What do I do in the air realm?

Try shooting the cannon.

As you might have guessed, your goal has something to do with the tornado.

When you first enter this realm, you probably won't have much luck hitting
the tornado -- it's too good at avoiding the red beam the cannon shoots out.

But that's only because you haven't set the cannon properly.

Have you tried looking down from this screen?

Look through the pipe and you'll see some balls on a grid.  These balls
control how the cannon shoots.