Overpressure

Blast effects are usually measured by the amount of overpressure, the pressure in excess of the normal atmospheric value, in pounds per square inch (psi).

After 10 seconds, when the fireball of a 1-megaton nuclear weapon has attained its maximum size (5,700 feet across), the shock front is some 3 miles farther ahead. At 50 seconds after the explosion, when the fireball is no longer visible, the blast wave has traveled about 12 miles. It is then traveling at about 784 miles per hour, which is slightly faster than the speed of sound at sea level.

Peak overpressure	Maximum Wind Speed
50 psi	934 mph
20 psi	502 mph
10 psi	294 mph
5 psi	163 mph
2 psi	70 mph

As a general guide, city areas are completely destroyed by overpressures of 5 psi, with heavy damage extending out at least to the 3 psi contour.

These many different effects make it difficult to provide a simple rule of thumb for assessing the magnitude of injury produced by different blast intensities. A general guide is given below:

Overpressure	Physical Effects
20 psi	Heavily built concrete buildings are severely damaged or demolished.
10 psi	Reinforced concrete buildings are severely damaged or demolished. Most people are killed.
5 psi	Most buildings collapse. Injuries are universal, fatalities are widespread.
3 psi	Residential structures collapse. Serious injuries are common, fatalities may occur.
1 psi	Window glass shatters Light injuries from fragments occur.

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