From the Super Mario Wiki
In Mario Kart Arcade GP DX
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 will not 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.
Names in other languages
||Rayo Intra Nube
||Lightning inside Cloud
- This item works similarly to the Time Bomb from Mario Kart Arcade GP and Mario Kart Arcade GP 2. However, it shrinks opponents rather than exploding.
- This item makes a distinct sound when received from an Item Box that isn't heard from any other item in the game.