JorRX.c\L OF GEOPHYSICAL RESEARCH VOL. 68, No. 18 SEPTEMBER 15, 1963

Radio Emission from Clouds

J. DOYNE SARTOR

National Center for Atmospheric Research Boulder, Colorado

Abstract. Radio emission from clouds, not associated with lightning, has been observed and recorded as such on several occasions. Since the source of this radiation cannot be the lightning discharge, a study has been started to explain its nature and origin. Preliminary studies have revealed nLdio emission from colliding charged water drops at frequencies of the order of tens of megacycles, possibly higher. Microwave radiation is known to result from the contact of charged mercury drops with a grounded probe. We find microwave radiation from the contact of mercury drops with glass, plexiglass, or a pool of mercury. Plans are discussed for using the mercury drop radiation phenomenon as a control in studying the radiation from cloud drops .

Radio emission from clouds, excluding that directly associated with the lightning discharge, has been documented a number of times. Gib­son [1957J observed 0.86-cm radiation (35 Gc/s) that he felt could only be of electric origin from some well-developed clouds. The strength of the weaker radiation he received corresponds to the expected thermal emission from such clouds, but the upper limit could be expressed as a radiation temperature 150 de­grees in excess of the thermal noise. Fleischer et al. [1962] observed electromagnetic radia­tion from thunderstorm clouds at many wave­lengths, taking care to eliminate any radiation directly associated with lightning strokes. Kim­pura [1963J surveyed recent studies of at­mospheric noise. In summarizing the electro­magnetic emission from clouds, Kimpura makes the following statements: 'As the frequency increases to 2 Me/s, more and more radiation pulses appear. .. . At 4 to 12 Mc/s the radiation becomes practically continuous .... At higher frequencies than 100 Mc/s, continuous and in­termittent radiation independent of any proc­ess like stepped leaders is observed.' HO(J(J and Semplak [1961J observed 5-cm (6 Gc/s) noise coming from convective clouds that was on oc­casion one to two orders of magnitude above thermal noise. Later HO(J(J and Semplak [1963J, working near 4 Gc/s, found high noise tempera­tures associated with rain clouds which could be explained on the basis of scatter from the side lobe of a nearby transmitter. Whether the parlier observations at 6 Gc/s can be attributed mostly to scattered radiation at 6 Gc/s or to

emission from the cloud is a question which needs further exploration.

In each of the five studies mentioned above, only radio emission not dire�t1y associated with the lightning discharge was considered. Thus there must be another process in clouds which is responsible for these signals. They may come from corona discharge from the individual particles in high electric fields. This must be the case just before a lightning stroke, but it can hardly be the explanation for the radio emission of a more continuous nature over longer periods of time, whether preceding the lightning discharge or not. This higher-frequency radiation may be evidence of the electromagne­t.ic emission from a large number of drops col­liding in more moderate fields or with sub­stantial residing charges.

This suggestion has been explored in the laboratory. As a result , radio emission has been observed from the collision between freely fall­ing, isolated, charged water drops. The experi­ment was conducted inside a Faraday cage with a Hammarlund HQ 180 radio receiver having a frequency range from 0.5 to 30 Mc/s and a

small loop antenna. The charged drops were produced with an arrangement of downward­pointing hypodermic needles as shown in Figure 1. The cable from a high-voltage power supply was connected directly to the needle on the left in the picture. The field from the charged needle induced an opposite charge on the drops as they formed upon leaving the needle on the right. The flow of drops was adjusted so that the two streams did not collide when no charge was

5169

5170 J. DOYNE SARTOR

Fig. 1. Hypodermic needles for production of cbarged water drops.

added. When the ch arge was applied the drops

moved slowly together as they fell, colliding some 20 or 25 em below the ends of the needles. On collision, a radio pulse was heard on the

loudspeaker and seen on an oscilloscope con­nected in parallel. The observed signal appeared

above the noise at a frequency of n.bout 20 to 25 Mc/s and increased in amplitude as the frequency increased to 30 MC/5, the limit of the receiver. The size of the drops varied from

2 to 4 mm in diameter. Figure 2 is a typical osci lloscope trace.

