You're mistaken. The post-tornado scenes of "Wonderbolts Academy" show the Mane 6 standing on the Academy's airstrip, which is build on a plateau. Take a look at this image from the episode, and you'll see that it's solid rock, not cloud.
Hey there! Uh… I'm kind of a musician and I wanted to use this beautiful image for the artwork of my new song. Would I be allowed to do it? I'll make sure to link here in the description, or to your DA page if you prefer. Here's a sample of my music: [link]