The major forms of lightning include forked lightning, streak lightning, ribbon lightning, and bead, or chain, lightning. Forked lightning refers to a flash in which multiple branches of a stroke are visible. Streak lightning is a flash that seems to illuminate a single jagged line. Ribbon lightning appears as parallel streaks of light. It is formed when wind separates the strokes of a flash. Bead, or chain, lightning is a flash that breaks up into a dotted line as it fades.
Some electrical flashes in the sky - such as heat lightning and sheet lightning - are not really separate forms of lightning, though they appear different in some ways. Heat lightning, often seen on summer nights, seems to occur without thunder. Actually, it is lightning that occurs too far away from the observer for its accompanying thunder to be heard. The people underneath what looks from a distance like heat lightning experiencing a normal thunderstorm. Sheet lightning appears as an illumination of a portion of the sky. But it is lightning whose distinct flashes either are too far away to be seen or are hidden from view by clouds.
A form of lightning called ball lightning differs greatly from ordinary lightning. Ball lightning appears as a glowing, fiery ball that floats for several seconds before disappearing. It has reportedly been seen during thunderstorms, usually after ordinary lightning has occurred. It is described as a red, yellow, or orange ball that may be as large as a grapefruit. It has been reported floating along the ground and inside houses, barns and airplanes. No one knows how or why ball lightning occurs, or what it consists of.
A glowing light called St. Elmo's fire may resemble ball lightning in some ways. St. Elmo's fire is caused by electrical discharges from a sharp object during a thunderstorm. It sometimes appears around airplanes, the masts of sailing ships, towers, and treetops.
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