From the Super Mario Wiki
The Thunder Cloud (originally Thundercloud and also known as 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. This item works similarly to the Time Bomb in Mario Kart Arcade GP and Mario Kart Arcade GP 2. However, it shrinks opponents rather than exploding. In Mario Kart Wii, unlike the rest of the items that appear in said game, it activates itself automatically when obtained from an Item Box, and cannot be thrown, unlike in the arcade games. This item makes a buzzer sound when received from an Item Box, a sound that is not heard from any other item in the game. It can be obtained if the player is anywhere from 3rd to 7th place, and in VS Races, they can also obtain it in 8th place.
When obtained, the player automatically receives a constant boost of speed. Also, drivers with a Thunder Cloud will not slow down when they go off-road. However, the Thunder Cloud eventually strikes and shrinks the player, having the same effects of the Lightning. 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, falls off the track, uses a Star, a Mega Mushroom or a Bullet Bill.
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