As already stated, a characteristic feature of the atomic bomb, which is
quite foreign to ordinary explosives, is that a very appreciable fraction
of the energy liberated goes into radiant heat and light. For a
sufficiently large explosion, the flash burn produced by this radiated
energy will become the dominant cause of damage, since the area of burn
damage will increase in proportion to the energy released, whereas the area
of blast damage increases only with the two-thirds power of the energy.
Although such a reversal of the mechanism of damage was not achieved in the
Hiroshima and Nagasaki bombs, the effects of the flash were, however, very
evident, and many casualties resulted from flash burns. A discussion of
the casualties caused by flash burns will be given later; in this section
will be described the other flash effects which were observed in the two
cities.

The duration of the heat radiation from the bomb is so short, just a few
thousandths of a second, that there is no time for the energy falling on a
surface to be dissipated by thermal defusion; the flash burn is typically a
surface effect. In other words the surface of either a person or an object
exposed to the flash is raised to a very high temperature while immediately
beneath the surface very little rise in temperature occurs.

The flash burning of the surface of objects, particularly wooden objects,
occurred in Hiroshima up to a radius of 9,500 feet from X; at Nagasaki
burns were visible up to 11,000 feet from X. The charring and blackening
of all telephone poles, trees and wooden posts in the areas not destroyed
by the general fire occurred only on the side facing the center of
explosion and did not go around the corners of buildings or hills. The
exact position of the explosion was in fact accurately determined by taking
a number of sights from various objects which had been flash burned on one