Dickey [1951J performed an experiment with charged mercury drops in which he observe(l the spectrum of the radiation they emitted when they struck a grounded probe within it

glass-encloscd dropping mechanism . He fonnd that the spectrum peaked at 11 frequency that was an explicit function of the size of the mer­

cury drop. The relationship he obtained in the laboratory was A. = 7D, where A. is the wave­

length at maximum power and D is the diameter of the drop. He theorized that this equation was an experimental manifestation and that the discharging mercury drops were acting as quarterwave antennas of length equal to the

distance along the surface of the drops, 7rD/2, thus giving the equation A./4 = 7rD/2, or

f... = 27rD (1)

We have reproduced Dickey'S experiment qualitatively without making a spectral analysis

of the radiation emitted from the discharge of

the mercury drops to the probe. Two 3D-Me/>

IF amplifiers were connected to three micro­wave crystn.l mixer detectors. In tillS way the radin.tion was surveyed at wavelengths of 3, l.24, and 0.35 cm. At en.ch of these wavelengths

a signal of the order of 10 db above noise was

observed. The signal was identified as definitely coming from the vicinity of the point of dis· charge by moving the source out of the antenna pattern, by modifying the flow of mercury drops , and by shielding the antenna opening with 11 copper plate .

It was noticed subsequently that even when no attempt was made to charge the drops, their contact with the sides of the glass-dropping mechanism resulted in microwave signals oi nearly the same amplitude as those from thf contact of the drops with the probe. A small bubble of glass in which a small amount oi mercury had been sealed served as a powerful source of radio emission. If the mercury was

swirled about the inside surface of the bubble, low-frequency radiation could be detected, bu

the microwave radiation was absent. Howerer,

if the bubble was shaken vigorously so that the mercury broke up into small drops, numerous microwave signals appearecl and the lower·

frequency signals increased in intensity and number.

Similar emission can be observed by dropping

mercury drops into a plexiglass container or by

vigorously shaking the plexiglass container,

causing the mercury sealed inside to break inio

RADIO EMISSION FROM CLOUDS 5171

drops ,yhich collide with the sides of the con­

tainer, ,yith themselves , and with the main body

of the mel'cnr y. Figure 3, an oscilloscope trace

of these signals, shows pulses t�-pical of all of the

methods of generation described above, those

produced by shakin g the glass bubble being the

Illost numerous.

It is not clear at this time what the energy

source for these s ignals is. The emission seems to occur at the t ime of momentary contact of

the mercurr with the glass or plexiglass, or

upon �on1"act of the mercury drops with the bod!- of mcrcurr liquid after having m a de con­

tact with onc of the dielectrics. If this is so, the source of the electromagnetic energy may be

the contact or triboC'lcctric potential of the mer­

cury for glass or ple:\lglass. Another possibility

for the source energy may be the formation and

dismption of the electric double-layer within

the liquid mercury. When the mercury is rep laced in these experi­

ments ,,-ith distilled or tap water, no emission

from drops colliding with the glass, plexiglass,

or other water surfaces could be detected at

micro\Yaye frequencies. If the water drops are charged and dropped against any conducting or

semiconducting surface (including a water sur­bee), of dimensions large with respect to the drops, strong emission is ob served at the lower

frequencies from a few megacycles per second

to the uPl,er limit of the receiver. The strength

of the�e 8ifrnals appears to be a function of the charge transferred . For example, if a stream of

equally charged drops impinges on one lead to

a capacitor, the strength of the signals emitted

e"entually begins to decrease and gradually

Fig. 2. Oscilloscope traces of electromagnetic radiation at 29.4 Mc/s from the collision between t'harged water drops. Vertical scale : 1 v per di vi­,ion. Horizontal scale: 1 msec per division.

