Static overpressure is one of the first atmospheric effects that any point outside the fireball radius experiences when near an atomic or thermonuclear explosion. When the detonation occurs, heating of the surrounding air by X-rays and other EM radiation causes a rapid expansion. This results in a blast wave (or shock wave, if you prefer) propagating outward from the detonation at high speed. At the forefront of this blast wave, and most notably in the mach front, there is a moving shell within which the atmospheric pressure is many times ambient,, as fast-moving air is 'compressed against' the unmoving atmosphere.

When a structure, for example, is subjected to this, it tends to implode rapidly, as the external air pressure will climb nearly instantaneously to multiple atmospheres of pressure. Very few structures are built or designed to withstand this sort of treatment, and they will typically begin to crush inward. This does not last long, however, for the static overpressure of the blast wave is soon replaced by the press of dynamic overpressure which will induce the structure to deform horizontally (blow over). The damage done by the static overpressure worsens the effects of the dynamic overpressure as the weakened supporting members of the building are unable to resist the 'push' of air leaving the blast area.

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