Fig. 17 The global electric circuit by C.T.R. Wilson.
as interpreted by Wilson. Electric field lines extend from the top of a
cloud to the highly conducting upper layers of our atmosphere (50km
and higher). Charge is presumed to leak along the field lines from the
top of the cloud to the surrounding ionosphere. Note that field lines also
go from the top of the cloud to ground thus leaking positive charge to
ground. Negative charge is brought to ground mainly by lightning
strokes and point discharge currents. The number of field lines between
the cloud top and ionosphere compared to the number between cloud
top and ground is an unanswered but crucial question which will
determine the amount of charge supplied to the global fairweather
circuit. For example, if the number of field lines going from the top of
the cloud to earth would equal the number of field lines returning from
earth to the bottom of the cloud, no current can flow to the ionosphere.