UNIVERSAL ASPECTS OF ATMOSPHERIC ELECTRICITY 
trical properties of that region will not be discussed 
here. This subject is treated in references [1] and [5]. 
Without the conductivity of the troposphere and 
stratosphere, which depends chiefly upon the cosmic 
radiation, atmospheric electrical phenomena would un- 
doubtedly be very different, and the universal distribu- 
tion of the fair-weather electrical phenomena that are 
now observed would doubtless not occur. 
At sea level the cosmic radiation forms ions in pairs, 
one positively the other negatively charged, at a rate, 
depending upon magnetic latitude, of 1.5 to 2.0 ion- 
pairs per cubic centimeter per second. This is practically 
the complete rate of ion formation over much of the 
ocean area and doubtless also over land in the polar re- 
gions, but in the lower atmosphere over most land areas 
the birth rate of ions is several fold greater than this on 
account of the additional ionization there by radioactive 
matter. The magnitude of the ionization rate near the 
earth over land is not readily determined and estimates 
vary between wide limits, but 10 ion-pairs per cubic 
centimeter per second may perhaps be taken as an ap- 
proximate representative average value. 
Despite the greater rate of production of ions, the 
electrical conductivity of the air over land generally 
does not exceed that at sea, and at some places, es- 
pecially near large cities, it is much less. For example, 
in the outskirts of Washington, D. C., it is about one- 
seventh the value at sea. This apparent paradox was 
resolved when it was found that im air which contains 
certain impurities, the normal small ions are trans- 
formed into large ions which drift more slowly in the 
electric field and thus contribute less to the conductiv- 
ity. These large ions are indeed so very sluggish that if 
all the small ions were transformed in this way the 
conductivity would be reduced to a very small fraction 
of the normal value. 
The electrical conductivity of a gas which contains 
ions of various types may be expressed as 
A =e kin, (2) 
where ¢ is the electronic charge, k is the ionic mobility, 
and n is the concentration of ions of each type. All 
ions are here assumed to carry a single electronic charge. 
The mobility varies as the inverse of the density of the 
gaseous medium and depends upon several factors in- 
cluding the character of the ion species, the sign of the 
ionic charge, and the “size” of the ion. Values ranging 
from 0.0003 to 0.0007 cm? y— sec for the mobility of 
large ions have been reported, whereas an average value 
for the small ions in the atmosphere at sea level is 
about 1.4 cm2 vy— sec~!, but the standard deviation of 
these values is rather large. Measurements made in the 
laboratory indicate that the mobility of the negative 
ion in air is about 1.3 times that of the positive ion. 
Since in the atmosphere the large ions appear to be 
formed chiefly at the expense of small ions, the air 
conductivity is reduced when air is polluted with sub- 
stances such as some products of combustion, which 
occur as molecular aggregates with a diameter of the 
order of 10-* cm. These, upon capturing small ions, be- 
come large ions with such low mobility that they play 
103 
an insignificant role in the conduction of electricity. 
The result of this is that the conductivity is reduced be- 
cause the terms kn (equation (2)) which apply for small 
ions are decreased more than the corresponding terms 
for large ions are increased. 
The concentration of small ions n, when equilibrium 
is established between the rate of production and the 
rate of destruction, is given approximately by the rela- 
tion 
q = an? + BnN, (38) 
where gq is the rate of production of small ions (ion-pairs 
per cubic centimeter per second) ; n is the concentration 
of small ions, either those positively charged or those 
negatively charged; NV is the concentration of the posi- 
tively charged, the negatively charged, and the elec- 
trically neutral large-ion constituents; @ is the coef- 
ficient of combination for small ions, with a value, at 
standard temperature and pressure, of about 1.6 < 107°; 
G6 is a parameter whose value is of the same order as 
that for a, but this value apparently depends upon 
some factors which are not yet identified. An average of 
values for 6 determined from the data of Cruise VII of 
the Carnegie is about 2 * 10~® [20]. 
In deriving this form of the ionic equilibrium relation, 
which is a simplification of more general relations [15],* 
assumptions have been made which restrict its applica- 
tion. But in many cases where the data used for N are 
the values measured with an Aitken nuclei counter, 
and those for n are the measures of small ion concentra- 
tion, this equation seems to be satisfied. The term con- 
taining NV in (3) usually is dominant in the lower at- 
mosphere over land and, especially in the vicinity of 
large cities, the term in n? is negligible, but at altitudes 
greater than 1 or 2 km in the free atmosphere, the latter 
term apparently is dominant during fair weather. These 
two terms are of about equal importance for the average 
conditions which prevail in the air near the surface over 
the oceans (N about 2000). Thus the equilibrium value 
of nis given by n = ~/q/a for clean air and by n = 
q/ (BN) for air that is polluted with many Aitken nuclei. 
The magnitude of air conductivity \ not only varies 
from place to place at sea level but it also varies from 
time to time. The average value of \ measured over the 
oceans on Cruise VII of the Carnegie was 2 X 10“ 
stat mho. Values over land are sometimes greater than 
those over the oceans, but smaller values are found at 
most places where measurements have been made. 
These values for land are so variable that an average 
has little significance. At Kew Observatory in the 
vicinity of London, \ appears to be about one-twelfth 
the value at sea and the variations are dependent to a 
large extent upon the varying pollution of the air. At 
the Huancayo Observatory near Huancayo, Peru, lo- 
eated at an altitude of 11,000 ft in a valley between 
ranges of the Andes mountains, the average A is large— 
about three times the average for the oceans—but it is 
less than should be expected for a station at that alti- 
3. Consult ‘Ions in the Atmosphere” by G. R. Wait and 
W. D. Parkinson, pp. 120-127 in this Compendium. 
