Ooh, this is beautiful! I love the poofy clouds ^_^ The colours are really light and airy and really help to give the feel of flight. Happy swan, too =D
I'd advise you, though, to have a few more (small) clouds overlapping the swan as well, because just having one cloud covering its tail looks slightly odd to me. Oh, and this is just a personal thing, but you could perhaps make the neck and beak a bit longer so it looks more like a swan and less like a goose. That's only a small thing though, overall the picture looks really beautiful. Keep drawing! =)
I really like the feel of this one, and you've done well with the airbrush to create the illusion of clouds.
I would recommend perhaps using more layers to complete your work, as It sort of looks like the tips of the wings are being "washed" by the cloud effect. Placing them on different layers may make it look much crisper.
But very nice job overall!