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Could I have some info on what would be realistic ranges for EMPs from nuclear bombs of different yields? I'm unclear about that. Of course it depends on altitude and blast yield, and some electronics can take more than others, but just in general what area would it affect?

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Starfish Prime had an EMP blast radius of approximately 900 miles. More if elevation is taken into effect.

This Wiki page shows a map with most of the US affected, which would be approximately 1500-2000 miles.

But of course this all depends on yield, height of detonation, bomb design. The Wiki hints at a super-powerful gamma detonation that would produce a much more intense EMP, but its existence is not confirmed.

You've got a lot of leeway to work with. For a limit, I'd suggest staying within the visual horizon, as suggested but not confirmed in the article linked.

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    $\begingroup$ It's worth noting that EMP does its damage by inductive coupling. Move a magnet (or an EMP-generated magnetic field) across a wire and you induce a current. The longer the wire, the more current can be induced. It's not generally the computer motherboards that are at risk... it's the power cables that conduct a massive induced current that blows the computer's circuits. Turn off the power strip before the EMP and your computer would likely survive. Hard drives, on the other hand, and all other magnetic media... all would be wiped clean. $\endgroup$ – JBH Aug 10 '17 at 3:14
  • $\begingroup$ I see. As for the starfish prime blast, would it be reasonable for a very rough estimate to say that a blast at the same altitude with four times the yield would be able to have the same effects at twice the distance (by the inverse square law)? $\endgroup$ – Tom Aug 10 '17 at 3:24
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    $\begingroup$ Yes, but the article notes (and correctly so) that everything from the natural planetary magnetic field over the location to weather at the time to the location of mountains and whether or not there is a high water table or not can and will affect the strength of the blast. If you're doing this for a story, I wouldn't bother with that level of detail (unless it is something useful for the plot of the story :-) ) $\endgroup$ – JBH Aug 10 '17 at 3:29
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    $\begingroup$ "The EMP at a fixed distance from an explosion increases at most as the square root of the yield." That 'at most' is pretty critical. The article then goes on into considering how much of the detonation is used for EMP effects (higher percent for smaller bombs, smaller percent for thermonuclear explosions...) And JBH is absolutely right. Any given nuclear EMP would be a calcu-guesstimated crapshoot. $\endgroup$ – Pyrotrain Aug 10 '17 at 3:32
  • $\begingroup$ Actually, that doesn't make sense now that I think about it. $\endgroup$ – Tom Aug 10 '17 at 3:45

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