I'll add to what other people are saying with a caveat.

Detonating a Nuke at ground level significantly reduces it's effectiveness and range. It's still going to be bad, but it won't be city levelling bad. A couple of miles at best for the worst explosion/fire damage, even if it's stronger than Fat Man.

There's a reason why the two bombs dropped on Japan were detonated at 500-600 meters(1600-1900 feet) above the ground.