Possibly reaching several kilometres height and depth, lightning is produced by cumulo-nimbus stormy clouds. A separation of charges in the cloud is caused by atmospheric turbulences and big differences of temperature (up to – 60 °C at 10 km altitude) the upper part of the cloud is made up of positively charged ice crystals and the base contains negatively charged water droplets.The base of the cloud influences locally the charge of the ground at the surface, attracting an equivalent quantity of electrical charges of opposed signs.
When it is highly charged, the cloud tries to dispel this charge in “exploding”: it discharges with charge exchanges either with the ground (lightning), either with other clouds or clouds zones (lightning flashes intra or inter-clouds). Millions of electric charges are then dissipated giving rise to currents, possibly reaching 500 000 A and several millions of volts.
It is important to note that currents of 30 mA under voltages of only 50 V can be dangerous for people, even mortal for values higher than 1 A!
From the cloud base, generally negatively charged, a low luminosity discharge known as the tracer, is released. This makes its way to the ground in leaps of some tens of meters.
As it approaches the ground, the highly charged tip of the tracer causes the electric field vertically below it to increase considerably. At about 200 m from the ground, jets of charges or “streamers” are released at points highly prone to lightning where the electrical field is the more intense (tree tops, chimneys, lightning conductors, …). The jet is transformed into a positive upward discharge which goes to meet the dart leader.
The streamer with the best triggering characteristics and which travels most quickly, will reach the leader and provides the electrical junction cloud-ground with the formation of an ionized channel. This favoured way causes strong electrical discharge of several amperes named “return stroke”. Within the space of 0.2 s to 1 s, several lightning strokes may be exchanged in continuous progression and at a very high propagation speed.
According to the polarity of the cloud (positive or negative charges at its base) and the direction of the discharge (ascending or descending), four types of discharges can occur. Under our latitudes, statistical measures in situ proved that more than 90% of the discharges are descending lightning strikes type negative.
Installed in compliance with certain guidelines, lightning conductors are designed to give, whatever type of lightning, excitation and propagation characteristics, better than all other elements nearby. It doesn’t attract (or push) lightning, but protects the structure against its effects, diverting the lightning current to the ground and ensuring its flow.