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Summer Thunder and Lightning

During the summer, the ground and the air above it are usually warm. Sometimes a layer of cool air will form above the warm air. Warm air is light and wants to rise. The cool air, on the other hand, is heavier and wants to fall. The resulting instability causes violent movement of air masses, with the heavier cool air sinking to the ground and the lighter warm air rising rapidly.

The rising warm air becomes saturated with moisture and droplets of water appear as a cloud begins to form. As the warm air continues to rise, the cloud grows and strong winds develop.

The water vapor changing into liquid results in the release of heat energy, which adds to the cloud’s growth. Eventually, rain begins to fall.

A thunderstorm cloud, called a cumulonimbus cloud, may be several miles across and up to eight miles high !

High altitude winds shred the top of the cloud, producing a flat-topped anvil shape. The upwards flow of air has been replaced by a strong downward movement generated by the precipitation / rain.

All that violent air movement that formed the thundercloud also created electric charges in the cloud. The upper portions of the cloud acquire a positive charge, and the bottom becomes negatively charged.

As the cloud passes over the ground, the negatively charged cloud bottom induces a positive charge in the ground. This ground charge follows the storm like an electrical shadow. This ground charge increases, but the air is poor conductor of electricity, so no electricity flows until HUGE electrical charges have built up.

When the electrical potential is great enough, lighting will strike out from the cloud to the ground. How much electrical potential you may ask? it could be as much as 100 million volts.

from the Smithsonian Institution Smart Lab weather kit