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This is interesting and counterintuitive — because of how the energy proportions itself (e.g. visible vs. infrared), kiloton-range nuclear weapons are brighter than megaton-range weapons, apparently.pic.twitter.com/AgRnRpBY8Y
Or rather, "energy is proportioned," to be a bit more grammatical. (Monday mornings, am I right?) Same document says that 45% of the fireball energy in kiloton-range shot was in visible spectrum, while it is only 25% in megaton-range shots.
I have looked for a graph or equation that would give peak brightness as a function of yield, but not found anything obvious. Which is itself kind of interesting.
I did find this interesting article from Nature, 1962, on the brightness of nuclear weapons — has some nice illustrations:pic.twitter.com/3E9BlUfBzV
It has this graph which seems wrong to me — it assumes that the amount of visible energy is directly proportional to the yield, but that's not right. Article ends with a nice bit about whether aliens in nearby star systems could see nukes going off on Earth (probably not).pic.twitter.com/vnZUAramyr
Poked around a bit and found these interesting diagrams (which are related to flash blindedness) which seem to indicate that indeed, lower-yield nuclear weapons are indeed brighter than higher-yield ones. Again, kind of an interesting and unintuitive fact.pic.twitter.com/EOBQ5n8xtL
Wonder if this effect is due to sorround atmosphere? Would same effect occur in space. Know the double flash is dependent on atmosphere. Could this be too?
Glasstone says that brightness is greatly increased in high-altitude detonations. Which makes sense — less energy being deposited into atmosphere means reduction in blast and thermal. Would imagine outer space even brighter for same reason.pic.twitter.com/dZsB97pPcH
Already answered my question, this did!
Oh no, you've opened the Pandora's Box that is different senses of "brightness" - luminance vs illuminance vs radiance vs irradiance vs radiosity, and flux and spectral counterparts.... Replies here are already impossible to parse.
Also answers a question I didn’t know I had until i read it: how did they analyze the chemistry/physics of the particles during the first seconds after ignition? Watching spectral lines of course! Do you know how they were recorded? Had to be similarly high fps as the videos 
Looking at MegaElectron Volt spectral lines - that's why they set up a 9000 foot causeway for Ivy Mike https://nuclearweaponarchive.org/Usa/Tests/Imkshotsetup.jpg …
And, not in the first seconds, but the first microseconds. (Light travels 1 km in 3 microseconds. The fireball maybe goes a few meters in the same time.)
wouldn't the rate of acceleration of doublings determine the brightness?
Looking at it from the point of view of a blackbody, wouldn't a high-yield weapon produce, rather than infrared, more ultraviolet than visible light? Same result, a greater comparative brightness of low-yield weapons. Or maybe it's b/c inverse square law wrt fireball surface...
This is misleading. I think this is about the immediate (prompt) radiation. When the extremely hot fireball escapes, it can be seen briefly until the X rays ionize the air around it. Then, it expands as a shock wave, which heats up a larger volume of air. Thus the double flash.
Always consider the effects of absorption and emission.
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