"I'm going to go out to the site and test the nuclear device which will launch the rocket [because it might fail]. If you see a Hiroshima Mushroom you do not come to the site; instead, turn around and leave immediately."

A brief summary of what nuclear weapons do.

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Short-term (first month) Effects

For this discussion, we will detonate a 1 megaton nuclear weapon in Los Angeles. 500 South Buena Vista Street, Burbank, California (Lat 34.155744, Lon -118.326766note You can follow along here on Google Maps if you like note or use this handy-dandy simulator ) will be ground zero and the bomb will detonate on the ground.note What, you thought we'd let you get away with Hannah Montana? This is what is called a "ground burst" and is easier to estimate the damage and casualties than an air burst, but it should be noted that if nuclear weapons were to be used for military purposes (and the two times they were used) the explosions will all but certainly be air burst.note Low-yield ground bursts might be militarily useful if the objective is to limit the radius of destruction, target military infrastructure with minimal collateral damage to nearby population centres, or to do things like deeply and irreparably crater a site  but the enhanced fallout would be a high cost. But we want to give a reasonable explanation for this article, so we'll use the simpler explanation of a ground burst

The Flash

The first thing you get with a nuclear explosion is the light flash. This is very, very bright and if you are close enough (21 km (13 miles) on a clear day and 85 km (53 miles) at night for a 1 megaton nuke) will cause at least temporary flash blindness. Unless you're Sarah Jane Smith and probably for her too, this is not good at all. But heaven help you if you look up at the sky while the bomb goes off.

That's not the worst of the problems — there is also thermal radiation to consider. If you're within 11 km (7 miles) (Beverly Hills), you're going to get a bad sunburn on exposed skin. 9 km (6 miles), permanent scars. 8 km (5 miles), third degree burns. In and around ground zero itself, an area roughly a half-mile across, the temperatures will (for a few seconds) be hotter than the sun! If you're sunbathing on the beach, you're toast in both senses of the word.

Some reports claim that 50% of deaths at Hiroshima and Nagasaki were due to flash burns, though the methodology of separating these from the burns caused by the firestorms - which caused the bulk of the deaths - is dubious.

It should be noted that the thermal pulse will travel in a straight line. If you are standing behind a concrete wall at, say, 5 kilometers and somehow are unscratched by the shockwave (see below) then it may feel like you are in a oven for awhile but you will come out just fine. You will have a problem if you are standing behind something more combustible like, say, a heavy curtain since the object you are standing behind absorbs the heat from the pulse instead. The problem consists of the fact the object you were standing behind is now on fire with all that entails.

Overpressure

Then you'll get the blast wave. This is measured in terms of the sudden increase in pressure. 10 psi of overpressure equals your building getting hit by a 470 km/h (294 mph) wind.

This is the big problem. The following things will probably happen:

Within 2.45 km (1.53 miles), most of Burbank, including our target, the Mt. Sinai Memorial Cemetery and a couple of parks, will just be gone. Nothing is going to survive on the surface.

Out to 4.50 km (2.81 miles), any ordinary house is gone and reinforced structures are going to be severely damaged. This would include Universal Studios and parts of North Hollywood.

Beyond here, you'll get damage to everything out to Beverly Hills and Panorama City. Windows will shatter as far away as Santa Monica (~18 miles away).

The blast wave is affected to a limited extent by terrain and atmospheric conditions. Whether your house is on the side of a hill facing the explosion or not could mean the difference between it being torn off its foundation or merely damaged so much that it collapses a few seconds later. Detonating a bomb inside a valley will confine the blast to a greater degree but increase the destruction within the affected area because the blast would be deflected off the valley walls. Atmospheric conditions are even less predictable but also have even less consequential effects.

For your average human, it's not the overpressure that will kill you. It's being crushed by a collapsing building. Or being shredded by flying glass. Or bricks hitting your head. Or being thrown against a wall in a way that Goa'uld could only dream of doing with their hand device.

The overpressure will put out many of the fires started by the thermal pulse, however it will also create a lot of new fires from things like toppled lamps and broken gas mains. It's possible for a fire storm to ensue as fires merge together.

The Mushroom Cloud and Fallout

After the fireball disperses, you will see the mushroom cloud start to form from condensing vapor. This contains water, debris and general radioactive nastiness. It's not just a nuclear explosion thing: any large explosion will produce one, as well as volcanic eruptions or a meteorite impact. There's a 1937 description of an explosion in Shanghai that references a mushroom — 8 years before the first nuclear explosions. (All that's required to create a mushroom cloud is enough heat applied in a short enough time; nuclear weapons just happen to be especially good at this.) Furthermore, one account of the eruption of Mount Vesuvius in Italy in 79 AD described it as having the shape of a pine tree; pine trees in Italy have a similar shape to mushrooms ◊ .

