d on the ground is
negligible. A large number of beta rays are also emitted during this
time, but they are unimportant because their range is not very great,
only a few feet. The range of alpha particles from the unused active
material and fissionable material of the bomb is even smaller.

Apart from the gamma radiation ordinary light is emitted, some of which
is visible and some of which is the ultra violet rays mainly
responsible for flash burns. The emission of light starts a few
milliseconds after the nuclear explosion when the energy from the
explosion reaches the air surrounding the bomb. The observer sees then
a ball of fire which rapidly grows in size. During most of the early
time, the ball of fire extends as far as the wave of high pressure. As
the ball of fire grows its temperature and brightness decrease.
Several milliseconds after the initiation of the explosion, the
brightness of the ball of fire goes through a minimum, then it gets
somewhat brighter and remains at the order of a few times the
brightness of the sun for a period of 10 to 15 seconds for an observer
at six miles distance. Most of the radiation is given off after this
point of maximum brightness. Also after this maximum, the pressure
waves run ahead of the ball of fire.

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.