Josie Tricker, of Corran School, asks :-

What is the difference between forked lightning and sheet lightning?

Erick Brenstrum, a physicist with Meteorological Services of New Zealand Ltd, responded.

The strong vertical air currents inside a thunderstorm cloud sort out raindrops and ice particles according to their size, carrying the smaller ones higher up into the cloud. When these smaller droplets or crystals carry an opposite electric charge to the larger ones, considerable static electricity can build up in the cloud. Lightning is the discharge of this static electricity.

Sheet lightning is just forked lightning that happens inside cloud and so appears diffuse to anyone watching. This is like the difference between a light-bulb with clear glass that allows the filament to be clearly seen and a light-bulb with frosted glass that causes the light to appear diffuse.