underpressure

From under- + pressure.

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underpressure (plural underpressures)

1. insufficient pressure; a marked drop in pressure
   • 2007, Vladimir A. Rakov, Martin A. Uman, Lightning: Physics and Effects, page 378:

     The expanding-hot-channel mechanism predicts an initial positive pressure change (an overpressure) while the electrostatic-pressure-relief mechanism predicts an initial negative pressure change (an underpressure).

• overpressure