During rapid decompression, the fog in the cabin is caused by which phenomenon?

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Rapid decompression makes cabin air expand and cool quickly. The air inside the cabin carries water vapor from passengers and the surroundings. When the temperature drops below the dew point, that water vapor condenses into tiny liquid droplets, forming the visible fog. This is moisture condensation—the water vapor turning into droplets due to cooling. It isn’t smoke or fumes, which would require combustion or chemical release, and it isn’t steam from beverages, which comes from hot liquids rather than a rapid pressure drop.