AND ON THE ELECTRIC FIELD OF THUNDERSTORMS. 
115 
of the whole tube.* If the thickness of the stratiform cloud were small, intense 
fields might result within the cloud and discharges might even occur; each flash 
would discharge only a small area of the cloud, of dimensions comparable with the 
thickness of the cloud. 
The characteristic striated and mammatiform appearances frequently observed on 
the lower surfaces of stratiform clouds associated with thunderstorms may be due to 
intense electric fields produced as above suggested, the electrical attraction between 
the upper and lower charges giving rise to convection currents. 
If the ionization above and below a stratiform cloud in the field of a primary 
thunder-cloud is unequal, the cloud will acquire unequal upper and lower charges and 
thus carry a resultant charge. For example, a stratiform cloud above a cumulo¬ 
nimbus cloud will intercept the flow of ions from the upper atmosphere and become 
charged with electricity of opposite sign to that of the upper pole of the shower-cloud, 
a steady condition not being reached until a potential difference between the thunder¬ 
cloud and the upper atmosphere is concentrated almost entirely in the region below 
the stratiform cloud. Lightning discharges between the stratiform cloud and the 
head of the primary thunder-cloud below will be likely to occur. 
In the absence of any such cloud above the primary thunder-cloud, the great 
diminution of the mobility of ions or electrons dragged out of the conducting layers 
as they penetrate into the denser regions of the atmosphere will have a very similar 
effect; the concentration of charge will be greatest where the change of conductivity 
with the height is most rapid. We may in fact, as suggested in Section VIII., 
consider that the conditions are much the same as if a conducting protuberance were 
drawn out from the conducting layer towards the summit of the thunder-cloud. It 
does not seem unlikely that discharges may sometimes occur between this protuberance 
and the top of the thunder-cloud. In a previous paper some evidence was obtained 
suggesting the occurrence of discharges of very great vertical length ; possibly these 
may have been of the type we have been considering. 
* This action of a layer of cloud, in particular of a ground fog, in increasing the vertical electric field 
within it has long been recognised in the case of the potential gradient of fine weather. E ester and 
Geitel, ‘Meteor. Zeitschr.,’ 17, p. 230, 1900; Geitel, ‘ Physikal. Zeitschr.,’ 17, p. 455, 1916. 