In Robert A. Heinlein's story, The Moon Is a Harsh Mistress, the Lunar colonists fighting the earth's governments drop a large steel-encased rock - say the equivalent of something the size of a Greyhound bus - on a site in the middle of the desert. The impact, when it lands, generates a gigantic flash and huge mushroom cloud. Someone at a meeting of the rebel government asks Manny Garcia, the protagonist of the story, why the colony violated civilized behavior and used nuclear weapons. Manny explains that it wasn't nuclear at all; it was simply the force of impact of a multi-ton unbraked object dropped on earth at the speed of gravity, 32 feet per second per second. For the mushroom cloud, if you throw rocks at a pond you're going to get ripples; if you drop a boulder in the desert, you'll get a rise of dust. For the spark, it's the same thing when you strike a hammer on an anvil, you get a spark. A mushroom cloud is just the (enormously enlarged) equivalent of a dust cloud from dropping a rock on the ground. Just the biggest dust cloud ever created by mankind. You hit anything with a big enough impact - nuclear or non-nuclear - you will get a spark and a mushroom cloud.

This mushroom cloud disperses in a matter of hours. During that time, fallout starts raining down on the ground. Radioactive fallout consists of what's left of the bomb, stuff caught in the fireball that's been made radioactive by the bomb's intense neutron radiation, and a whole host of new and exciting isotopes created in the explosion itself. Most of it has half-lives short enough to disappear within hours, days or weeks. This is the idea behind fallout shelters - not to spend the rest of your life down there, but to wait in a shelter for a couple of days until the worst of the fallout has disappeared. However, stuff like strontium-90 (half-life of 29 years) or caesium-137 (30 years) have half-lives short enough to be really radioactive, but long enough to stick around and cause trouble for decades. It's worth noting people in the radiation biz generally use seven half lives as the rule of thumb when getting to 'zero' radiation, and that's not counting radioactive daughter products. This is why the (control) Vaults in Fallout were set to open after twenty years. (Radiation in Fallout works in ways that make nuclear physicists drink, but it's still worth noting.)

Let's explain the term 'half-life' in the context of nuclear physics, which you've probably heard before, but may not be sure of what it means.note Not to be confused with the iconic franchise of the same name. It's the amount of time it takes for half of something radioactive to disappear; or, more accurately, to decay into something else.note For some nuclides, that something else is harmless and non-radioactive, but for others, whatever it decays into is also radioactive. And possibly even worse. Say for example that you have a radioactive sample with a half-life of one month that would kill you after an hour's exposure (1,000 rems (10 Sv) within an hour is a guaranteed fatal dose of radiation). Then, after 30 days, only half of the sample would remain and it would now need two hours of exposure to kill you. After 60 days, four hours, after 90 days, eight hours, after 120 days, sixteen hours and so on. Now, if its half-life is ten years, then it's going to take an awfully long time before it's safe to be anywhere near it (unless in lead suits, that is). On the plus side, the longer the half-life, the less radioactive something is.

The initial radiation may well kill you, but at the distance it would, you're dead anyway from the other effects. The fallout stuff can cause hair loss, infertility, cataracts, tumors, heart failure and generally a nasty death, much earlier than planned. To get something of an idea, watch Threads.

When it comes to radiation, it depends on the size of the nuclear weapon. For smaller nukes, especially rather small tactical devices, immediate radiation accounts for much more of the damage they inflict than the (still very substantial) explosive effects, although delayed radiation from fallout is insignificant. For larger nukes, of course, the radiation pulse is absorbed by the atmosphere before it can reach anyone who wasn't killed by the explosion, but delayed radiation is another matter entirely. For some types of small nuclear weapons, especially neutron bombs, the immediate radiation is the main kill mechanism, because the energy distribution from the blast is designed to favor that; that's another story, though.

If you're unlucky enough to be targeted with a neutron bomb, the neutrons might make other material radioactive through a phenomenon called neutron activation. While this might happen with any nuclear bomb, it's only with enhanced-radiation bombs that it's likely to be a more significant problem than fallout and blast damage (for normal thermonukes, anything close enough to be neutron activated is likely to be blown apart or incinerated instead, so the small amount of matter that's made radioactive just contributes slightly to fallout).

The Doomsday Device

At one point, a design was on the drawing board for a nuclear device designed to produce extra fallout. It was called the cobalt bomb. The design called for the explosive core of the bomb to be surrounded by a tamper made of (non-radioactive) cobalt-59. When the bomb went off, the neutrons zipping out of the reaction would turn the cobalt-59 into radioactive cobalt-60, and then the blast would hurl these tiny fragments of cobalt-60 far and wide. It would have been a "dirty bomb" on steroids, and enough of them could have contaminated the entire surface of the Earth with radioactivity. Even its designers referred to it as a Doomsday Device.

