From the Super Mario Wiki
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Sand cloud, also referred to as sand storm, is an attack that Yoshis are able to perform in Super Mario World and Super Mario World: Super Mario Advance 2. Any colored Yoshi who holds a Yellow Koopa Shell in its mouth is able to cause a small earthquake every time the player lands after a jump, which defeats any enemies who are on the ground nearby. After Yellow Yoshis become available in the game's Special Zone, the player is able to use a Yellow Yoshi to eat any colored Koopa shell to attack using sand clouds.
The sand cloud ability will last as long as the player keeps the Koopa shell in the Yoshi's mouth.