A fraction of a second after a nuclear explosion, the heat from the fireball causes a high-pressure wave to develop and move outward producing the blast effect. The front of the blast wave, i.e., the shock front, travels rapidly away from the fireball, a moving wall of highly compressed air.
New York City Example
A 150 kiloton bomb constructed by terrorists is detonated in the heart of Manhattan, at the foot of the Empire State Building. The bomb goes off without warning at noon time. It's a clear spring day with a breeze to the east.
- There is no warning. The population has not been evacuated nor sought shelter. Both measures could reduce casualties.
- There is clear weather, with visibility of 9 miles (16 km).
- This is an isolated attack, leaving the rest of the country free to respond.
- A large percentage of the day time population is outside - 25%.
- The daytime population density is roughly uniform and about 125,000 per square mile.
- The shock wave will spread out uniformly in all directions, being minimally affected by structures.
Blast Effects on Humans
Blast damage is caused by the arrival of the shock wave created by the nuclear explosion. Humans are actually quite resistant to the direct effect of overpressure. Pressures of over 40 psi are required before lethal effects are noted.