When does sheet lightning occur?

Sheet lightning - usually occurs during a storm as the result of a discharge of excess electricity within a single thundercloud. The outline is obscured and the result is a diffused light spread over a large area of the sky in contrast to the vivid spiral or ribbon like flashes of chain forked or zigzag lightning. The most favourable conditions for sheet lightning are provided when the electric field is equal throughout the area. Unlike other forms of lightning it does not reach the ground and the channel cannot be distinguished. What is often referred to as sheet lightning is merely the lightning up of the sky by flashes occurring beyond the horizon. All lightning is the natural discharge of large accumulations of electric charges in the atmosphere. It may take place between neighbouring clouds or between cloud and earth. Just before the discharge the cloud's electric potential is often built up by the action of falling raindrops or other natural processes.