Black smoke on buildings before nuclear blast?

I noticed while watching some old United States bomb test footage - - that black smoke seemed to linger around the buildings/houses a few seconds before being blasted to smitherines… What is that? Why does it occur?

The smoke is from the paint and wood burning due to the light flash, it’s blasted when the shock wave finally comes by.

Its due to the heat flash, not light, which precedes the blast wave by a few seconds.

More correct to say the heat gets there before the pressure wave does. They both start at the same time. Infra-red travels at the speed of light while a pressure wave loafs along at the considerably slower speed of sound.

Could you please specify what this “heat” is made from if it is not light and can travel faster than the blast wave?

Sure. The heat is made up of Electromagnetic Radiation of a wavelength longer than the longest wavelength of red on the Visible Light band of the EM spectrum.

How hot is it? Would it disintegrate flesh?

I think there are some good examples of what it can do to flesh from Hiroshima and Nagasaki survivors.

Why do I get worried about lieu asking about nuclear detonations and disintegrating flesh?

For something really spooky you can find pictures from the Hiroshima and/or Nagasaki blasts where there are ‘shadows’ of people on walls. I say ‘shadows’ because the shadow is reversed out…a lighter spot on a dark background. This happened when the heat flash started to burn the building in question but someone was standing in front of it so as to absorb the heat themselves (and thus protecting part fo the wall whihc was in theri shadow) leaving a permanent shadow on the wall next to them.

The HEAT is NOT made up of Electromagnetic Radiation of a wavelength longer than the longest wavelength of red.

Energy from the blast that reaches the houses first is made up of Electromagnetic Radiation. Heat, is the energy characterized by the rapid vibration of electrons and molecules in the material being burned by the Electromagnetic Radiation.

This radiation by the way, consists of broadband radiation that is both visible and not visible. I see nothing wrong with calling light. Though not a complete statement of facts, it got the point across to the OP.