Frise
Bandeau Lafoudre 
 

Phenomenology


Lightning is defined as an electric discharge, accompanied by a substantial current. The most common cause is the separation of positive and negative charges in a cumulonimbus cloud formation.

These storm clouds are often created after cold fronts. As they approach, the mass of cold air pushes the warmer air upwards, creating turbulence and thermal convection, which as it rises creates storm clouds or cumulonimbus clouds.

Diagram showing the distribution of positive and negative charges in a storm cloud
Diagram showing the distribution of positive and negative charges
in a storm cloud
In parallel to these  aerodynamic effects,  the positive and negative electric charges separate inside the cloud. How lightning initially forms is still a matter of debate. The top of the "anvil" cloud is made up of ice crystals and is positively charged, whereas the lower part of the cloud is made up of water droplets and is negatively charged. Quite often, a pocket of positive charges is surrounded by negative charges. It is difficult to give precise estimations of the total size of the charge formed in this manner, as they can vary from tens to hundreds of coulombs.

When a cumulonimbus cloud develops and creates negative charges in its base, this has an effect on the earth's electric field which inverses and increases substantially.  In good weather conditions a positive field can develop and a conductive charge or positive streamer can develop at around 150 V/m.  When the charge reaches between 10 - 15 kV/m at ground level, this is a precursory sign that a lightning charge is imminent.