The thousands of atomic bombs exploded on Earth (2015)

First one was cuban missile crisis that had a lot of escalation, but most poignant was probably dropping depth charges against submarines armed with nuclear torpedoes (https://en.wikipedia.org/wiki/Cuban_Missile_Crisis). On that front - thank god Khrushchev chickened out.

Second one was a false detection by Soviet early warning system which should have triggered an immediate retaliation, but was stopped by one person with critical thinking skills (https://en.wikipedia.org/wiki/1983_Soviet_nuclear_false_alar...).

The possible responsible parties is interesting.

https://en.wikipedia.org/wiki/Vela_incident#Possible_respons...

So the Tsar Bomba was designed to be twice as large as it actually detonated. What the hell?

See, a thermonuclear bomb like this is (in the Teller-Ulam design) a three stage weapon: first, the primary (a regular atomic bomb, generally a sphere of plutonium that is imploded really fast) goes. The primary is a fission bomb, where a bunch of fissile material (Pu-239 or U-235- or technically U-233 but that's never been done at scale- Pu239 is easiest to isolate so its most common) is rapidly pushed together and generates a whole bunch of energy. This is what destroyed Hiroshima and Nagasaki, this is an "atomic bomb." This energy release comes from the chain reaction of atoms splitting: one U or Pu atom splits into a chain of daughter atoms and gives off a lot of energy- and extra neutrons which hit other atoms and cause them to split, setting off a chain reaction of energy release when enough U/Pu atoms are dense enough.

And then the energy from that release is directed into a bunch of fusionable material (probably lithium-deuteride) which causes those atoms to get so hot they start to fuse. And that fusion (like in the sun itself) gives off much more energy- as a rough estimate let's say something like 10x the amount of energy from the primary.

And then there is a third stage, the uranium tamper. You take some depleted uranium (U238- which is what is left behind when you pulled out all of the U235 from the natural uranium) and you direct the energy of the fusion into that. And that causes the U238 to do fission (which it won't do normally, unless you dump a whole lot of neutrons into it). This is, basically, doubling the amount of energy released. It's this uranium tamper- which is basically free, once you have an enrichment program up and running- which was replaced by lead in the Tsar Bomba test, so it only produced half the energy it could have.

As a note, fission turns out to be where most of the fallout is generated, and fusion doesn't generate a much fallout. The infamous "neutron bombs" of the 1970's and 1980's were a common name for basically, a thermonuclear bomb without that Uranium tamper (e.g. something like a Tsar Bomba but smaller) which produced very little fallout relative to its size. This worried some people, who thought that a conventional NATO-Warsaw Pact war might go nuclear if the West German government could be convinced to allow the use of such a bomb on their territory because it had less fallout. This also explains why the uranium tamper wasn't included on the Tsar Bomba test, because that would have produced absurd amounts of nuclear fallout.

Also as a general rule large bombs aren't very efficient, because you are destroying everything in a sphere volume: to double the radius of destruction you need to increase the energy by a factor of 8. Once Minuteman came along, and there were a thousand very accurate ICBM's available (and then Minuteman III which added MIRV's so quickly tripling that- and the Soviets responded with SS-11) it was more efficient use of the expensive fissile material to put more smaller warheads around, because the delivery systems were no longer the bottleneck.

Isn't this the basic idea of ICBMs / MIRVs with multiple warheads? Basically nuclear cluster bombs.

So the last test at which it is feasible that someone watched with their own eyes would be the Chinese nuclear test at Lop Nor on 16 October 1980, at 04:30:29.67 UTC.

That was a test of the JL-1 missile, which was fired off and then flew all the way through reentry to nuclear detonation. At the time, a lot of people in the West doubted that Chinese missiles were capable of delivering a working warhead (1) so they proved their capability by firing an actually armed SLBM and it detonated in the atmosphere, exactly like it would do against a city.

1: Much like a lot of people discount North Korea's missile delivery today. No one doubts that Israel, France, Russia or the UK have missiles that can deliver nuclear warheads successfully (the US, like China, has actually done a full SLBM launch-to-warhead detonation test, the only two countries to do so- though the US Operation Frigate Bird was before the LTBT). I wonder what it is that connects China and North Korea, and makes Westerners doubt their abilities in ways that they don't doubt the French or the UK?

Presumably the operational devices are more consistent and higher yield than the early pre-fusion cores. There are outliers on both sides in the test data.

Much of the soot generated from a nuclear blast comes from the surrounding environment, cities presumably, which seems to be less flammable than in the past(armchairing here).

Time of year will always be a factor.

I'm sure there are things I'm not thinking of off the cuff.

Most tests were underground after the first decade or so once the effects of fallout were better understood. Plus underground testing reduces the affected radius and conserves area for additional testing.

Also note that most tests were "small" yield weapons rather than the really big hydrogen bombs. It's easy to forget how large the range of power there is among different nuclear weapons. E.g. Tsar Bomba was 4000 times more energetic than the bomb dropped on Hiroshima.

There is also the secondary impacts that would start immense uncontrolled wildfires all over the planet that would produce much of the nuclear winter effect that is assumed, due to the amount of soot and ash it would produce. But most people would not live long enough to experience the nuclear winter anyways after the explosions, the fireball, the uncontrollable wildfire, the marauding, and the starvation and disease.

If you lived long enough to make it to nuclear winter, you would probably just wish you had died long ago. If you had anyone at all with you, you might have to even make hard decisions about who you can take with you and who to leave behind, and if you managed to somehow get to somewhere where you might be able to feed yourself and survive near the equator, you would only really be living a primitive subsistence life. If you happened to have a female with you, you may try to rebuild the human population, whether she wants to or not. Yes, it’s very grim; but maybe even optimistic. But even if that is pessimistic, at best you will find yourself in a world where there is nothing left that you could rely on for contemporary existence. No communications, no electronics, nothing but manual transportation once any surviving fuel is used up on any surviving carbureted engines, no running water, and you would have to fight off savage roaming marauders, who want all your things and will have thrust everyone into a hellish version of Madmax, etc.

See any of the actual first hand accounts of what happened in the New Orleans and the Superdome after hurricane Katrina for a tiny little taste of what you would be facing.

The nuclear winter would be the least of your problems.

1) Big bomb. The mushroom cloud must go up to above the maximum altitude for rain.

2) Soot. You need a city or the like underneath the bomb to produce a bunch of soot to get lifted into the sky.

Either one by itself (we've seen really big bombs, but they're not dropped on places that will make a bunch of soot, Saddam's oil fires created enough soot but nothing lifted it above the rain, the bombs on Japan weren't big enough to lift the crud above the rain) has little effect.