If a large enough amount of energy is dumped in a small enough volume of Earth's lower atmosphere, events follow a standard pattern:
- The air within the volume is fully ionized and heated to extremely high temperatures;
- the resulting blackbody radiation mostly passes through the plasma, until it encounters non-ionized air at the boundary of the volume;
- this air ionizes in turn: the fireball grows rapidly, until the boundary temperature drops to several thousand degrees, at which point much of its blackbody radiation can propagate over longer distances without being absorbed by the intervening air.
- Meanwhile, the fast-moving particles in the plasma form a shockwave: the lower the initial energy, the higher the proportion that will be deposited in this blast rather than as radiation.
Clearly at some point this model breaks down. For example, conventional (non-nuclear) explosions do not concentrate energy in a small enough volume to produce a ball of plasma, even if the total energy of the explosion is very high. There is speculation around the possibility of pure fusion nuclear weapons, which could overcome the lower yield limit caused by needing a fissile primary stage; but even if these could be made arbitrarily small, a low enough hypothetical yield would simply be insufficient to ionize any air or even form a shockwave.
My question then is: how would the above behaviour change as the initial volume increased, or as the energy load decreased? I'm imagining a region in "energy-volume space" in which the standard description is approximately valid. Does it have a sharp edge, or a gradual transition to other behaviour? What do various "locations" on the transition look like?