From the Super Mario Wiki
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 Mario Kart Arcade GP DX. In Mario Kart Wii, unlike other items, it activates itself automatically when obtained from an Item Box.
When obtained, the player automatically receives a constant boost of speed. Also, drivers with a Thunder Cloud won't slow down when they go offroad. 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 them be shrunk instead. It can be destroyed when the character holding it drives into a cannon, uses a Star, a Mega Mushroom or a Bullet Bill.
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