Simultaneously, I both like and dislike how this drawing seems to have so much space, no doubt caused by how the characters and the island are really the only objects. On one hand, the spaciness strongly invokes the feeling of being high up in the sky and of flying, which I like. The emptiness makes it feel like they've been traveling in search of this island for a long while.
But at the same time, the spaciness doesn't seem to fit the fact that they're in the middle of a thunderstorm. Storms are very turbulent, dangerous, and--for someone flying through it--obtrusive. The fact that the clouds and lightening are just casually hanging in the background with little contrast doesn't get that across. As a solution, I would say keep the airiness and low contrast around the island in the middle, but around the edges where the characters are, have higher contrast with clouds that seem to be closer in on them.
I like how the lightening is drawn; its form gives it a lot of motion. Though, I think that the lightening bolt in the right hand corner isn't lighting up the clouds directly around it enough. Because it's a decently sized light source from this angle, it should cause harsher shadows and brighter illumination of the things nearby.
I have no complaints about the characters. They're both draw very well. The position that the girl is in makes her look fittingly anxious, which is a nice touch.
Overall, very nice work.