This is the brilliant flash that we recognize as lightning. Generally, however, this entire process happens so fast that the human eye cannot separate its components, and we see only a complete flash. After the initial return stoke, the channel through the air remains very conductive, and thus provides an easy pathway from the cloud to the ground. Oftentimes, several subsequent strokes will follow this same channel, with each stroke draining the negative charge from higher up in the cloud. These strokes follow within milliseconds of each other and produce the flickering effect that is sometimes observed.
Thunder is produced when the air heated by the return stroke rapidly expands into the air around it.
Since the return stroke begins at the ground, thunder begins at the ground. The rumbling of thunder is caused as the sound from the different sections of the lightning bolt reach the observer at different times. The distance to the lighting strike can be estimated by counting the number of seconds it takes from the time you see a flash until the time you hear the thunder. Dividing this number of seconds by five will give a good estimate of the number of miles between you and the strike. The proper installation of lightning rods on buildings and trees can save immeasurable amounts of monetary as well as sentimental value by rechannelling the energy of a direct stroke.