrcDidly. Bibilashvili et al Llkl point out, that the water content of
such a part of the cloud reaches colossal values, up to 20 g/m .

List [333 studied the growth of hail when blown over by air
containing supercooled droplets and crystals. He showed, that hailstones
grow mainly at the expense of drop sticking. As a 6ide result of his
experiment, there was the revelation of the fact, that the electric poten-
tial of hail reached then discharge voltages.

Schaefer [3^3 conducted experiments in creating oily and watery
mist with the help of a nozzle. He noted that during formation of coarse
drops, flying through a cloud of tiny droplets, there occurs a disturb-
ance of the electric field, often leading to an explosion probably as crib -
able to electric discharger from coarse drops.

Shishkin showed theoretically Ll3 , that the growth of raindrops
during their merging at fall must be attended by a rise of drops 1 potential <
He assumes that field intensities, sufficient to start discharges, can be
be induced that way.

The electric potential xl of the coarse particles suspended in
the cloud, whose radius is R, and which are formed by settling over them
of supercooled cloud droplets of radius r, having a potential Uy, can be
described by the formula

"s-Mf) 2 < 6 >

which is easy to obtain in the assumption, that the space charge of the
tiny drops, from whose combination the coarse particles grew, is preserved.

14.

At the 5 mv electrokinetic potential on tiny cloud droplets, with a 3
radius of cloud droplets and 3 mm radius for the coarse ones, we find
that the electric potential on coarse particles is 5 000 v. The magnitude
of the charge on such a drop will be 1,6 •10~^k«

At cloud water content of 10 g/ar in the zone of radar spot
visibility, we obtain 80 such droplets per nr of air, and in each cubic
kilometer there will be 8*10 such droplets. The space charge, con*
centrsted in such accumulation of coarse drops is found to be equal to
120 k per 1 km .

We apparently obtained overrated values of space charges in a
thundercloud. Charge values of several hundreds of coulombs are seldom
encountered in thunderclouds. Nevertheless, the computations character-
ize the intensity of the phenomenon. The overrated values of the volume
density of charges were obtained because formula (6) does not take into
account the charge outflow from the coarse drops. Meantime, this outflow
becomes more substantial as their potential rises. At field intensity of
500 v, it is close to glow discharge and appearance of streamers* Assuming
the velocity of the ascending current at the center of the jet in the
lower part of the cloud to be lOm/sec, the air being saturated by moist*
ure at 15° C temperature, we shall obtain that 3«**g/nr will be condensed
at every minute. Let us admit that the radius of drops is ecual to 10 mk
The number of such droplets, forming in 1 cm 5 per minute will be 800.
Assuming, that because of selective sorption of ions OH", there emerges
on each oi them a charge characterized by an electrokinetic potential
near 5 mv, which corresponds to 17© e^e^entarv charges , we shall obtain
a generation near 4 k per minute in every cubic kilometer within
the lower part oi the cloud. 'As already mentioned, part of this charge
settles over the coarse particle accumulation in the middle part of the
cloud, while the other part is carried to its upper part. Here the par-
ticles lose the negative ions OH" on account of equilibrium state or
slow evaporation conditions and acquire a positive potential and charge
relative to air.

15.

This loss of negative charge can be estimated at ~1 — 2k/knr min.
The positive electricity of the upper part of the cloud is generated with
about the same intensity. Data by M. and R. Reiter [22] f having shown
that the pH of thunderclouds constitutes 6.37 as an average, while pH
of other forms of precipitations does not exceed 5.75 — 6.14, serve as
an indirect indicator that the negative charge of a thundercloud is gene-
rated as a consequence of selective adsorption of ions OH at violent
condensation. And this happens at the time, when an increased acidity can
be anticipated in the thundershower because of the appearance in the air
of nitrogen oxides, consequence of lightning discharges. The thunderstorm
phenomena at this stage are already intensely manifest. Strong thunder-
storm fields are observed in the upper half of the cloud with lightning
discharges between the upper positive and lower negative charges. There
are cases of thunderstorms, described in literature, when these phenomena
occur prior to precipitations and appearance of summit contour
i.e. prior to passage of elements constituting the upper part of the
cloud to crystalline state C353 • During such a stage scarce lightnings
to ground may begin to take place, though still seldom. The second stage
of the thunderstorm differs little from the first. The fact of the matter
is that concentrations of large drop elements begin to form already prior
to passage of cloud summit elements to crystalline state; these cores
form alor^ the axis of ascending currents; meanwhile, ice particles appear
in the supercooled pert of the cloud. From the investigations by Chestnaya
end ?iaytsev Cl53, it may be seen that the formation process of coarsened
elements end of the core with increased water content is already apparent
in cumulus clouds of comparatively lesser thickness. Thus, all evolves
around the quantitative aspect of the matter. The accumulation of charges
and the electric fields generated by them reach a level sufficient to
promote an active manifestation of thunderstorm electricity precisely
during the above-described second stage of development of a powerful
cumulo-nimbus cloud. This coincides with the penetration of cloud summit
to great heights, with partial and total formation of the anvil, and
mainly with the formation of the core of suspended, radar-detectable
particles.

