104 
tude even if cosmic radiation were the only ionizing 
agent. At Huancayo, A undergoes an 80 per cent de- 
crease in about one hour between 6 and 8 a.m. during 
the dry season (Fig. 5). The low value continues during 
the daylight hours and is followed by a gradual increase 
which begins at about sunset. The abrupt decrease of 
is accompanied by a corresponding increase in the 
count of Aitken nuclei. 
Apparently on the days when this occurs a shallow 
stratum of very stable air is established at night. The 
nuclei, initially present in the air near the surface, 
coalesce and settle out during the night and any radio- 
active matter exhaled from the ground is entrapped. 
Thus there are two factors which tend to gradually in- 
crease the conductivity at night. In the morning when 
the stable air layer breaks up and mixing sets in, nuclei 
presumably come down from the higher air and such 
radioactive matter as may have been entrapped is dis- 
persed throughout a greater depth of air. Measurements 
indicate that the rate of ionization in daytime is about 
80 per cent of that for nighttime at this station. Ac- 
cordingly, the greater part, about 80 per cent of the 
abrupt decrease of \ in the morning, is attributable to a 
corresponding increase in the pollution of the air by 
substances which can form ions which drift very slowly 
in the earth’s electric field. 
These are examples of the kind of information now 
available which seems to justify the statement that 
most of the large changes of air conductivity, not only 
changes with time but also with position, are asso- 
ciated with changes in the purity of the air. 
Temperature and pressure, although important fac- 
tors where change of altitude is involved, effect only 
minor changes in air conductivity at sea level. The fact 
that the conductivity is greater in summer than in 
winter at a number of places may be attributable partly 
to a temperature effect. Calculations indicate, however, 
that for an annual temperature range of 30C, the cor- 
responding range of conductivity for pure air would be 
18 per cent of the mean, but the actual range is much 
greater than this—other factors apparently are in- 
volved. 
Some observations indicate that the content of radio- 
active matter in the air over land is greater im summer 
than in winter and the greater conductivity in summer 
is probably in part a consequence of this, but the in- 
formation about the annual variation of the radioactive 
content of air, or about the rate of ion formation q, is at 
present insufficient for making an appraisal of the quan- 
titative importance of this factor. One would expect an 
effect of this kind only im regions where the exhalation 
of radioactive matter from the soil is hindered more 
during the winter season than during summer, owing 
to the prevalence of such conditions as a snow cover, 
frozen damp soil, or unfrozen waterlogged soil. The 
chief part of this annual variation of \ in the vicinity 
of large cities is attributable to a corresponding varia- 
tion in the pollution of the air and there is some evidence 
that this factor plays a dominant part in effecting the 
annual variation of conductivity at most places on land 
where observations have been made. 
ATMOSPHERIC ELECTRICITY 
The air conductivity near the earth is also affected by 
the electric field. During normal weather the concen- 
tration of negative ions near the surface decreases as 
the field strength increases. This occurs because nega- 
tive ions drift away from the earth when the potential 
gradient is positive (field strength negative) and few, 
if any, such ions are supplied to the air from the earth. 
This effect of the electric field upon air conductivity 
is very pronounced when, as during storms, the field 
strength is large. Examples are given in Figs. 4 and 6. 
In Fig. 6, beginning at about 14" the negative con- 
ductivity (Az) is negligible during most of the follow- 
ing hour while the positive conductivity (\;) is about 
normal. But shortly after 15515", \» abruptly returns 
to a normal value and at the same time ), almost van- 
ishes. This condition continues for about fifteen 
minutes, then, at 15'30™, \» again vanishes and ), re- 
turns to a normal value. Six other such alternations 
occur before the storm ends at about 17"20™. 
Most of the changes of the electric field, which cause 
these marked changes in conductivity, are not clearly 
seen on the electrogram for potential gradient in Fig. 
6. That correspondence is better illustrated in Fig. 4 
during the interval 0" to 3 when the electric field, being 
less intense, was clearly recorded most of the time. 
Many of the changes of air conductivity are not ac- 
companied by noticeable changes in air-earth current 
density. It is only when the change in conductivity ex- 
tends throughout a considerable range of altitude that a 
marked correlation is seen. These cases must be taken 
into account when one is examining data for the broader 
universal aspects of atmospheric electricity. For ex- 
ample, in estimating the magnitude and the character of 
the variations of the supply current from measurements 
of 7, allowance must be made for abnormalities which 
depend wholly upon local circumstances. The most sig- 
nificant of these abnormalities, for areas of fair weather, 
depend upon local modifications of the distribution of 
air conductivity with altitude. 
Air conductivity depends upon altitude in a com- 
plicated way. The value at an altitude of 18 km (60,000 
ft), during the notable flight of the balloon Explorer IT, 
was 100 times the average for sea level. The chief factors 
involved here are (1) the intensity of cosmic radiation 
increases with altitude, (2) the rate of ion formation for 
a given ionizing radiation decreases directly as the air 
density, (3) the mobility of the ions varies inversely 
as the air density, (4) the rate of ion destruction in pure 
air of a given ion concentration decreases with altitude, 
varying directly with the 14 power of pressure (ap- 
proximately) and inversely as the % power of absolute 
temperature, (5) the concentration of radioactive mat- 
ter exhaled from the earth over land decreases with 
altitude, (6) the pollution of the atmosphere usually de- 
creases with altitude, and (7) the dependence of 8 
(equation (3)) upon temperature and pressure doubt- 
less plays a part of unknown magnitude in determining 
the variation of \ with altitude in the lowest kilometer 
or so. 
The last three factors apparently are relatively in- 
significant at altitudes greater than one or two kil- 
