This phenomenon results from the accumulation of electrical charges in cumulonimbus clouds
In situ measurement 
results indicated that potential differences within or between Cb clouds could 
reach ... between the cloud and the ground surface as negative charges 
accumulate at the... [DOC] [TLE] Static Electricity 4: Static Electricity and Lightning - Science NetLinks [PAR] Protons carry a positive charge (+), and electrons carry a negative charge (-).