Summer and thunderstorms go hand in hand, but how do they form?

Thunderstorm in the distance

The weather during the summer months can be very dramatic, with plenty of heat and energy in the air producing some big thunderstorms, but how do they form?

It's all to do with heat and moisture. As air rises it cools, causing the water vapour in the air to cool and condense into liquid water droplets and make clouds. Some clouds are small and produce no rain, some are really high and made of ice, and some are huge cumulonimbus clouds that produce hail and thunder.

A cumulonimbus cloud

Within one of these thunderclouds there are huge currents of air moving up and down, dragging heat and moisture into the cloud, allowing them to become massive. The moving air carries water droplets to the top of the cloud (which can be 20,000 to 30,000 feet high) where they freeze and circulate and bump into other ice particles. Some of these collisions make hail.

Currents of air within a thundercloud

As hail and other, smaller ice particles bump against each other they become electrically charged. The hail gets a negative charge and the ice particles become positive. The heavy hail sinks to the base of the cloud and the lighter ice particles are drawn to the top on those air currents. The cumulonimbus cloud effectively ends up like a giant battery.

Positive and negative charges within a thunderstorm

As the negative charge at the bottom of the cloud increases it becomes too much and has to discharge somehow. This is when we see lightning and hear thunder, either from the base of the cloud to the ground, or within the cloud itself.

Lightning travels at 270,000 mph and as it moves through the air it creates a huge amount of heat almost instantly - some 30,000°C which is five times hotter than the surface of the sun! It's this that we hear as thunder after we see that brilliant blue flash of light.