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They were air-detonated, I believe 200 meters above ground level, so the damage was less than the maximum potential of the bombs.

Nagasaki is more solemn, the ground zero area has rubble visible in a glass display area, and ruins of a brick pillar covered with scorch marks from the historic Urakami Christian Church

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air detonation is for max damage. "The air burst is.. to allow the shockwave of the fission or fusion driven explosion to bounce off the ground and back into itself, creating a shockwave that is more forceful than one from a detonation at ground level." https://en.wikipedia.org/wiki/Air_burst

I assume the original commenter meant maximum specific damage - essentially the air detonation causes a bunch of shockwave structure (pre cursor waves I think?) that result in much increased overall damage over a much larger area at the cost of a reduction in direct damage at the point of detonation (essentially vaporising basically just a few buildings/block vs functionally demolishing an entire city). There’s a documentary called (iirc) “trinity and beyond” that covers a lot of this a at least to the level of understanding of a non-physicist like me :)

I agree, max damage was the wrong term. I was envisioning the temperatures to melt steel structures that exist closer to the detonation.

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