Definition: (DOD, NATO) An air pressure wave which moves ahead of the main blast wave for some distance as a result of a nuclear explosion of appropriate yield and low burst height over a heat-absorbing (or dusty) surface. The pressure at the precursor front increases more gradually than in a true (or idea,) shock wave, so that the behavior in the precursor region is said to be nonideal.
precursor front
From Department of Defense, for About.com
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