They dropped Little Boy on Hiroshima but had it timed to explode at about 1,900 feet in the air or so.
What, though, if it had been triggered to explode only upon hitting the ground? How would this have changed the damage and casualities of the city?
Like all nuclear weapons, most of the energy would have then gone into excavating a large hole. The physical damage to structures and the immediate number of casualties would have likely been far less, but it would have likely produced more radioactive contaminated soil (aka “fallout”). Depending on the prevailing winds, this could have resulted in either more long-term casualties or fewer.
Air explosions result in far more extensive immediate blast effects, because the heat and radiation can reach a larger footprint unimpeded.
The effect of the actual bomb was to immediately light most of the city on fire, which was compounded by the light residential construction used in the city.
To expand on my answer, air blasts produce more physical damage and casualties at a given distance from the blast for soft targets (like cities).
Ground-level blasts produce a far smaller radius of destruction. However, they are more effective at destroying hardened targets (like missile silos).
What robby said, except I’d remove the caveat of it being nuclear weapons. Conventional explosives act the same way. This is why a lot of explosives tests set the energetic material on a table or suspend it from a string. Just like wind has a lower velocity closer to the ground, the overpressure wave does not have as much energy next to the ground.
The only situation I can think of to help that make sense, as it’s a bit counter intuitive, is to hold your fist out, palm down, and open it. Easy, right? Now, put your fist on a flat surface, and try to open it. It takes more energy because you’re not just putting energy into moving your fingers, you’re also having to resist the force of the surface, too.
In regard to fallout, I remember reading somewhere (no cite) that the fallout of the bomb was reduced by mountains surrounding the city I imagine if the bomb had hit the ground some of the fallout would still be caught in the mountains and the fallout damage would have been about the same.
You can’t remove the caveat of a nuclear explosion because of the radiation factor. Even if you’re outside the blast area you are still exposed to massive amounts of radiated energy which will destroy the cells of the body.
You’re probably thinking about the mountains in Nagasaki shielding part of the city from Fat Man’s blast.
Neither of the bombs dropped on Japan had appreciable fallout over Japan due to the fact that they were airbursts; the only real fallout was that of the bomb materials themselves and whatever dust got kicked up into the fireball.
Basically Robby has it right- Little Boy would have created a decent sized crater, and the blast/heat/prompt radiation would have been greatly decreased.
On the other hand, the crater, and the area immediately nearby would have been highly radioactive, and so would an area downwind for a good distance, because rather than just the bomb materials, all the material from the crater and whatever else that got caught up in the fireball would have been vaporized, highly radioactive, and would have settled out of the air in a short period of time, spreading that radiation as fallout.