To summarize, radiation comes in two bursts--an extremely intense one
lasting only about 3 milliseconds and a less intense one of much longer
duration lasting several seconds. The second burst contains by far the
larger fraction of the total light energy, more than 90%. But the
first flash is especially large in ultra-violet radiation which is
biologically more effective. Moreover, because the heat in this flash
comes in such a short time, there is no time for any cooling to take
place, and the temperature of a person's skin can be raised 50 degrees
centigrade by the flash of visible and ultra-violet rays in the first
millisecond at a distance of 4,000 yards. People may be injured by
flash burns at even larger distances. Gamma radiation danger does not
extend nearly so far and neutron radiation danger is still more limited.

The high skin temperatures result from the first flash of high
intensity radiation and are probably as significant for injuries as the
total dosages which come mainly from the second more sustained burst of
radiation. The combination of skin temperature increase plus large
ultra-violet flux inside 4,000 yards is injurious in all cases to
exposed personnel. Beyond this point there may be cases of injury,
depending upon the individual sensitivity. The infra-red dosage is
probably less important because of its smaller intensity.

CHARACTERISTICS OF THE DAMAGE CAUSED BY THE ATOMIC BOMBS

The damage to man-made structures caused by the bombs was due to two
distinct causes: first the blast, or pressure wave, emanating from the
center of the explosion, and, second, the fires which were caused
either by the heat of the explosion itself or by the collapse of
buildings containing stoves, electrical fixtures, or any other
equipment which might produce what is known as a secondary fire, and
subsequent spread of these fires.

The blast produced by the atomic bomb has already been stated to be
approximately equivalent to that of 20,000 tons of T.N.T. Given this
figure, one may calculate the expected peak pressures in the air, at
various distances from the center of the explosion, which occurred
following detonation of the bomb. The peak pressures which were
calculated before the bombs were dropped agreed very closely with those
which were actually experienced in the cities during the attack as
computed by Allied experts in a number of ingenious ways after the
occupation of Japan.

The blast of