SFERICS 
By R. C. WANTA 
U.S. Weather Bureau, Upton, N. Y. 
Sferics (less commonly spherics) is a contraction of 
the word atmospherics meaning natural electrical phe- 
nomena detected by radio methods. Sferics have vari- 
ously been known as clicks, grinders, sizzles, X’s, strays, 
parasites, and sturbs, and other names. They comprise 
natural static which interferes especially with ampli- 
tude-modulated radio wave reception. Hlectromagnetic 
waves at radio frequencies reaching the earth from 
outside the atmosphere, of solar or other origin, are 
sometimes included within the meaning of the term, but 
will not be discussed here. Sudden electrical discharges 
resulting in redistribution of charge within and between 
clouds, between clouds and the air space above or 
below, and between clouds and earth, give rise to electro- 
static, induction, and radiation fields. It is the last 
which principally form sferies at a distance, and with 
which we are especially concerned here. The signifi- 
cance of sferics in meteorology is due to their origin in 
relatively intense convection intimately involving water 
vapor. 
The reception of sferics preceded even the artificial 
generation of radio waves for transmission over long 
distances. A. S. Popov, using an elevated wire, noticed 
the association of lightning flashes with the indications 
of a coherer at Cronstadt in 1895 [9]. The history of 
sferics during the remaining years of the last century 
and the first two and a half decades of this century is 
sketched in two papers, by Cave and Watson-Watt [9] 
and by Watson-Watt [40]. Sferics were positively cor- 
related with cyclones, polar fronts, thunderstorms, and 
cumuliform clouds, and less definitely, with showery 
precipitation. Rudiments of later methods of sferics 
study appeared—not only observation of frequency and 
intensity of sferics, but also coordinated observation of 
bearings of sources of sferics by means of direction 
finders. Sferics provided a key which opened the door to 
an understanding of radio wave propagation. Investi- 
gators in more than a dozen countries explored the 
nature and sources of atmospherics, and official inter- 
national recognition soon was given to this new field of 
meteorology. Lugeon has recently written an historical 
account, which includes a summary of the more im- 
portant international resolutions concerning sferics [17]. 
Is the source of sferics necessarily lightning? In 
Popov’s ease the answer seemed clear, but a complete 
answer has not yet been given. Nevertheless, Watson- 
Watt’s dictum that no valid evidence contradicts the 
hypothesis that all sferics originate in lightning still 
stands [4, 41], at least as regards sferics received at a 
distance [24]. Besides the positive correlations already 
mentioned, supporting evidence was found early in the 
diurnal and annual variation of sferics; sferics originat- 
ing over land exhibited afternoon and summer maxima, 
whereas those originating over water exhibited an early 
morning maximum. Good correspondence was obtained 
between prevailing areas of sferics origin and the distri- 
bution of thunderstorms given by Brooks [6]. Further- 
more, the general features of the wave forms of sferics, 
as determined, for example, by Appleton and Chapman 
[2], Lutkin [19], and Schonland and Laby and their 
collaborators [15, 31, 33], corresponded with the details 
of the lightning discharge. In this connection, see 
Hagenguth’s article! elsewhere in this Compendium. 
The meteorologist may feel safe in treating all sferics 
arising more than some one hundred miles from a sferics 
receiver tuned to very low radio frequencies as evidence 
of violent convection such as occurs in thunderstorms. 
The observation of sferics has taken several forms: 
determination of direction of arrival, measurement of 
intensity and rate of occurrence, and display of the 
wave form of individual sferics. From Brooks’ estimate 
that approximately 40,000 thunderstorms occur over 
the globe per average day and from the fact that day 
and night ranges of sferics at radio frequencies of the 
order of 10 ke sec? exceed, respectively, 1000 and 
3000 miles, some idea is gained of the average frequency 
of occurrence of sferics. 
In English-speaking countries the most commonly 
used device for determining the direction of arrival is 
a direction-finding system, justly associated with the 
name Watson-Watt, which uses two stationary loop 
antennas at right angles to one another. Each loop 
characteristically receives the maximum signal in a 
direction along its own plane, the signal intensity at 
angles of arrival other than zero varying as the cosine. 
The input circuits are tuned in the neighborhood of 
10 ke sec“, corresponding to the quasi-frequencies of 
the lightning stroke. The signal in each loop is sepa- 
rately amplified and then added vectorially on an 
oscilloscope screen to produce a straight line represent- 
ing the direction of arrival with 180° ambiguity. More 
recently, ambiguity of azimuth has been eliminated 
electronically. The signals are most often observed 
visually, less often recorded on stationary or continu- 
ously moving film. This form of sferics receiver has 
been used by numerous investigators in many coun- 
tries [4, 11-13, 17, 32, 41]. (For an early analysis of 
such observations, see [39].) The Watson-Watt system 
was employed in networks of stations during World 
War II in England, the United States, the Caribbean, 
the western Pacific, China, Japan, and elsewhere. Radio 
or telephone intercommunication of stations comprising 
a network permits synchronized observation of the 
same instantaneous source. One station generally desig- 
1. See “The Lightning Discharge” by J. H. Hagenguth, 
pp. 1386-148. 
1297 