16.

St age of Maximum Discharge Inte nsity (Fig, 2§) . The distinctive
side of the tliird stage of a thunderstorm resides in the fact, that pre-
cipitation particles, having grown to large sizes, begin to fall to
ground with a velocity, exceeding that of the updraft. This avalanche-
like fall of the accumulation of coarse particles, charged to a high
negative potential and carrying the accumulated charge of several tens
or hundreds of coulombs, leads to a sharp electric field increase in the
space cloud-ground. The high potential of drops favors the beginning of
ionization and the appearance of lightning , starting with intense and
frequent strokes to ground. At the same time the cloud is found to be a
negative electrode, the ground being positive. The large drops or ice
particles, falling from the upper layers and carrying negative charges,
feed these negative discharges to ground.

Assuming the radius of a coarse particle being 3 sun* and its
potential limited to 5mv, each drop will carry a charge equal to
1.75 • 10*9 it. Let the intensity of the thundery rain be 10mm for 30 min.
Calculation shows that 2500 drops of above-dimensions should then fall
on a square meter. The total charge, contributed by these drops from
above to cloud base, will be 5 .k m/kxar min. This is entirely sufficient
for feeding discharges of 2 — 7 k intensity with a frequency of one
lightning flash in 2 minutes per souare kilometer.

Thus, the proposed theory and scheme of thunderstorm development
provide the possibility of establishing cuantitatively the intensity of
thunderstorm events. At the same time it becomer clear, how the value
of vertical motions in the cloud can facilitate the development of thunder-
storm phenomena. The value of the gravitational factor of heavy shower
precipitation is also ascertained. Tverskoy Cl73 was first to clearly
draw attention to the necessity of bringing forth both these factors
in order to explain the thunderstorm phenomena. It would seem, however,
that here we would be facing contradiction with the generally well known
fact, that thunderstorm precipitations bear more often positive charges
and the ordinary showers negative ones. It is also well known that the value
of drop potential does not attain the described magnitudes. This apparent

17*

contradiction may be explained by the fact, that during ordinary heavy
rains without thunderstorm activity, the negative charges having accumula-
ted on drops, are partially preserved prior to their fall on ground. As to
thunderstorm precipitations, they may either lose their charge during
lightning strokes, or undergo recharging.

The cloud base is often situated at an altitude of 1500— 2000 m.
As the precipitations overcome the space separating the cloud base from
the ground, they partially evaoprate. At the same time, their droplet
charges are also being lost to a significant measure. This takes place
as a consequence of increased deeorption of ions at evaporation, as well
as on account of current conductance, the letter being substantial in the
space free from clouds.

In the pattern of charge distribution adopted by Simpson and his
associate, the presence is noted of a small region with positive charge
in the lower part of the storm. According to Williams data L30], this
seat is located mainly ahead of the zone of thunderstorm precipitations.
The cause of formation of that lower seat of positive charge may be in
the inflow of air to the leading edge of the storn: in the lower kilometer
atmosphere layer from regions unperturbed by the thunderstorm. As was
shown in [2] , the air contains under "normal conditions" a positive space
charge of atmospheric aerosol; at ita convergence, a small core of positive
charges can be formed under the thunderstorm. This is also helped by forces
of electrostatic induction, caused by a suostantial negative charge of
the basic part of the thundercloud.

Attenuat ion ot^e of a Thunderstorm (Fig. 2 Q

The onset of this stagq of a thunderstorm takes place when the
ascending currents, feeding the thunderclouds, decrease or attenuate. As
was shown by Fedorova Ll83 , who utilized the data of radar observations,
thunderstorms attenuate when they emerge in daytime either in a valley
or a low plain, or also in a maritime bay etc., and in nighttime, when .
unfavorable conditions occur for them above seacoast elevations and also in
a series of other cases when, due to local circulation, there occur descend-
ing compensation currents.

18.

The development of the thunderstorm itself leads at a specific stage of its
expansion to the emergence in the cloud of descending currents conditioned
by the hydrodynamic effect of the falling shower* As the rainfall from the
thundercloud progresses, carrying with it the main part of its negative
charge, and as there develop in the cloud, besides seats of ascending cur-
rents, those of descending ones, the pattern of charge distribution becomes
more and more mixed. In some seats the negative charge is preserved, in
others it becomes positive as a consequence of discharges, rainfall and
descending motions. Accordingly, the ligthning discharges during the atte-
nuation stage of a thunderstorm take place mainly inside the cloud. We
already noted that the transition from the state of violent condesation
to that of equilibrium or evaporation induces the vanishing of the negative
charge and the appearance of a positive one.

