Laundry dries by the wet-bulb gap, not by temperature or humidity
#0044 Home & daily life
Fact rank
A
The principle that a drying object stays at the wet-bulb temperature was confirmed in university lecture notes (secondary), and the figures were calculated with the psychrometer formula. No measurements comparing laundry drying across seasons were found, so not rated S
Wow rank
S
Shows with numbers that the intuition "laundry dries faster in summer" is reversed in the shade
Usefulness
A
Useful when choosing the day or place to hang laundry, and for putting air flow on laundry dried indoors
Verification: Supported by evidence
AI verification reports the evidence it found; it does not guarantee that a claim is true.
How fast laundry dries depends not on the temperature or the humidity alone but on the gap between the two thermometers of a psychrometer (the difference from the wet-bulb temperature). In the shade or indoors, a dry, clear winter day (5°C, 40% humidity, a gap of 4.4°C) dries laundry faster than a muggy summer day (30°C, 80% humidity, a gap of 2.9°C).
Why
Wet fabric loses heat as its water evaporates and cools to the wet-bulb temperature, below the air temperature. The drying force is the difference in water vapour between this cooled surface and the air, so even when it is hot, the difference is small if the humidity is high.Evidence
Secondary Lecture notes by Professor R. Paul Singh (food engineering, UC Davis). During the constant-rate drying period, the product temperature stays at the wet-bulb temperature of the surrounding air link
Secondary Lecture notes from RGPV, a state technical university in India. When radiant heat reaches the drying object, its temperature rises above the wet-bulb temperature (why sun-drying falls outside the calculation) link
Primary FAQ for children by the Japan Meteorological Agency. The laundry index seems to be calculated from temperature, humidity, wind speed, solar radiation and so on; it is calculated by private weather companies, not the agency, so the formula is not published link
Notes and limits
湿球温度と差は乾湿計の式で計算した（5℃・40％は湿球0.6℃、30℃・80％は27.1℃）。布の表面と空気の水蒸気の差に直すと冬0.29kPa・夏0.19kPaで、日陰では冬の方が約1.5倍速い。梅雨（25℃・90％）は差1.3℃で、さらに遅い。日なたでは日射が布を温めるので、この計算から外れ、夏が追いつくこともある。風は表面の湿った空気を入れ替え、速さを何倍にも変える。空気が抱えられる水蒸気の上限は、気温が10℃上がるごとに約2倍弱（1.7〜2倍）になる。「洗濯指数」は気象庁ではなく民間の気象会社の計算。
Derivations
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