洗濯物の乾く速さは乾湿計の温度差で決まる/en

#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

Posted by: Mamechi editors ・ Verified: 2026-10-30 ・ Method: AI + human ・ On Wikipedia?: Not on Wikipedia

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

Secondary Weathernews article (2025, interviewing a laundry expert at Lion). Blowing air on laundry with an electric fan dried it in a third of the time link

Notes and limits

The wet-bulb temperatures and gaps were calculated with the psychrometer formula (wet bulb 0.6°C at 5°C/40%, 27.1°C at 30°C/80%). As the vapour difference between the fabric surface and the air this is 0.29 kPa in winter and 0.19 kPa in summer, so in the shade winter dries about 1.5 times faster. The rainy season (25°C, 90%) has a gap of 1.3°C and is slower still. In the sun, solar radiation warms the fabric, so this calculation no longer holds and summer can catch up. Wind replaces the moist air at the surface and can change the speed several-fold. The most water vapour air can hold just under doubles (1.7–2 times) for every 10°C rise. The "laundry index" is calculated by private weather companies, not the Japan Meteorological Agency.

Derivations

Derived from: —

Derived facts: —