Question: What causes lightning?

Naomi Green answered on 11 Mar 2015:

Great question! I love thunderstorms and lightning, I think they are beautiful to watch. I’m not so keen on being outside in them though 😉 how about you?

In certain weather conditions there will be warm air trapped under cold air. As the warm air rises it cools down and some of it will change from a gas to a liquid and form water droplets and form rain clouds. Sometimes this happens very quickly and the water droplets clump together and freeze creating ice crystals. Eventually these droplets and crystals become too heavy to be supported by the warm air which is rising from beneath and they fall as hail. As the hail falls through the cloud it rubs up against other smaller crystals, which are not yet heavy enough to fall. The smaller crystals are positively charged and the big hail crystals become negatively charged by rubbing against them. So we now have small positively charged hail crystals at the top of the cloud and big negatively charged crystals at the bottom of the cloud. The negatively charged crystals are attracted to the Earth’s surface or buildings and trees on it. Eventually the difference in charges between the cloud and Earth becomes too big and an electrical current flows between the two to balance out the difference in charge.

That electrical current is what you see as lightning. As the electrical energy is discharged it rapidly heats up the air in the cloud making it expand. As the water droplets bump into each other it makes a noise and that produces the thunder that you hear at the same time.

The electrical current will always try and take the easiest route and that is why lightning often hits tall buildings because they are closest to the clouds! Tall buildings have to be designed with lightning protection to prevent them being damaged in a storm.

I’m sorry if that was a long answer but it is quite complicated!