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