One of the most iconic images from the Fallout video game series is a picture of Vault Boy holding his arm out and giving a thumbs up ◊ . He's not celebrating or approving of anything— he's testing to see if he's in the fallout radius. Convention has it that if you can hold your arm out at full length and completely hide the mushroom cloud on the horizon behind your thumb, then you're outside of the fallout zone. If you still see the cloud outside of your thumb, then you're not safe. Of course, this method of testing is depending on the idea that the size of the cloud is directly proportionate to the fallout it will create. Not a very reliable testing method in the event of an actual detonation.

Mr. EMP

There is also electromagnetic pulse. The mechanism is rather complicated, needless to say; when it comes to the effects, they can be felt with relatively low-yield and low-altitude bursts, although over a smaller area than if one were to, say, detonate a 25 Mt device 400 km over Kansas. That might knock everything that depended on electronics in orbit out of commission and destroy every unhardened electronic device in North America, actually. Your car would probably refuse to start, for instance, because the electronics it depends on to function would be fried. They found about this in the 1950s and 1960s, when they were conducting high-altitude nuclear tests. Another issue with high-altitude bursts is particle radiation becoming trapped in the Earth's magnetic field; this leads to temporary, although nasty, artificial radiation belts.

The EMP occurs when the intense flux of gamma radiation from a nuclear explosion produces an ionized region in the surrounding medium via a mechanism known as Compton scattering. The gamma rays strip electrons off of things, producing Compton electrons and positively-charged cations. The electrons are much lighter than the cations and same sign charges repel; the electrons travel to the outer parts of the deposition region while the cations stay in the central part. The outer parts are negatively charged; the inner parts are positively charged. Because this deposition region is never symmetrical or spherical (it could only be so under ideal conditions) there is a net vertical electron current (that is, there's a net flow of electrons. This is in the opposite direction as the conventional current, which is positive, and obviously vertically-oriented). This produces an intense pulse of broadband electromagnetic radiation, which is the EMP, which radiates outwards at the speed of light. Electromagnetic waves have notationally infinite range, but in practice are limited by the inverse-square law and atmospheric attenuation. Anyway, this electromagnetic radiation can be picked up by conductive objects in the same manner that an antenna picks up a signal, and once transmitted to electronics, damaging them. In fact, the EMP is so intense that it can lead to very strong currents, although for only a very short duration, in things that normally aren't very conductive. Close to the Earth, the ground, which conducts electricity well, allows the electrons an alternative return path to the central deposition region, which results in an intense magnetic field in the air and ground, but in those areas there's more to worry about from the actual explosion. Effects from the emitted EM radiation can be felt over a greater area.

The mechanism is a little different for high-altitude bursts. The deposition region ends up being in a large region of the upper atmosphere. Only the gamma rays that travel to what becomes this region have much of an effect; otherwise, there's not much to interact with. Anyway, across this very large area Compton electrons are produced. Phillip J. Dolan, in The Effects of Nuclear Weapons, wasn't very specific; he just said that "[the] electrons are deflected by the earth's magnetic field and are forced to undergo a turning motion about the field lines...[this causes] the electrons to be subjected to a radial acceleration which results, by a complex mechanism, in the generation of an EMP that moves down toward the Earth." The electric field is rather less intense, but for obvious reasons the areas affected are much larger. This way, electronics across entire regions may be damaged. You can check the relevant publication out here , while a detailed explanation by an expert in the field is here .

As implied above, you can harden electronic devices against EMP, with things like a Faraday cage. You can recognize a Faraday cage as being similar to the shield with holes on it that covers the glass window in the door of your microwave oven; if it wasn't there, the microwaves would pass through the glass and cook you.

Ground Burst vs. Air Burst

We've had our bomb go off on the ground. If it was detonated in the air, you'd get more damage due to fewer buildings being in the way. More importantly, far less fallout is generated as a smaller fraction of the fireball will intersect the ground. If none of it intersects the ground, it is termed an air burst. For our 1 megaton bomb, this requires a detonation height of at least 3,000 feet.note about 914.4 meters

Air bursts cause a phenomenon known as the Mach Effect. When you detonate a bomb in the air, it creates a spherical shock wave (the direct wave). When the wave hits the ground, it literally bounces off, creating a second shock wave that moves faster than the direct one. Chances are, this second wave will overtake the first and combine, producing a skirt around the base of the shock wave bubble where the two shock waves have combined. This skirt sweeps outward as an expanding circle along the ground with an amplified effect compared to the single shock wave produced by a ground burst.

