AND ON THE ELECTRIC FIELD OF THUNDERSTORMS. 
113 
potentials fall short of what are required to produce lightning discharges, there is no 
reason to suppose that the vertical currents are of an altogether different order of 
magnitude. If any considerable proportion of shower-clouds are of positive polarity 
the upper atmosphere will receive an excess of positive electricity which may possibly 
be sufficient to maintain the positive potential of the conducting layers and to supply 
the normal downward current of the fine-weather regions. The total current which 
must be supplied for this purpose is, as Simpson* has pointed out, of the order of 
1000 amperes for the whole earth. 
It is not necessary to suppose that only isolated clouds of the cumulo-nimbus type 
contribute to the current between the ground and the upper atmosphere. If we 
consider a cloud from which heavy rain is falling and assume that the conditions are 
uniform over a large area, the case is in fact somewhat simpler than that of the 
cumulo-nimbus cloud ; the general results are the same. 
We may suppose that a steady condition is reached in which the vertical electric 
field within the cloud (and thus the potential difference between its upper and lower 
surfaces) has a value, which depends on the rate of rainfall and other factors; it is 
assumed to be independent of the sign of the polarity. Even if this potential 
difference is only or of that reached in thunder-clouds the effects may be 
important: the E.M.F. which tends to drive a current between the ground and the 
upper atmosphere is still from 10 to 100 times the normal potential of the upper 
atmosphere above fine-weather regions. 
The difference between the mobilities of the positive and negative carriers dragged 
out of the conducting upper atmosphere will again cause clouds of positive polarity 
to differ from those of negative polarity in (1) the greater magnitude of the vertical 
current (positive for the cloud of positive polarity); (2) the smaller magnitude of the 
potential (positive) at the upper surface and greater magnitude of the potential 
(negative) at the lower surface of the cloud ; and thus (3) the greater intensity of the 
potential gradient (negative for the cloud of positive polarity) below the cloud, this 
again tending to cause a larger part of the vertical current below the cloud to be 
carried by ions liberated at the ground and thus to produce a more complete 
discharge of the (negatively-charged) rain. 
XXII. Influence of the Nature of the Earth's Surface below a Thunder-cloud 
or Shower-cloud. 
The dissipation of the lower charge of a thunder-cloud or other rain-cloud by the 
upward stream of ions liberated by point discharges or by splashing at the earth’s 
surface must depend largely on the nature of that surface, on whether for example it 
consists of desert, snowfield, grassland, forest, lake or sea ; and again the effect of the 
nature of the covering of the earth’s surface may depend on the sign of the electric 
field. 
* Simpson, ‘Nature,’ December 12, 1912. 
VOL. CCXXI.-A. R 
