An airburst is the explosion of a bomb through the air rather than on the ground.
airburst
If you are asking specifically about the two bombs dropped on Japan in WW2, they were fuzed for airburst, so they did not hit the ground. Both exploded between 1500 ft & 2000 ft altitude. Airburst was selected to maximize blast & thermal flash damage effects area.
Greater damage from blast and thermal radiation.
The Tunguska event happened on June 30, 1908. It was an airburst event; no crater. (A wonderful discovery sequence followed)
It will if it is fused for airburst. This is selected to maximize the area and severity of blast and thermal flash effects.
There are too many factors to give you one simple answer. It will depend on the yield (size) of the weapon, where it is detonated (on ground, low airburst, high airburst) etc. A 150 kiloton weapon, low airburst, would cause 3rd degree burns at about 5.4 kilometers, and cause total destruction of ordinary buildings (from blast) out to 4 kilometers. Current day nuclear weapon are LESS powerful than the VERY large weapons from the 50s and 60s- better guidance systems can place the weapon on target within a matter of feet- in the 1960s, it was a matter of miles.
A MK-3 Plutonium Implosion Fission Bomb, yield ~21KTons, airburst @ ~1,800 feet. See link for more info.
July 16, 1945 Gadget, Trinity Test tower shot, NM - PlutoniumAugust 6, 1945 Little Boy, Combat airburst, Hiroshima - UraniumAugust 9, 1945 Fatman, Combat airburst, Nagasaki - PlutoniumJuly 1, 1946 Able, Crossroads Effects Test airburst, Bikini atoll - PlutoniumJuly 25, 1946 Baker, Crossroads Effects Test shallow underwater, Bikini atoll - PlutoniumCanceled 1946 Charlie, Crossroads Effects Test deep underwater, Bikini atoll - Plutoniumetc.Take your pick, either the first atomic explosion (Plutonium) or the first combat use (Uranium).
Airburst and altitude, Surface burst, Subsurface burst and depth (dirt, rock, or water)? Different in each case.
1945, the delivery system was composed of a B-29 bomber with Silverplate modifications and gravity bomb casings with RADAR altimeter airburst fuses.
That will vary dramatically with the design of the bomb and whether it is an airburst or a surface burst.
An airburst, surface burst, or shallow subsurface burst. If the burst is too high to entrain dirt or water the cloud will be composed entirely of material from the bomb itself.