There are also nuclear warheads designed for ground penetration, i.e. to destroy missile silos. It's been estimated that the USSR pointed a gigaton worth of nuclear weapons just at Cheyenne Mountain, deciding to destroy the entire mountain. Should the US and Russia engage in a nuclear war, Stargate Command is toast.

By comparison, the combined total yield of all nuclear testing to date (as of early June 2008) is 510 megatons, or 0.51 gigatons. The largest individual nuclear weapon ever, the RDS-220 or Tsar Bomba ("Emperor Bomb"), was designed with a yield of ~100 megatons but had this reduced due to fallout concerns to a yield of 50-58 megatons (sources differ) when tested in 1961 — that is, one quarter of the Krakatoa eruption. At this point, the total yield of all the warheads targeted on Cheyenne Mountain exceeds total US strategic megatonnage.

As regards to weapon yields, it is worth noting that:

The Tsar Bomba (RDS-220) was huge and unwieldy (27 tons, 8 m long, and so thick that the bomb bay doors of Tu-95 bomber had to be removed to fit it there) and so not a very practical weapon.

Multiple smaller blasts are vastly more efficient at spreading the damage of a huge area, and it is likely that where blast waves met enormous forces would result. Furthermore, widespread destruction is a little more likely to result in a firestorm. The trouble with that is avoiding having the bombs destroy each other. But hey, it's not like this is rocket science or anything.



Casualty Figures

Precisely how many people would die, be crippled, or be wounded would depend on a lot on circumstances, for example:

Time of day: If it's a weekday and people are at work, the area would have more people in it.

Weather: sunny day, more people outside, but the blast has it easier to dissipate. Cloudy day, fewer people outside, but the heat waves may get reflected by the clouds back towards the ground, resulting in greater damage. Windy day, fewer people outside, but wider fallout. Wind direction is important too.

Warning: a sudden strike would kill far more people than one after several days of conflict leading to a nuclear exchange. Whether air-raid sirens went off first (if they exist) or emergency broadcasts occurred first would be important too. A general note on warnings- for a strike launched from the USA, the USSR or PRC would get about 20 minutes warning before the first explosions - something that contributed heavily to Soviet and Chinese paranoia in the '60s in general and the Cuban Missile Crisis in particular, given that their own nuclear missiles of the time took several hours to be deployed and fueled. From the Soviet Union the People's Republic of China would have gotten about five minutes' warning (and vice versa), and from the USSR The UK would have gotten four - just enough time to make a nice cup/pot of tea. But not to drink it.

The availability of medical personnel and assistance, although they would be overwhelmed in even the smallest exchange. A 1978 study calculated that treating the wounded in a single-weapon attack upon the USA's city of Detroit would exceed the total medical resources of North America - the study anticipating several hundred thousand victims with severe burns versus an intensive-burn-unit capacity of less than 3000 people in North America. Treating the wounded would have been further complicated by the destruction of most, if not all, of Detroit's medical infrastructure in the attack.

Estimates for total dead and wounded depend very much on 1) which country was attacked and 2) with what. The latter would depend on who launched the 'first strike' in a nuclear war, with a 1978 US study calculating in a full nuclear exchange she would lose some 70-90% of her population if the USSR struck the USA first - versus only 50-70% if the USA attacked first. The same study thought the USSR would also only lose 60-80% in a full exchange, even if the USA struck first, due to the greater dispersal of her population and much better measures for protecting her population from nuclear strikes. Below a full exchange the report was inconclusive, stating that deaths would be dependent on the numbers of weapons used and against what. At the very lowest end the report posited several million dead for a US strike upon St Petersburg/Leningrad, or a couple of million dead in a Soviet strike on Detroit. The UK's 1955 Strath Report thought that only 1/3 of the population would die if only ten weapons were used upon the UK, but in hindsight this figure was extremely optimistic with later estimates putting dead at close to 100% - 'the survival of the British nation' was not a realistic possibility in the event of even the most limited nuclear exchange, nor would it be for similarly densely-populated regions such as Japan.

It is to be noted that the distinction between a 'counter-value' (attacks on industry, infrastructure) and 'counter-force' (attacks on military targets) nuclear strike can be an academic one in some countries. For example, the UK is so small that almost every population centre in the country is within the blast radius, CEP (Circular Error probability, the amount to which the weapon is likely to be off target), firestorm-radius, or fallout-zone of its military targets. The Soviet Union, by contrast, kept their nuclear weapons and facilities well away from their cities due to both security and environmental considerations (many of their bases were extremely cold). A lot of this was because the USSR was several hundred times larger than the UK and its major population centers were even further apart than the USA's (even though it was more than twice as large as the USA by land area, the Soviet Union's population was only slightly smaller and was mostly concentrated in European Russia and the Ukraine).