Before lightning is formed, the cumulonimbus cloud of thunderstorms must become electrically charged. In most rain clouds, the bottom of the cloud is negatively charged and the top is positively charged. It is not known how the cloud becomes charged, but scientists have formed numerous theories to try to explain this phenomenon. These theories have been divided into two main categories: those that require ice and those that do not. However, meteorologists are leaning toward the theory that requires ice because lightning is not often seen unless ice has formed in the upper layers of the rain cloud.
The first theory describes how ice gains a negative charge while frozen and unfrozen water keeps its positive charge. Another theory shows how when large droplets fall swiftly, they gain a negative charge, while slowly falling water gains a positive charge.
Theories that do not require ice gives the explanation that the cloud gets its charge by attracting negative charges from the ionosphere. These negative charges are pushed to the base of the cloud by strong downdrafts, while the positive particles are pushed upwards by warm air within the rain cloud.
Eventually the negative charge of the base of the cloud gives the earth a positive charge. When the electrical potential reaches approximately ten thousand volts per centimeter, ionization occurs along a narrow path and the result is a flash of lightning. The negative particles descend from the base of the cloud to the ground.
However, most lightning flashes are not a single event, but rather numerous strokes followed by a leader stroke. There can be up to 42 strokes to a lightning bolt. The time between successive strokes is 0.02 seconds. The average bolt lasts only one fourth of a second.