As storm clouds develop, the temperature at the top of the clouds becomes much cooler than that at the bottom. For reasons that scientists still do not understand, this temperature difference results in the accumulation of negatively charged particles near the base and positively charged particles near the top of the storm cloud. This buildup of electrical charge causes a high-voltage discharge in the form of lightning. What is sheet lightning, and how does it differ from forked lightning? Discuss
Source: The Free Dictionary