Just out of sheer curiosity. If Nikita Khrushchev claimed the Ruzzians had a 100 megaton warhead, how would that compare to what Enola Gay was carrying?
You can use Ground Zero 2 to compare effects though the largest man made bomb is 50 megatons.
The yield on Little Boy (a gun-type [sup]235[/sup]U fission device) was between 15-18 kT. The amount of damage and damage radii aren’t linear to the yield as much of the initial energy is absorbed and converted with non-linear effect, but a 100 MT nuclear device certainly would have turned the greater metropolitan Chicago area into a large steaming lagoon for Lake Michigan.
However, I should point out that while the USSR tested a multistage thermonuclear device (known in Western literature as ‘Tsar Bomba’, constructed per Andrei Sakharov’s “Third Idea,” an independently derived design analogous to the Teller-Ulam design in the West) which had a demonstrated yield of ~52 MT and a potential yield of ~100 MT (estimated), this was neither a practical weapon nor was anything of this scale ever deployed by any nuclear power. The largest nuclear device deployed in an ICBM RV by the Soviet Union was an 25 MT (estimated) device on the monstrous R-36M ICBM (NATO reporting name SS-18 ‘Satan’), intended to be used against hardened missile and strategic installations. This is probably in the neighborhood of the largest practical device; in terms of destructive power over an area, deploying a spread of smaller devices in the 500 kT neighborhood is a lot more effect.
Stranger
Indeed, the point of having such a single large warhead was probably to turn Cheyenne Mountain into Cheyenne Crater. 
Even for a nuke that would be pretty hard. The Cheyenne mountain really is a mountain and there would be a lot of solid granite to remove before it becomes Cheyenne Lake. The facility itself, the Cheyenne Mountain Operations Center (which is currently mothballed, as NORAD Ops is now on Petersen AFB) is basically a series of shock-isolated half-inch thick steel cans under 2000 ft of granite, intended to survive a near-hit by a very large (>25 MT weapon.
However, if you take out the missile wings, you take out the ground-based counterforce capability, and that was what the 25 MT weapon was designed to do. It does not address, though, the Submarine Ballistic Missile Fleet (“41 for Freedom”) or the Strategic Bomber Wings maintained by the Strategic Air Command, which form the other two legs of the defensive nuclear triad.
Stranger
True, although I understand that Soviet doctrine with respect to Cheyenne and other known command centers was something like “explode the largest nuke we have at ground level as close to the site as possible. Once the smoke and dust clear enough for another warhead to survive, do it again. Repeat until reasonably certain said facility no longer exists, or until our own arsenal has been destroyed.”
Anyway, to go back to the OP, although the Tsar Bomba was impractical as standard arsenal weapon, aren’t there a few special instances in which you would want one really big device? Like maybe the nuke hidden in the basement of the Soviet Embassy in Washington?
The problem with this is it assumes counterforce capability. The reality in a strategic nuclear exchange is that you can’t be certain of keeping weapons–especially those in known, fixed positions–secure from attack, so it is just better to go ahead and launch. This tends to invalidate game theory strategies (and therefore actual war strategies) that assume limited exchange; there is no point at which you are safe stopping “just here”. Also, there is a backup to the NORAD Operations Center in CFB North Bay in Ontario, Canada, as well as the Looking Glass airborne command center that would provide command and control functions should the NORAD operations centers be destroyed.
Now, the Soviets did develop the Fractional Orbital Bombardment System, which would put a weapon in fractional orbit from an unanticipated direction, allowing delayed attack that was (by the standards of the day) virtually undetectable and unstoppable, but that was more as a surprise attack weapon, not counterforce. Both the US and USSR dabbled with space-based offensive weapons, but the vulnerability of such weapons and the difficulty with servicing them made them impractical.
The Tsar Bomba was huge for a bomb–roughly the size of a CONEX container–and weighed about 10 tonnes. You would have great difficulty smuggling this into an embassy via diplomatic pouch. 
To destroy D.C. or another major city or surface installation, you’d really want an airburst at ~5k feet or higher to maximize damage area, especially from the thermal pulse and air pressure wave. The only application for a massive ground-based device I can think of is excavation, like blasting a really big duck pond. Even for this, a number of synchronized smaller devices would be more effective. The Tsar Bomba was really intended to just one-up the competition.
Stranger
That’s what they WANT you to think. Have a friend who was once assigned chaplain duty at Cheyenne Mountain and was disappointed they never gave him access to the Stargate level.
You overestimate the Mountain. “Near-hit” is your operative clause; a direct hit by a much smaller warhead would still likely do it in, and no nation ever assigned one warhead to anything that deserves two (or more). Soviet accuracy was never as good as ours, but by the late Cold War period they had gotten better, and when you’re lobbing megatons vice kilotons, your CEP margin of error is much relaxed.
That’s part of the reason the place is in mothballs now; in anything less than a full scale nuclear war, other C2 facilities are cheaper and just as survivable, but any nuclear strike in 2009 is going to result in a bunch of Americans and Canadians buried under a mountain of pulverized radioactive rock.
The bigger, more frightening question is whether the earth and it’s atmosphere would survive a full blown series of strikes & counter-strikes. The original team of scientists were concerned even one nuclear event could burn off the atmosphere. Thankfully, that didn’t happen.
You can imagine the jolt if even 500 strikes hit here and another 500 in Russia within the same hour. Would we even have an atmosphere? Could if possibly shift earth’s rotation? Most people that were targeted wouldn’t be around to care. But countries that weren’t targeted would.
