The Thunder Cloud (also called Lightning Cloud) is an item from the Mario Kart series, appearing in Mario Kart Arcade GP, Mario Kart Arcade GP 2, Mario Kart Wii, and the upcoming Mario Kart 7. Unlike other items, it activates itself automatically when obtained from an Item Box.
When obtained, the player automatically receives a constant boost of speed. However, the Thunder Cloud eventually strikes and shrinks the player. The Cloud, however, can be passed on to another kart by bumping into any other racer before the lightning strikes. To use the item efficiently, racers have to bump another player and let her or him be shrunk instead. It will be destroyed when the character holding it drives into a cannon.