114 
MR. C. T. R. WILSON: INVESTIGATIONS ON LIGHTNING DISCHARGES 
Point discharges will occur most frequently and give rise to the largest currents 
over forests and lands covered with vegetation ; also on mountain summits and ridges, 
owing to the increased intensity of the electric field through proximity to the charged 
cloud. Ionization by splashing of rain on the ground and the relative number of 
positive and negative ions liberated thereby is likely to be very different over the 
various surfaces. Of special interest is the question of the amount and nature 
of the ionization at the surface of the ocean under heavjr rainfall. 
Over an area in which the surface ionization was large we should expect an 
increased vertical current, a diminution of the charge carried to the ground by rain, a 
diminution in the intensity of the electric field of the cloud, and in consequence a 
diminution in the tendency for lightning discharges to occur. 
The holding up of charged rain-drops by the electric field and the diminution of the 
field by the dissipation of cloud charges by forests and other sources of surface 
ionization are perhaps not negligible factors in the local distribution of rainfall. 
Mr. L. F. Richardson,^ describing some of the phenomena observed during the 
passage of a line squall in France, on September 6, 1917, remarks “the cloud was 
noticeably darker over the Forest of Argonne than over the grasslands of 
Champagne.” 
XXIII. Secondary Thunder-clouds. 
It has thus far been assumed that the source of E.M.F. is within the cloud in 
which the lightning discharges and other electrical effects are manifested. It is easy 
however to imagine conditions in which a cumulo-nimbus cloud, which acts as 
electric generator, may supply electrical energy to quiescent clouds and produce in 
them intense electrical fields and even lightning discharges. 
Consider for example a horizontal stratiform cloud which intersects lines of force 
connecting the poles of a cumulo-nimbus cloud; the stratiform cloud might be a 
lateral extension of the shower-cloud. The electrical conductivity within the 
stratiform cloud will, through capture of the ions by cloud particles, he very small 
compared with that of the free air above or below the cloud. The current from the 
poles of the primary thunder-cloud will cause an accumulation of charges of opposite 
sign at the upper and lower surfaces of the stratiform cloud. This accumulation of 
charge will continue—unless the field within the cloud previously reaches the sparking 
limit—until a steady condition is reached, when the vertical field within the cloud is 
sufficient to maintain a current equal to that which enters its upper and lower 
surfaces. The potential difference finally existing between the upper and lower 
surfaces of the stratiform cloud might amount to a considerable fraction of that 
between the top and bottom of the shower-cloud, the ratio being that of the resistance 
of that portion of a tube of flow which lies within the cloud to the resistance 
* Richardson, ‘Quart. Journ. Roy. Meteor. Soc.,’ 45, p. 112, 1919. 
