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To everyone saying Sandstorm shouldn't have a form because "clouds don't interact with sand":
Look at Weather Ball's animation. When Castform launches it up, it starts out Normal. Once it's up in the sky, it transforms into whatever type the weather is, then comes down to attack the opponent.
Sandstorm affects Weather Ball while it's up in the sky and makes it Rock type. Therefore, it should affect the cloud-based Castform.
