system
Given the following post, respond as if you're commenting on Reddit:
human
Why are nuclear bombs activated above their target and not on ground impact?
gpt
They can be detonated at any height, depending on what whoever planned the attack is trying to achieve. Some factors that would be considered:

**Fallout**

When a nuclear device detonates, anything within a vaporization radius of the device will be turned to vapor. The vapor of that matter will mix with the vapor of the device debris. That's how you get what's called fallout - various types of matter get mixed in with the highly radioactive device debris, get irradiated by neutron radiation, and can be transmuted into other radioactive elements. The more matter that's in that vaporization radius, the more and more dangerous fallout you get. So detonating high enough that the vaporization radius doesn't touch the ground will minimize fallout, and detonating on or right above the ground will maximize it though vaporizing all of those cubic meters of earth. In most cases, you would prefer to minimize fallout.

**Thermal Radiation**

This is non-ionizing radiation put out by the bomb, that travels in a straight line, doesn't go through things, and heats up whatever it hits. It can burn people to death, start fires, melt things, etc. If the device detonates too low, buildings and terrain may shield some of the surrounding area from thermal radiation, while a detonation high enough to be above these things will expose more things to it. You detonate low to get the maximum destruction to things very close to the detonation, and detonate higher to spread the destruction out more at the expense of a little less effect right under the bomb. Of course, even the lesser destruction from being under a higher-altitude detonation is more than enough to completely destroy most things.

**Blast wave**

This is a shock wave of outgoing high-pressure air. It tends to knock down buildings and other structures through high winds and pressure and inflict blast injuries to people. It spreads in a kind of similar way to the thermal radiation, but is more complex due to reflections, interference, air pressure at detonation altitude, and other factors. There's charts for what altitude to detonate at for various effects at various ranges.

**Physical Vaporization**

Back to the vaporization mentioned in the fallout section. The other side of that is that, if you are attacking a hardened target, like something deep underground or not very affected by heat and blast, like train tracks, etc, it may be necessary to physically vaporize the target to ensure the intended destruction. So back to a ground or near-ground detonation.

**Summary**

It all depends on exactly what you're trying to attack. Destroying a nuclear missile silo is different from destroying a military base or navy yard, which is different again from going for maximum casualties in a city, and any number of other types of targets one might want to attack.