How does thermal radiation from a nuclear explosion cause burns and fires?

Approximately 35 percent of the total energy yield from a nuclear airburst is released as thermal radiation, which consists of an intense, near-instantaneous emission of infrared, visible, and ultraviolet light from the fireball.12

Because this energy travels at the speed of light, it reaches distant regions almost immediately. When this radiation strikes the human body, the absorbed energy can cause severe skin and eye injuries, often referred to as flash burns.231

Regarding fires, the thermal pulse is capable of igniting combustible materials—such as wood, plastics, and petroleum fuels—at significant distances from the detonation point. These individual ignitions can spread and coalesce into massive, uncontrolled fires known as firestorms. In a firestorm, the intense heat generates a convective column of rising hot gases that draws in fresh air from the periphery, creating powerful inward-rushing winds that consume virtually all combustible material in the area. While the initial thermal pulse starts these fires, the subsequent shock wave can further spread them.14