How hot is a nuclear explosion?
The explosion itself is not the source of the heat emitted from
a nuclear detonation. Instead the heat is the source of the
explosion. Compare this with thunder following a lightning strike.
In the nuclear bomb temperatures of about 20 million degrees
fahrenheit are produced causing the emission of x-rays which
ionizes the air preventing any more light or IR emission until the
bomb cools enough that it no longer is emitting x-rays. In
lightning the temperature of the ionized conducting air channel is
only about 90000 degrees fahrenheit.
As to the temperature of things around the fireball from a
nuclear detonation, directly beneath the fireball temperatures can
reach about 7000 degrees fahrenheit. To give an example by
which to compare this heat, free flowing magma, or melted rock,
averages a high of 4000 degrees fahrenheit.