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Blowing wind exerts pressure on the objects that are in its way. The amount of pressure exerted by wind on an object depends on the wind's speed and density, and the object's shape. If you know these three variables, you can easily convert wind speed to pressure in pounds per square inch (psi). Before attempting this calculation, it is useful to know that the density of dry air at sea level is roughly 1.25kg per cubic meter and that every object has a drag coefficient (C) that can be estimated based on its shape.
The is a definite relationship between the overpressure and the dynamic pressure. The overpressure and dynamic pressure are equal at 70 psi, and the wind speed is 1.5 times the speed of sound. Below an overpressure of 70 psi, the dynamic pressure is less than the overpressure; above 70 psi it exceeds the overpressure. Since the relationship is fixed it is convenient to use the overpressure alone as a yardstick for measuring blast effects. At 20 psi overpressure the wind speed is still 500 mph, higher than any tornado wind.
As a general guide, city areas are completely destroyed (with massive loss of life) by overpressures of 5 psi, with heavy damage extending out at least to the 3 psi contour. The dynamic pressure is much less than the overpressure at blast intensities relevant for urban damage, although at 5 psi the wind speed is still 162 mph - close to the peak wind speeds of the most intense hurricanes.
Humans are actually quite resistant to the direct effect of overpressure. Pressures of over 40 psi are required before lethal effects are noted. This pressure resistance makes it possible for unprotected submarine crews to escape from emergency escape locks at depths as great as one hundred feet (the record for successful escape is actually an astonishing 600 feet, representing a pressure of 300 psi). Loss of eardrums can occur, but this is not a life threatening injury.
The danger from overpressure comes from the collapse of buildings that are generally not as resistant. The violent implosion of windows and walls creates a hail of deadly missiles, and the collapse of the structure above can crush or suffocate those caught inside.
These many different effects make it difficult to provide a simple rule of thumb for assessing the magnitude of harm produced by different blast intensities. A general guide is given below:
1 psi Window glass shatters
Light injuries from fragments occur.
3 psi Residential structures collapse.
Serious injuries are common, fatalities may occur.
5 psi Most buildings collapse.
Injuries are universal, fatalities are widespread.
10 psi Reinforced concrete buildings are severely damaged or demolished.
Most people are killed.
20 psi Heavily built concrete buildings are severely damaged or demolished.
Fatalities approach 100%.