106 
km (48,000 ft) were made in 1948 by Gish and Wait 
during ascent or descent for electric surveys over 
thunderstorms (a joint project of the U.S. Air Force 
and the Department of Terrestrial Magnetism of the 
Carnegie Institution of Washington). These unpub- 
lished values are on the whole consistent with those 
shown here except that they tend to be smaller.* 
The values of \ versus altitude, shown in graph B 
(Fig. 2) were calculated for the case m which the air 
contains no large ions and the cosmic radiation is the 
only ionizing agent. The agreement between these and 
the measured values is one of the considerations which 
leads to the conclusion that throughout much of the 
high atmosphere pollution is negligible and cosmic ra- 
diation is the chief ion-producing agent. 
The relatively large air conductivity at high alti- 
tudes and the increase with altitude are features upon 
which several other aspects of atmospheric electricity 
depend. For example, (1) the suggestion that thunder- 
storms are the source of the supply current could not be 
seriously considered if } were not a rapidly increasing 
function of altitude, and (2) the decrease of electric 
field strength with altitude and the character of the 
diurnal variation and that of some of the other varia- 
tions of field strength depend on these aspects of 2X. 
For cases in which the electric field strength or the air- 
earth current, or both, depend upon the over-all effect 
of the conductivity throughout a vertical column of the 
atmosphere, it is convenient to use the concept of elec- 
tric resistance, rather than electric conductance, of the 
column. 
The resistance of a vertical column of the atmosphere of 
1 em? cross section and extending from the earth’s sur- 
face up to a certain height is obtained by integrating the 
reciprocal of \ from the surface up to that height. A 
value of “columnar resistance” r, obtained in this way 
from the values of \ shown in Fig. 2, is 10" ohms for a 
column extending from sea level to an altitude of 18 
km. If this is regarded as representative for the whole 
earth, the total effective resistance R from the surface 
to the 18-km level would be 200 ohms. Most of this 
resistance is offered by the lower part of the column. 
The highest eight kilometers contribute only five per 
cent of the total, whereas the lowest two kilometers con- 
tribute 50 per cent. 
Estimates of r up to altitudes considerably greater 
than 18 km, based on observed values of cosmic-radia- 
tion intensity and the indicated trend of that radiation 
at altitudes not yet explored, indicate that the value of 
r for a column extending up to the ionosphere would not 
exceed by more than 10 per cent that for 18 km, pro- 
vided the small ions at those higher levels are not trans- 
formed into larger, less mobile types, in appreciable 
number. Such estimates may at least set a lower limit 
for r between the earth and the ionosphere. An upper 
4. Subsequent to the preparation of this article, results of 
the measurements described above have been published and 
will be found in “Thunderstorms and the Earth’s General 
Electrification,’ by O. H. Gish and G. R. Wait, J. geophys. Res., 
55:473-484 (1950). 
ATMOSPHERIC ELECTRICITY 
limit, not exceeding by as much as 50 per cent the 
value up to an altitude of 18 km, seems to be indicated 
by the extent to which air pollution of local orign— 
say from a city like Washington, D. C.—diminishes the 
air-earth conduction current through diminution of air 
conductivity in the lower atmospheric strata. 
The columnar resistance is apparently 15 to 20 per 
cent greater near the equator than in middle latitudes, 
owing chiefly to a dependence of the vertical distribu- 
tion of cosmic-radiation intensity upon latitude (Fig. 1). 
This is of interest here because it provides an explana- 
tion of the tendency, found by S. J. Mauchly, of the 
potential gradient measured at sea to be about 17 per 
cent less in the equatorial belt than in belts of higher 
latitude, whereas the air conductivity at the surface was 
found to be practically independent of latitude. This 
variation of r with latitude is not, however, as large as 
the variation from place to place on land. Over an 
urban area the value of r may be several times that in 
the relatively pure air over open country. 
When all these circumstances are considered, it seems 
likely that an average value of r is about 107! ohms and 
that the effective resistance R over all fair-weather 
areas of the earth is not far from 200 ohms. Here RF is 
equivalent to r/S where S is the area of the earth. It is 
assumed here that the total area over which electrical 
storms are in progress at a given instant is negligible— 
the data for thunderstorms of the earth collected by 
C. E. P. Brooks mdicate that the total storm area is 
usually less than 0.0088. 
The electrical conductivity of air in the ionosphere is 
apparently so great that electric fields there must be 
very small and can be disregarded in this discussion. 
No direct measurements of \ have been made in the 
ionosphere, but a reliable estimate of the order of magni- 
tude is obtained by using the measurements, made by 
“Tadio” methods, of the equivalent ion density, to- 
gether with estimates of air density at the corresponding 
altitudes. The value for X, estimated in this way for an 
altitude of 70 km, near the lower boundary of the 
ionosphere, is about 10’ stat mho em, or the resistiv- 
ity, the reciprocal of conductivity, is less than 10° ohm 
em. According to this estimate, the relaxation time, 
namely 1/(47\), at this altitude is less than 10° sec. 
This means that a local concentration of free electric 
charge cannot persist here for an appreciable time; it 
would, indeed, be diminished to 0.01 per cent of the 
initial value in 0.1 usec. 
Similar circumstances occur in the earth: The re- 
sistivity of most of the material near the earth’s surface 
is less than that for air in the lower ionosphere—that of 
ocean water is less than 100 ohm cm. Hven for geologi- 
cal structures where the highest values of earth-re- 
sistivity (108 to 107 ohm cm) have been measured, the 
relaxation time is of the order of microseconds. In con- 
trast to this, the relaxation time for air near sea level is 
generally greater than 400 sec while for air at 18-km 
altitude it is about 4 see. Because of these circumstances 
it seems permissible to describe the world-wide aspects of 
atmospheric electricity with the aid of a spherical 
condenser model as follows. 