The answers to your questions are yes and no, respectively. The amount of energy that even a thousand 1 MT devices can deliver is an more than an order of magnitude smaller than the amount of energy absorbed by the Earth every hour. Now, the effect of aerosols and suspended particles from the thermal effects and resulting surface fires are another story and may have significant short and intermediate term impacts upon the climate, though the long term effects of “nuclear winter” are almost certainly overstated. The net impulse on the Earth would be absorbed by the flexible crust and the semi-liquid mantle without any measurable net influence on the Earth’s progression in its orbit.
Stranger
The Earth’s atmosphere has survived asteroid impacts in the thousands-of-megatons range. Nukes are powerful from a human standpoint but most people don’t really understand just how humongous the Earth is. For a standard of comparison, a really large multi-megaton nuke releases about as much energy as a hurricane or a mid-sized earthquake.
ETA: what SoaT said.
Interesting. Good to know the Swiss banks will still be around. I doubt they are targeted by either side.
I grew up at the height of the duck and cover campaign. They weren’t kidding when they said mutual destruction. There might be a few isolated small towns and nothing more in both our countries.
Was this column updated without a note? My notes suggest that the USSR wouldn’t break up for awhile after 1978.
The problem isn’t craters or melting, it’s the pressure blast. Hiroshima I have variously heard estimated between 7 and 20 kilotons, or about 1/10,000 of a 100-megaton bomb. It flattened most of the city for miles around. The bomb was set off at altitude to have that specific effect. The famous observatory at ground zero apparently survived because the copper-sheet dome roof vaporised before the pressure wave arrived, so the steel beams did not get a lot of pressure. The pressure wave hit from above not the side, so the walls stayed up.
What it didn’t do in, fires and a firestorn finished. One story was that if any decent sized city was nuked with anything over 10Mt, the flash would start brush fires and knock over houses for 50 miles around, and the emp pulse would knock out most electronics for 100 to 200 miles.
In the scenario of nuclear war, it’s always fun (??) to speculate what was planned. The best estimates were each side had closer to 20,000 nuclear weapons than 10,000. You’re Brezhnyev, and once you’ve hit the USA and NATO everywhere you can think of 2 or 3 times, take out any industrial capacity or storage facilities or airports or harbours, what else do you do?
Better take out anywhere that might give aid and comfort to the US effort - Israel, Australia, NZ, South Africa (in those days)… Argentina or Chile? Brazil? Anyone else who might get uppity about a defenceless Soviet Union? China especially? The Americans are trying to destroy your missiles while you try to take out them, so it’s shoot now or forever hold your peace. No second chance.
The odds of a small war - you hit one city, I hit two, it’s your turn to decide about retaliation… is pretty low. You certainly won’t blow the atmosphere off, but it would be pretty dusty, smokey and glow-in-the-dark for a while. The question would be whether anything would live through all the fall-out; I doubt humans would survive more than a few weeks afterward.
I think part of the problem is that in North America, barely touched by WWII, we didn’t really grasp the paranoia of a country overrun twice in the lifetime of its leaders. Regardless, I guess MAD actually worked in the end as a deterrent.
I’m impressed with the comments here, but I’d like to add a couple of points about Cheyenne Mountain. When I toured there as a cadet in 1971 they stated that their mission only required them to survive the first 45 minutes of a nuclear war. (Actually, it may have been 30 minutes or an hour. Memory fades)
During that same tour we were told that a nuclear war, although it would certainly devastate civilization, would not do so in exactly the way most people thought. In fact, the combined world nuclear stockpile in 1964 (after which total explosive force actually DECLINED as weapons became more accurate) was not sufficient to destroy an area the size of Texas with blast alone.
I also want to repeat a mention from one of the posts above: An attack on Chicago would NOT produce a significant crater because air bursts (in which the fireball never touches the ground) are much more effective in producing destruction than surface or sub-surface bursts. This is why there is no crater in Hiroshima or Nagasaki.
One book I found very interesting from a societal perspective was Warday by Whitley Streiber & James Kunetka. It describes the fictional account of two journalists traveling the US following a US/Soviet nuclear exchange. The overall picture is a nation in the grips of something like the Great Depression times 1.5, with a few cities gone or abandoned (not just a generic economic depression, but specifically very like the 1930’s in terms of available technology and agricultural production).
The limited nature of the fictional exchange strains credibility, and some of the technical details now seem almost endearingly naive & anachronistic in a 50’s sci-fi sort of way. But apart from that, the societal & cultural realism are IMO very well thought-out and present a very chilling read.
Yes, I read “War Day”. Quite a good book, and entirely plausible. One of the secondary effects of the war was loss of the crops in the mid-West because modern strains of wheat must be cultivated artificially. With the loss of the crops a dust bowl ensued. It was proposed to airdrop grass seed in radioactive areas, but there was not enough grass seed to do the job.
It was a popular book which disappeared almost completely after a couple of years.
Another good book was “Alas Babylon” (Pat Frank, IIRC?). It describes a small Florida community trying to keep things together as the nation pulls out of a nuclear war about 1963. Details like the diabetics slowly dying, some lowly congressman as the official inheritor of the presidency, gasoline running out, the usual marauders taking advantage of disorder.
Post-apocalyptic anarchy novels were common in 1960’s and 1970’s SF.
The problem wasn’t one big bomb flattening Texas, etc. The problem would probably be the sheer number of bombs, and aimed at what? Were they ALL aimed at missle silos and naval bases, or would it be necessary to destroy the nation’s (and the US’s allies’) entire industrial production capability? Can you leave standing the capacity to make airplane parts or even brass cartridges? The hydro dams? A well-placed cobalt bomb would guarantee that Texas and downwind would not be used for agriculture any century soon. Maybe in 40 or 50 years they’ll declassify all those war plans…
Where there’s industry, there’s people. And you have that next 30 or 45 minutes to decide whether to launch all 17,000 warheads or sit and vaporize quietly and peacefully without retaliation.