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