How thunderclouds produce lightning

As you probably know, lightning is really not much more than a very large and powerful spark. It is caused by a discharge of electrical energy that has built up inside a thundercloud. During a thunderstorm, processes involving things such as vertical winds and different-sized water droplets cause electrical charges to build up in various parts of the cloud. When these charges become powerful enough, they discharge, causing a stroke of lightning.


Credit: NASA/Global Hydrology and Climate Center

For more info: http://thunder.msfc.nasa.gov