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Heat lightning is the name used for the faint flashes of lightning on the horizon or other clouds from distant thunderstorms that do not have accompanying sounds of thunder. This occurs because the lightning occurs very far away and the sound waves dissipate before they reach the observer. Heat lightning was named because it often occurs on hot summer nights and to distinguish it from lightning accompanied by audible thunder and cooling rainfall at the point of observation.

Lightning results from the discharge of negative ions created from the friction of ice and water particles bumping into each other at the bottom of a cloud. Heat lightning can be an early warning sign that thunderstorms are approaching. In Florida, heat lightning is often seen out over the water at night, the remnants of storms that formed during the day along a sea breeze front coming in from the opposite coast.

From wiki.