why are atom bombs or nuclear Bombs detonated above certain Height clearance from ground? Why not directly detonate it on the ground?
In: Physics
You can do either in war or testing. There are differences in the effects:
* For a ground burst, you get a LOT of intense blast pressure (i.e., destroy everything) in the immediate area around ground zero, but the ranges of medium (i.e., destroy houses) and light (i.e., break windows) pressure are decreased. You also get a lot more downwind radioactivity (fallout) because the soil, dirt, etc., mixes with the radioactive byproducts of the bomb that go into the cloud, and that makes them “fall out” of the cloud relatively quickly, concentrated and radioactively “hot.” Your thermal radiation (the heat wave) will be largely blocked by buildings, mountains, etc. (it is line-of-sight).
* For an airburst, you can have less intense blast pressures, but your range of medium and light pressure can go much further (both because they are less blocked, but also because you can have complex interactions between the initial blast wave and the blast wave that is reflected off of the ground, and this can amplify the blast wave if you know what you are doing). The thermal radiation is much less hindered because it’s come down on your target, not moving horizontally through it. And if you detonate it at a height in which the fireball doesn’t mix with the dirt/debris/etc., you get basically no intense downwind radioactivity.
* For a very high airburst, like in or near the boundary of outer space, you get almost no pressure, radiation, or heat on the ground, but can have complicated electromagnetic effects (EMP) that can hurt electrical infrastructure, block radar, and things like that.
For any given target or goal, you might prefer a different aspect of the above (or some variant between them — the “boundaries” are less clearly defined as I have made them out to be). If you are attacking a “soft” target, one that can be destroyed through medium/light blast pressures, then you want an airburst. Things like cities. So Hiroshima and Nagasaki were airbursts, because the goal was to destroy as much civilian building as possible (as is reflected in the notes of the planners).
If you are attacking a “hard” target, like a bunker or trying to crater a runway or destroy an underground missile silo, you want a ground burst.
The fallout effects could also be a relevant consideration; if you are attacking a target near an ally and fear that the radioactivity could hurt them, or blowback on you, you’d prefer an airburst. If you are testing a bomb and don’t want downwind radioactivity, you might do it high in the air. (Though since the 1960s, almost all nuclear tests have been underground, because that not only reduces intense downwind fallout, but the more diffuse “global fallout” that you get with airbursts as well — “global fallout” is a subtle raising of global radioactivity, as opposed to the intense but localized “you can’t live there anymore” of “local fallout.”)
Most US nuclear weapons could be and can be set for any of the above options; they are “FUFO” (Full Fusing Options), as the jargon calls it.
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