= Solution
<wetting> <liquid> in narrow pores and gaps between fibers can have a concave meniscus. Its signed curvature is negative, so the <Kelvin equation> permits equilibrium at a <vapor pressure> below that above a flat <liquid> surface. \b[<capillary condensation> in sufficiently small, wettable gaps explains moisture accumulation at high relative humidity.] Increased humidity brings the <vapor> closer to flat-surface saturation and permits condensation in larger gaps. Surface <adsorption> can precede a bulk-like meniscus; large open spaces do not automatically fill at every humidity. The interpretation assumes a <wetting> contact geometry and does not require the entire cloth to be below the ordinary planar dew point.
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