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.