Evaporative drying

Why do clothes dry faster on a windy day than on a merely warm one?

Two washing days. The first is still and warm — twenty-eight degrees, no breeze, the air heavy after rain. The second is grey and twelve degrees with a steady wind. Most people would hang the washing out on the first; in practice the second day usually wins, and often by hours.

If heat were what dried clothes, that ought to be impossible. So heat is doing something, but not the thing we assumed. What has to happen, physically, for a wet shirt to become a dry one?

Water has to leave the fabric as vapour. At the surface, individual molecules are jostling; a few at any moment have enough energy to break free of their neighbours and enter the air. That is evaporation, and it happens at any temperature — no boiling required.

But traffic runs both ways. Molecules already in the air strike the wet surface and are recaptured. So drying is not an escape rate; it is a net rate, escapes minus returns. Ask what sets the returns and you have the whole mechanism: how much water vapour is already in the air touching the cloth.

Physicists put a number on both sides. A wet surface at a given temperature can, at most, hold the air above it at a particular vapour pressure — the saturation value. The actual air has some vapour pressure of its own. Drying runs on the difference. Close that gap and evaporation stops dead, however warm everything is: that is why a bathroom mirror stays wet in a steamy room.

Now the crucial move, and it is the one intuition misses. Which air matters? Not the air across the garden — the air within a millimetre or so of the cloth. In still conditions that thin skin saturates almost immediately, because it is fed by the very surface it sits on. From then on, water can only leave by diffusing through that stagnant film to reach drier air beyond, and molecular diffusion is desperately slow. On a still day the shirt is not short of energy. It is sitting inside its own private fog, and the fog is the bottleneck.

What does wind do? Nothing to the water and almost nothing to the energy. It strips that saturated film away and replaces it with whatever the day's air happens to be. The steep difference is restored, continuously. Removing the bottleneck turns out to be worth more than heating the loaders.

Does temperature matter at all? Yes, and quite strongly — the saturation vapour pressure climbs steeply, roughly doubling for every ten degrees in the everyday range. But notice it acts on one side of a difference, and the other side moves with it: warm air on a muggy day is already carrying a great deal of vapour. What controls drying is the gap, which is why relative humidity is so often the better predictor of a washing day than the thermometer.

One consequence follows immediately. Evaporation costs energy — the escaping molecules take it with them — so a drying surface runs cooler than the air around it. That is the chill of stepping out of a shower. It also means drying can be limited by two things: the vapour gradient, or the supply of heat to replace what the vapour carries off. On a cold, still, sunless day both are poor at once.

And below freezing? Clothes still dry. Ice has its own vapour pressure, small but real, and if the surrounding air is drier than that, water leaves the ice directly as vapour without ever becoming liquid. The washing freezes stiff on the line, and then quietly sublimates itself dry.

A few refinements tie the steps together.

"Warm air holds more moisture" is a serviceable shorthand, but it invites the wrong picture — air does not sponge up water, and the vapour would be there in the same amount with no air present at all. What temperature really sets is how much vapour can persist before condensation balances evaporation. The shorthand survives; the sponge should not.

It also follows that wind and humidity are not rival explanations. Wind's job is to keep delivering air of the ambient humidity to the cloth; if that ambient air is nearly saturated, a gale delivers a great deal of air that cannot accept much water, and drying stays slow. Wind removes a bottleneck it did not create — it cannot make a humid day into a dry one.

Two honest limits. Which constraint actually binds — the vapour gradient or the energy supply — shifts with conditions, so no single rule of thumb covers every washing day; sunshine on a still day helps by raising the fabric's own temperature, and therefore the gap. And a garment is not a puddle: once the free surface water has gone, the remainder is held inside fibres and has to migrate outward first, which is why the last stage of drying is always the slowest, whatever the weather.

• Why a tumble dryer heats and tumbles, and which of the two is doing more work.
• How the wet-bulb temperature turns this same balance into a measure of how survivable a humid heatwave is.

Every term the collection defines is gathered in the glossary.