Fig. 3. Microwave (1.24 em) pulses from mer­cury drop-glass contact. Vertical scale: 1 v per division. Horizontal scale: 20 /Lsee per division.

disappears as the capacitor charges. The num­ber of such drops that will emit a signal de­

pends on the capacitance. Professors M. Brook and E. J. 'Vorkman, in private communications,

have advised me of a natural phenomenon that agrees with these observations. They report that

growing icicle s on the eave of a building, that

drip to form a 'stalagmite' type of growth on

the ground, will occasionally shock-excite a

radio through a nearby antenna. The signals

from oppositely charged water drops colliding with one another are much weaker because the water drop has a much smaller capacitance than

the conducting probe, an ordinary capacitor, or a grounded icicle. The major cause for the difference in the emission frequencies of the

two liquids is probably the considerable differ­ence in the conductivities of water and mer­cury, although in this instance the effective con­ductivity of water is a complex function of its surface characteristics. Since the surface tension of mercury is rough ly 6Yz times that of water , water drops of dimensions approximately equal to those of the mercury drops are much easier to deform, especially by electrostatic forces.

The lengthening of a drop in one dimension may also have a considerable effect on the relation between particle size and radiation frequency, such as that e},.'"j)ressed in (1) for mercury drops.

There are applications of the results of these observations that could provide interesting

methods for studying cloud electrification and associated parameters from the troposphere and from space-including the possibility of obtain­ing information about. the atmospheres of other

5172 J. DOYNE SARTOR

planets. It is possible that the early stages of rlectrical activity in a cloud could be studied and an estimate of the median size of the parti­cles involved made through a wavelength-size analysis as suggested by (1). Because of the small dimensions of the drops, the relationship implies that the higher frequencies may be more important. Thus measurements could be made from satellites through the ionosphere nnd, because of the close relationship of cloud electrification and precipitation intensity, used to identify precipitating clouds from above. 'Tol­bert and Straiton [1962] suggest that discharges between particles charged triboelectrically in the atmosphere of Venus may explain the high noise temperatures observed from that planet. The observations of strong radiation from un­charged mercury drops colliding with dielectrics described in this report, offer some substantia­

tion of the Tolbert and Straiton suggestion in that the radiation they hypothesize has been confirmed.

Future research plans include the quantita­tive study of the radiation from colliding parti­cles; we shall concentrate primarily on water drops while using the more easily evaluated observations from mercury drops as a guide. The results of these stuuies will be used in a general study of the interrelationship between cloud electrification and the growth of precipita­tion.

Aclcnowledgment,�. I wish to acknowledge the considerable contributions of Mr. Jack Tefft and

Mr. Alan Miller in the acquisition and operation of the equipment used in this work. The use of the glass bubble containing mercury was suggested bv a research endeavor under the direction of D; James P. Lodge, and the original bubble wa� borrowed from him.

REFERENCES

Dickey, F. R., Jr., The production of millimeter waves by spark discharges, Tech. Rept. 123 Cruft Laboratory, Harvard University Jul� 1951.

' .

Fleischer et aI., Effects of the atmosphere on radio astronomical signals, Final Rept., AFCRU2-311, 55 pp., Rensselaer Polytechnic Institute Troy, New York, April 30, 1962.

'

Gibson, J. E., Some observations of microwave radiation from clouds, M em. Rept. 693, 4 pp., U. S. Naval Research Laboratory, Washington, D. C., March 27, 1957.

Hogg, D. C., and R. A. Semplak, The effect of rain and water drops on sky noise at centimeter wavelengths, Bell System Tech. J., 40, 1331-1348, 1961.

Hogg, D. C., and R. A. Semplak, Measurement of microwave interference at 4GC due to scatter by rain, Proc. IEEE, 51(3), 499,1963.

Kimpura, Atsushi, Electromagnetic energy radio ated from lightning, Proc. Intern. Conf. on At. mospheric and Space Elec., 3rd, Montreaux, Switzerland, May 1963.

Tolbert, C. W., and A. W. Straiton, A considera. tion of microwave radiation associated with particles in the atmosphere of Venus, J. Geo­phys. Res. 67(5), 1741-1744, 1962.

(Manuscript received June 10,1963; revised July 5, 1963.)