Local fallout is dust and ash from a bomb crater, contaminated with radioactive fission products. It falls to earth downwind of the crater and can produce, with radiation alone, a lethal area much larger than that from blast and fire. With an air burst, the fission products rise into the stratosphere, where they dissipate and become part of the global environment. Because Little Boy was an air burst 580 meters (1,900 ft) above the ground, there was no bomb crater and no local radioactive fallout.

https://en.wikipedia.org/wiki/Little_Boy

Fat boy was dropped from 500 meters, so I guess there too was no fallout.

I don’t know though if this was known before the drop though.