After I posted a short video of a simulated 15 kiloton airburst, a viewer commented that they had checked another calculator and got different numbers. That is a fair point, and the answer is not that one tool is broken. Blast simulators make different modelling choices, and those choices move the rings a lot.
I build Kaboomly, a free nuke map that runs in the browser, so here are the four choices that matter most.
1. Airburst or surface burst
The same weapon behaves very differently depending on where it goes off. An airburst spreads the blast wave over a wider area, so the light-damage rings get bigger. A surface burst gives smaller blast rings but pulls soil into the fireball and creates local fallout. Some tools default to one, some to the other.
2. Burst height
Even among airbursts, height matters. There is a height that maximizes the area of a given overpressure, and it is different for every pressure level. Kaboomly does not optimize this. It takes one documented historical example, Hiroshima at about 1,670 ft for roughly 12.5 kt, and scales it with the cube root of the yield. A tool that picks a different height will show different rings for the same yield.
3. Which threshold a ring represents
A ring labelled "heavy damage" might be drawn at 5 psi in one tool and at 10 or 20 psi in another. Thermal rings can mean first, second or third-degree burns, and the distance changes a lot between them. Always check the legend before comparing two maps.
4. Scaling law and source data
Most public calculators, including NUKEMAP and Kaboomly, trace back to The Effects of Nuclear Weapons by Glasstone and Dolan (1977). Blast distances scale with the cube root of yield, so ten times the yield gives only about 2.15 times the distance. The fireball has its own scaling, about W^0.4. Tools that use rounded rules of thumb instead of the published curves will drift, especially at very small or very large yields.
An example
For a 15 kt airburst Kaboomly shows the 5 psi ring at about 1.6 km (1.0 mi) and the 1 psi ring at about 4.5 km (2.8 mi). If another tool uses a surface burst or a different height, its numbers for the same yield can be noticeably smaller or larger, and both can be correct for their own assumptions.
What I did about it
On Kaboomly every result shows the burst type and the thresholds, and there is a page explaining how the numbers are calculated with references. The site never shows casualty estimates, only the rings and the number of residents living inside an area.
If you compare simulators, compare like with like: same yield, same burst type, same threshold. The differences mostly disappear.
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