Storm formation.
When stormy clouds (usually cumulus-nimbus)
are being originated, a positive/negative charge separation
takes place inside the cloud.
The cloud´s lower part is negatively charged.
This process induces a positive charge accumulation in every
element on the earth, especially significative in metallic
points.
The cloud starts discharging this energy in
steps that last millionths of seconds. These sudden electric
variation provoke even a higher accumulation of positive
charges.
Now the corona effect is considerable; some
times, as a green light, as it happen to be in the ship masts
when seamen told about the Saint Thelmo fire. Also, many
sightseers have noted how their hairs stand on end when storm is
nearby.
Discharging path for the lightning strike is
already created. The cloud will discharge through this path: an
electric discharge, highly energetic, able to reach temperatures
of 30.000º C, that we see as a blinding light that we call
lightning, or lightning flash when it is produced between
clouds. Thunder is the sound produced by the explosion of gases
generated by this heating, along the discharge channel. Thunder
duration seems longer due to sound reflections and slower
because of lower sound speed (340 m/s) compared to light
(300.000 Km/s).