|Before an atmospheric discharge,
a thundercloud - cumulo nimbus - emits a series of ions known as down
tracers or leaders. The negative polarity leaders fall towards the
ground in successive fifty meter drops at speeds of 0.15 to 1 m per
microsecond (µs), whereas the positive leaders fall more steadily.
||As they approach
the ground, the tips of the highly ionised down leaders generate a
powerful electric field that can reach several hundred kilovolts per
meter. This powerful disturbance triggers the creation of opposite
polarity up leaders, primarily at high and prominent spots along the
||As the down and
up leaders join, an ionised channel to the ground fills with cloud.
A return arc, known as the atmospheric discharge, returns through
this ionised channel. It consists of several arcs between the ground
and the cloud.