Oh eat a dick as if there were a scenario where I see a mushroom cloud and DON’T run away
The idea isn’t completely accurate. There are cases where a cloud bigger than your finger is not an immediate threat- you’re upwind from it, for example, or it’s just an old Mushroom Cloud that has bloomed to 20,000 feet tall and even from a safe distance it might still be bigger than your thumb- And there are cases where a smaller-than-thumb mushroom cloud might be dangerous to you (it’s very new and hasn’t grown to full size yet, or you’re downwind of it).
Radiation should not be your first concern with a nuclear blast, though. If you’re close enough for the actual blast itself to lethally irradiate you, you’re close enough for the shockwave and heat to kill you too, and those things kill you MUCH faster than radiation does. Radiation is very slow and inefficient at killing, you see, with the lethal dose required to kill a human still taking up to 6 months to actually kill you. The amount required to kill you instantly is so much greater than the amount required to ensure death that very few nuclear weapons can actually kill anyone due to non-thermal radiation. The three things you need to be worried about are the flash, the shockwave, and the thermal pulse. If you’re killed instantly, they won’t even list radiation as a contributing factor on the cause of death because it would be superfluous.
Why do you need to worry about those things, but not the radiation? That’s simple: The atmosphere is opaque over long distances to short-wave radiation but transparent to longer wave radiation. It’s why we can see the stars and use radio waves but don’t get fried by all the gamma radiation and X-ray radiation the Sun puts off. (Well, that and the magnetosphere, but the magnetosphere isn’t going to help in this case).
Visible light and thermal energy are both much longer wave than the gamma radiation and x-rays, so they will travel further in amounts that can hurt you. You’re looking at deep tissue sunburns, blindness if you were looking in the direction of the flash when it occured, and deep tissue regular burns from the superheated air. Then there’s the shockwave. It’s a little slower, moving only in machs instead of speed-of-light, but it will blow out your eardrums, blow you into a wall, blow you through a wall, blow walls into you, throw shrapnel…you get the idea. Shockwave = bad.
So. Your first and immediate concern with nuclear weapons is the bright flash, the shockwave, and the thermal pulse. Lets say you survive those, by being in a sufficiently sturdy structure or far enough away they don’t kill you and having the good fortune of not looking in the direction of the blast when it occured. Congratulations! Now do you worry about radiation? No. You have a firestorm to escape from. Fire is much faster and much more efficient at killing you than radiation is, and it’s a much more present threat considering a nuclear bomb can flash-ignite things miles away (depending on the size of the bomb). Even if you survived by ducking into a sufficiently sturdy structure, that structure may now be on fire.
If the building WAS flash-ignited by the nuclear blast, you have to escape from the burning building while making sure you grab a wet towel or something to breath through so you don’t burn your lungs when you get outside. Congratulations, you’re probably also irradiated. If you escape the burning area, though, there is a pretty good chance you might live- you weren’t close enough to be killed instantly, after all.
If the building was NOT flash-ignited, congratulations! All you have to do is get the hell out of the area downwind of the blast within the next 30 minutes before the dangerous stuff gets to you. If it starts raining black inky rain, you do three things:
1. Do not drink the black rain. It doesn’t matter if your throat is burned and you need cooling liquid relief. It doesn’t matter if you think the rain tastes like licorice or chocolate. It doesn’t matter if you’re a turkey and instinctively look up whenever it starts raining. Do not drink the rain.
2. Get under some sort of cover as quickly as possible. Minimize your exposure to the black rain. Cover up with as many thick layers of clothing as possible- Gamma radiation will go through clothing, but alpha and the weaker levels of beta particle radiation will not. Every bit of covering helps.
3. KEEP MOVING. The longer you stay in that area where the black rain is occuring, the higher your radiation dosage is, because the black rain is the most potent and immediate form of nuclear fallout.
If you survived all this, your next concern should be getting to a hospital somewhere that hasn’t been nuked and/or taking lots of Prussian Blue. Congratulations, you’ve survived a nuclear blast! Your next biggest challenge is surviving dehydration and whatever disease outbreak occurs now that the infrastructure in that area has been blown to hell. Worst case scenario, cholera outbreak will kill more people than the nukes themselves did.
*The above information may be missing a few key pieces of information or may be partially inaccurate. The above information was gleaned from several documentaries about how people survived Hiroshima, which was a woefully underpowered bomb and does not represent the full power of a modern nuclear device. The above information will not work on lawn gnomes.*