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Here are a couple of facts about lightning and atmospheric electricity, 
taken from "An Introduction to Atmospheric Physics (2nd Ed.)" by 
R.G. Fleagle and J.A. Businger.

Average "fair weather" atmospheric current over entire earth: 
  about 4 X 10^(-12) A sq. meter  or 0.00001 Amps per square mile  
  (positive current downward)

Lightning strokes typically begin as a downward discharge of negative charge
called a pilot leader, of roughly ~100 meters length.  The pilot leader 
extends itself downward further as replenishing charges are conducting downward
through the ionized path.  Eventually the pilot leader is close enough to the
earth's surface to create a large potential gradient exceeds the breakdown
potential of the air in the vicinity of sharp objects.  This induces a positive
streamer to advance upward from the surface, where it meets the pilot leader
at a height of 5-50 meters.  When the two meet, a conducting path is formed 
between cloud and ground, and up this path rushes a wave of ionizing potential
(called the return stroke) at a speed of about 100,000,000 meters/sec.  
This fully ionizes the channel, and the negative charge in the cloud base
"rushes earthward through the brilliantly luminous channel and discharges 
roughly the lowest kilometer of the cloud."  After a recovery time of about
1/20 sec., the potential redevelops, allowing a "dart leader" to reactivate
the channel for another round of current exchange.  The total number of strokes
averages about three or four in the space of less than a second.

Typical charge transferred in lightning strike: about 20 Coulombs