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. This reflected blast wave can merge with the incident shock wave. This merging phenomenon is called the "Mach effect". The overpressure at the front of the Mach wave is generally about twice as great as that at the direct blast wave front. This footage is from a conventional explosion.