Question: What causes lightning?

Kevin Motherway answered on 12 Nov 2014:

As a thunderclouds build up an electric charge due to the movement of ice particles in the clouds especially in really tall cumulonimbus clouds. Whenever you have particles in motion you’ll get a charge building up, just like rubbing a balloon on the sleeve of your jumper. At the top of the cloud you may get a positive change and then a negative charge at the bottom. Once the difference between the positive and negative builds up to a critical level a current or lightning bolt will form, heating the air along its path and ionising the gas into a plasma (a state of matter hotter than a gas where the electrons are ripped off atoms and then those atoms can be used to transfer the current). The hot plasma glows white or even blue hot and that’s the lightning flash. The rather explosive formation of the plasma causes a massive shock wave or pressure wave that you experience as thunder.

Whenever you have a negative or positive charge accumulating at the base of a cloud it also induces the opposite charge on the surface of the earth. Just like in the cloud, if the difference between the charge at the base of the cloud and the surface of the earth gets to a critical level the electrical charge flows down from the cloud or up from a point in the ground (yes lightning can go UP to the cloud) and the same mechanism for the flash and thunder as above occurs.

Lightning conductors on buildings help transfer current from lightning strikes down from the cloud (or up from the earth ) safely without damaging buildings. The very tip of lightning conductor is actually radioactive so it will ionise the air (and liberate electrons in it) in an invisible cloud around the tip of the conductor and attract any lightning strikes away from any high point on the building it is trying to protect!