Lightning - Step leaders and return strokes

It all begins when the atmosphere becomes unstable. In the summertime, the sun's energy heats the earth's surface, which in turn heats the atmosphere close to the ground. This warmer air close to the ground is bouyant relative to the air above it. If this instability becomes great enough, columns of warm air may begin to rapidly burst upward through the atmosphere in a process called convection. As the massive volumes of air rise, they condense in the cooler surrounding air. If there is sufficient moisture in the air, this process creates the enormous, towering cumulonimbus clouds (see diagram at above right), and a thunderstorm is born. An approaching cold front can also force warm, moist air at the surface upward, initiating thunderstorm development.

While there are several theories, the exact mechanism of lightning generation within a thunderstorm is not yet known. Strong updrafts in a thunderstorm carry water droplets into the subfreezing air high in the atmosphere. It is thought that electrification of a storm is related to the freezing of these small droplets of water as they are carried high into the cumulonimbus cloud. Some have also theorized that condensation of water vapor into water droplets during the convective process is the source of charge generation. Whatever the source, the large cloud eventually develops regions of positive and negative charge- usually positive charge high in the cloud and negative charge at the base. The negative charge at the cloud base induces a 'shadow' of positive charge on the ground, much like a magnet induces polarity on a metal paper clip.

The storm soon becomes supercharged with electrical energy as the convective activity continues. When the insulating air between the regions of opposite charge can no longer hold the two apart, a lightning flash begins to develop.