The Mach Stem
If the explosion occurs above the ground, when the expanding blast wave strikes the surface of the earth, it is reflected off the ground to form a second shock wave traveling behind the first. This reflected wave travels faster than the first, or incident, shock wave since it is traveling through air already moving at high speed due to the passage of the incident wave. The reflected blast wave merges with the incident shock wave to form a single wave, known as the Mach Stem. The overpressure at the front of the Mach wave is generally about twice as great as that at the direct blast wave front.
At first the height of the Mach Stem wave is small, but as the wave front continues to move outward, the height increases steadily. At the same time, however, the overpressure, like that in the incident wave, decreases because of the continuous loss of energy and the ever-increasing area of the advancing front. After about 40 seconds, when the Mach front from a 1-megaton nuclear weapon is 10 miles from ground zero, the overpressure will have decreased to roughly 1 psi.