Originally posted by Prof. T.C McAbe
But it actually gets more tricky with regards to the other major bomb effect — blast pressure. Blast pressure is basically a heavy shockwave that knocks down buildings, crushes people, and is the most immediately potent effect of the bomb. It is the effect that the choice in the detonation height of the bomb was most specifically tailored to, because it is the most affected by the change of height.
Here is why. On the surface, the blast behaves more or less like the other effects — it goes in a straight line horizontally, getting dissipated as it affects buildings. But in the air, something special happens. The shock wave going straight down from the exploding fireball doesn't stop when it hits the ground — it reflects back upwards. You can actually see this in some of the high-speed nuclear test photos. When it reflects back upwards, it interacts with the existing shock wave and adds to its power, forming what is known as a "Mach stem." Here is a diagram illustrating this principle. Now instead of one shockwave moving out, you have effectively two shockwaves on top of each other (or, more specifically, a new shockwave with about twice the strength of the original one).
Three things
1. Hulks shockwave penetrated the planet to its core instead of merely bounce off. This means it was vastly stronger.
2. Assuming it bounced off, the stronger shockwave created is still a product of the total force. The force that initially created the shockwave was concentrated. Then this force created shockwave energy that spread out over a distance (became less in concentration). But when combining with the other shockwave it becomes more concentrated (but never to the degree as the original force).
3. Primes, PC Superboy feats are better, and Superman's black hole feat is within the vicinity if not better 😉