Thunderstorms form in cumulonimbus clouds called thunderheads. Huge electric sparks, or lightning, heats the air. This produces the noise named thunder. Thunderstorms usually feature strong winds and heavy rains. Thunderstorms begin when strong updrafts, or upward rushes of heated air, form inside clouds. Then the rising air rubs against the air going down, building up static electricity. When the weather is hot and humid with a(n) cold front approaching, thunderstorms are probable. These conditions cause the warm air to rise rapidly.