Eh, there's not a lot of fallout if the blast is in the air. Fallout is when the blast vaporises the ground which only happens if the blast HITS (or blows up close to) the ground Of course, if it blows up on the ground the shockwave doesn't go as far, so its a tradeoff of damage vs radiation.
It is much less than a ground detonation, but an airburst creates such an updraft that it acts like a giant vacuum cleaner, sucking everything up into it. That is where the mushroom cloud comes from.