FLASH BURN

FLASH BURN
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 side only.
To illustrate the effects of the flash burn, the following
describes a number of examples found by an observer moving
northward from the center of explosion in Nagasaki.
First occurred a row of fence posts at the north edge of the
prison hill, at 0.3 miles from X. The top and upper part of
these posts were heavily charred. The charring on the front
of the posts was sharply limited by the shadow of a wall.
This wall had however been completely demolished by the
blast, which of course arrived some time after the flash. At
the north edge of the Torpedo works, 1.05 miles from X,
telephone poles were charred to a depth of about 0.5 millimeters.
A light piece of wood similar to the flat side of an
orange crate, was found leaning against one of the telephone
poles. Its front surface was charred the same way as
the pole, but it was evident that it had actually been ignited.
The wood was blackened through a couple of cracks
and nail holes, and around the edges onto the back surface.
It seemed likely that this piece of wood had flamed
up under the flash for a few seconds before the flame was
blown out by the wind of the blast. Farther out, between
1.05 and 1.5 miles from the explosion, were many trees
and poles showing a blackening. Some of the poles had
platforms near the top. The shadows cast by the platforms
were clearly visible and showed that the bomb had detonated
at a considerable height. The row of poles turned
north and crossed the mountain ridge; the flash burn was
plainly visible all the way to the top of the ridge, the farthest
burn observed being at 2.0 miles from X.
Another striking effect of the flash burn was the autumnal
appearance of the bowl formed by the hills on three
sides of the explosion point. The ridges are about 1.5 miles
from X. Throughout this bowl the foliage turned yellow,
although on the far side of the ridges the countryside was
quite green. This autumnal appearance of the trees extended
to about 8,000 feet from X.
However, shrubs and small plants quite near the center
of explosion in Hiroshima, although stripped of leaves, had

obviously not been killed. Many were throwing out new
buds when observers visited the city.
There are two other remarkable effects of the heat radiated
from the bomb explosion. The first of these is the
manner in which heat roughened the surface of polished
granite, which retained its polish only where it was shielded
from the radiated heat travelling in straight lines from the
explosion. This roughening by radiated heat caused by the
unequal expansion of the constituent crystals of the stone;
for granite crystals the melting temperature is about 600
deg centigrade. Therefore the depth of roughening and ultimate
flaking of the granite surface indicated the depth to
which this temperature occurred and helped to determine
the average ground temperatures in the instant following
the explosion. This effect was noted for distances about 1
1/2 times as great in Nagasaki as in Hiroshima.
The second remarkable effect was the bubbling of roof
tile. The size of the bubbles and their extent was proportional
to their nearness to the center of explosion and also
depended on how squarely the tile itself was faced toward
the explosion. The distance ratio of this effect between
Nagasaki and Hiroshima was about the same as for the
flaking of polished granite.
Various other effects of the radiated heat were noted,
including the lightening of asphalt road surfaces in spots
which had not been protected from the radiated heat by
any object such as that of a person walking along the road.
Various other surfaces were discolored in different ways
by the radiated heat.
As has already been mentioned the fact that radiant heat
traveled only in straight lines from the center of explosion
enabled observers to determine the direction toward the
center of explosion from a number of different points, by
observing the “shadows” which were cast by intervening
objects where they shielded the otherwise exposed surface
of some object. Thus the center of explosion was located
with considerable accuracy. In a number of cases these
“shadows” also gave an indication of the height of burst of
the bomb and occasionally a distinct penumbra was found
which enabled observers to calculate the diameter of the
ball of fire at the instant it was exerting the maximum charring
or burning effect.
One more interesting feature connected with heat radiation
was the charring of fabric to different degrees depending
upon the color of the fabric. A number of instances
were recorded in which persons wearing clothing of various
colors received burns greatly varying in degree, the
degree of burn depending upon the color of the fabric over
the skin in question. For example a shirt of alternate light
and dark gray stripes, each about 1/8 of an inch wide, had
the dark stripes completely burned out but the light stripes
were undamaged; and a piece of Japanese paper exposed
nearly 1 1/2 miles from X had the characters which were
written in black ink neatly burned out.