Spark image

Lightning is a huge electrical discharge (a giant spark) either between one cloud and another or between a cloud and the ground. In a storm there are enormous convection currents in a cloud, water droplets and ice particles going up and down within it. This movement causes friction between the particles when they collide which causes a build up of electric charge in the cloud. When the charge is big enough the electric field produced ionises the air. The electrical resistance of the air 'breaks down', it is no longer an insulator and the charge is discharged as a spark.

Thunder is just the noise of the expanding and contracting air. The air is heated up by the lightning flash, expands and then cools so contracting. This expansion and contraction makes the cracking and rumbling sound that we call thunder.

The current in a lightning flash may reach 10 000 A and the temperature 20 000 oC, three times the temperature of the surface of the Sun.

© Keith Gibbs 2009