The vapour cone effect is caused by pressure differences around the aircraft. A sudden drop in pressure associated with high speed allows water vapour, which is normally invisible, to condense as fog.
蒸汽锥效应是由于战斗机周围的压力差所导致的，高速飞行时压力的陡然降低会使不可见的水蒸气凝结成雾气。
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They base this on the previously unconsidered drop in pressure that occurs when water passes from gas to liquid state in condensation.