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Moisture inside a tent doesn't automatically mean rain has passed through the fabric.
People release water vapor through breathing, while wet clothing, damp ground, and humid air can add even more moisture. When warm humid air contacts a colder tent surface, water can condense into droplets.
Ventilation helps reduce this buildup, although condensation can't always be eliminated completely.
A perfectly waterproof tent can therefore still have water covering parts of its interior by morning.
© REI Co-op