CONCLUSION

The above-expounded theory of a thunderstorm allows to compute the
intensity of charge generation in a thundercloud, to explain their spatial
dirtribution and the c v ?racter of lightning discharges at various stages
of the storm.

The multilateral data on thunderclouds, accumulated during the latest
years, and the established regularities of aerosol particle charging by way
of selective adsorption of ions H + and OH" as a function of aerosols 9 pH
and of condensation or evaporation processes, can now serve as a basis for
the explanation of the generation mechanism of thunderstorm electricity.

fThe violent condensation in powerful updrafts of a thundercloud leads to
the generation in its lower half of a negative space charge at the expense
of the preferential adsorption of ions OH" on the surface of cloud drops.

I In the upper half of the cloudj that is in the zone of stabilization and
drop evaporation together with crystals,! formation of a positive charge .
takes place at the expense of preferential yield of ions OH- from the

19.

surface of cloud particles. The energy for aerosol particle charging
originates from adsorption and condensation heat. The formation of a
core of suspended particles in the ascending current jet of air contain-
ing supercooled drops or hailstones leads to substrntial accumulation
and localization of the negative charge* The rapid fall of the thundery
rainfall from that region leads to field intensity increase between the
cloud and the ground and is attended by intense negative discharges from
the cloud to ground during the principal phase of the storm//

The power of thunderstorm electricity generation is found to be
so much the more substantial, that the ascending current are more intense
and the height of their spread is greater; it is also a function of
humidity and water content of the cloud.

»♦♦** THE END ♦*♦*♦

Contract No. NAS-5-3?60 Translated by ANDRE L.BRI CHANT

Consultants and Designers, Inc. - rt *. ., . -, rt £ir

Arlington, Virginia on 20 ~ 2l Merch 19&3

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\ I V

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Grosenergoizdat, M.-L. 1956.

[11].- I*M# I MIANITOV, A# P. CHPV/iYEV .- K voprosu ob usloviyakh perekhoda

moshchnykh kuchevykh oblakov v grozovyye (To the
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C123.- H>.I • WUL'FSO N.- Izv.A. N. SSSR, ser # geofiz. f No.l, 1957.

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po nablyudeniyam vo vremya dvukh poletov aerostatov
4 i 5 Marta 19^1 g. (Some data on the structure of
stratus clouds according to observations during
two aerostat flights on 4 and 5 March 19^1).

Trudy TsAO, vyp. 1, 19^7 •

[1*0.- N. Sh. BIBILASHVILI , V. F. LAPCHEVA .. et AL .- lev. A.N. SSSR, ser.

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[153.- H> S . SHISHICIN .- Oblaka, osadki i grozovoye elektrichestvo.

(Clouds, nrecipitations and thunderstorm electricity

GITTL, M. f 195^.

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Fizmatgiz, M. f 1958.

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[183 ♦- A,A »FED0R0VA «- vliyanii rel'yefa na povtoryayemost»livnevykh

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«-"-!•- Blanc ha rd D. C. Electrically charged drops from bubles in sea water and their
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RiiFKR^NCKS (continuation)

21.

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D03.-

C313.-
[32].-

C333.-
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' - ".y e 5. u . I **;r Moore 9 B.'Uiant electrical storm. Recent Advances ir Atmospht-
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Williams J. C. Some properties of the lower positive charge in thudei clouds. Re-
cent Advances in Atmospheric Electricity. Pergamon Press. London, New Jork,
Paris. 1958.

At 1 a s D . Radar lightning echoes and atmospherics in vertical cross-section. Recent
Advances in Atmospheric Electricity. Pergamon Press, London, New Jork, Pa-
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Moor c C B-. V o tin e g u t B. f Bo t k a. Results of an experiment to determine
initial precedence ol organized electrification and precipitation in thunderstorms.
* London, New Jork.

im HageWersuchskanal. tfervenica.

Recent Advances in Atmospheric Electricity. Pergamon ftress.
Paris, 1958.

List P , Wachstum von Eis-Wasscrgemischen
Physica Acta, t. 32, Nr 4, 1959.
Schacfer V. I. The electrification of oil and water clouds. Recent Advances in At-
mospheric tiectricity. Pergamon Press, London, New Jork, Paris, 1958.
At pert L . Comments on initial electrification progresses in thunderstorms. Journ
MeteoroTT No 5. 1967.

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