The ball of fire rapidly expands from the size of the bomb
to a radius of several hundred feet at one second after the
explosion. After this the most striking feature is the rise of
the ball of fire at the rate of about 30 yards per second.
Meanwhile it also continues to expand by mixing with the
cooler air surrounding it. At the end of the first minute the
ball has expanded to a radius of several hundred yards and
risen to a height of about one mile. The shock wave has by
now reached a radius of 15 miles and its pressure dropped
to less than 1/10 of a pound per square inch. The ball now
loses its brilliance and appears as a great cloud of smoke:
the pulverized material of the bomb. This cloud continues
to rise vertically and finally mushrooms out at an altitude
of about 25,000 feet depending upon meteorological conditions.
The cloud reaches a maximum height of between
50,000 and 70,000 feet in a time of over 30 minutes.
It is of interest to note that Dr. Hans Bethe, then a member
of the Manhattan Engineer District on loan from Cornell
University, predicted the existence and characteristics of
this ball of fire months before the first test was carried out.
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.
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