system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: With Atomic bombs, when they are deployed, why are they detonated so high off of the ground?

Hiroshima as an example, was detonated 1,900 feet above the city.
gpt
The air burst is usually several hundred to a few thousand feet (100 to 1000m) above the hypocenter to allow the shockwave of the fission or fusion driven explosion to bounce off of the ground and back into itself, creating a shockwave that is more forceful than one from a detonation at ground level. This "mach stem" only occurs near ground level, and is similar in shape to the letter Y when viewed from the side. Airbursting also minimizes fallout by keeping the fireball from touching the ground, limiting the amount of debris that is vaporized and drawn up in the radioactive debris cloud. For the Hiroshima bomb, an air burst 1,800–2,000 feet (550–610m) above the ground was chosen "to achieve maximum blast effects, and to minimize residual radiation on the ground as it was hoped U.S. troops would soon occupy the city".[2]

That's from the "Air Burst" Wiki.

Basically, it lowers the amount of residual radiation while increasing the actual force of the blast.  Perfect if you want to wreck a city... but occupy the area afterwards.