About Geometry and Initial Phase of Cloud-to-Ground Lightning
  Cloud-to-ground lightning is the most common among atmospheric discharges. Since electric fields in the vicinity of a thunder-cloud do not exceed 250 kV/m the physical process that triggers the lightning remains unexplained. [1, 2, 3] Recent measurements established a weak correlation between solar wind and incidence of lightning. [4] Here we show, that if an ionized path created by cosmic rays provides a trigger, the distribution of lengths between two successive forking points in a lightning channel (internodes) closely resembles the exponential distribution with average length between 415 m and 510 m. The results, if confirmed, imply that a thunder-cell may be an additional source of fast elementary particles that initiate lightning process. 
Ale\v{s} Berkopec
physics.ao-ph