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 pressure from the atomic bombs differed from that of ordinary
high explosive bombs in three main ways:

A. Downward thrust. Because the explosions were well up in the air, much
of the damage resulted from a downward pressure. This pressure of course
most largely effected flat roofs. Some telegraph and other poles
immediately below the explosion remained upright while those at greater
distances from the center of damage, being more largely exposed to a
horizontal thrust from the blast pressure waves, were overturned or tilted.
Trees underneath the explosion remained upright but had their branches
broken downward.

B. Mass distortion of buildings. An ordinary bomb can damage only a part
of a large building, which may then collapse further under the action of
gravity. But the blast wave from an atomic bomb is so large that it can
engulf whole buildings, no matter how great their size, pushing them over
as though a giant hand had given them a shove.

C. Long duration of the positive pressure pulse and consequent small
effect of the negative pressure, or suction, phase. In any explosion, the
positive pressure exerted by the blast lasts for a definite period of time
(usually a small fraction of a second) and is then followed by a somewhat
longer period of negative pressure, or suction. The negative pressure is
always much weaker than the positive, but in ordinary explosions the short
duration of the positive pulse results in many structures not having time
to fail in that phase, while they are able to fail under the more extended,
though weaker, negative pressure. But the duration of the positive pulse
is approximately proportional to the 1/3 power of the size of the explosive
charge. Thus, if the relation held true throughout the range in question,
a 10-ton T.N.T. explosion would have a positive pulse only about 1/14th as
long as that of a 20,000-ton explosion. Consequently, the atomic
explosions had positive pulses so much longer then those of ordinary
explosives that nearly all failures probably occurred during this phase,
and very little damage could be attributed to the suction which followed.

One other interesting feature was the combination of flash ignition and
comparative slow pressure wave. Some objects, such as thin, dry wooden
slats, were ignited by the radiated flash heat, and then their fires were
blown out some time later (depending on their distance from X) by the
pressure blast which followed the flash radiation.