LINE DRYING, OUTDOOR WORKSHOPS, AND UNHEATED GARAGES

Laundry that comes in damper than it went out

Why is my washing still wet after a day outside?

Because the air ran out of headroom. Drying needs the air to accept water, and once the temperature falls towards the dew point in late afternoon the air is nearly full — so evaporation stops and reverses. Anything left out past that point picks moisture back up.

Out at eight in the morning. In at six in the evening. Ten hours on the line, no rain at any point, and the sheets are heavier coming in than they were at lunchtime.

Nothing went wrong. The air simply stopped accepting water some time in the late afternoon, and then started handing it back.

Drying is a transfer, not an evaporation

Water leaving fabric has to go somewhere, and the somewhere is the air. So drying depends on how much room the air has left — the gap between what it currently holds and what it could hold at its temperature.

That gap is at its widest in the early afternoon, when the temperature peaks and the dew point sits flat where it has been all day. It closes through the evening as the temperature falls back towards the dew point.

When the gap reaches zero, the air is saturated. Evaporation does not merely stop — the fabric is now the coldest thing on the line, and water starts condensing onto it.

The day, hour by hour

A typical day with a 55°F dew point. The last column is what does the drying.
Time	Temperature	Relative humidity	Headroom in the air
8am	57°F	93%	Almost none
11am	66°F	68%	Opening up
2pm	73°F	53%	Widest
4pm	72°F	55%	Still good
6pm	64°F	73%	Closing fast
8pm	58°F	90%	Effectively none

Hanging out at eight was pointless — the air had nothing to offer for the first two hours. Leaving it out past six was actively counterproductive.

The useful window was roughly 11am to 5pm, and it was set by the dew point, not by the sunshine.

Why cold, clear days work so well

This is the part that seems backwards. A crisp 40°F winter day dries washing better than a muggy 70°F one, and the reason is that cold air holds almost no water.

Air at 40°F and 70% humidity carries about 4.6 g/m³. Warm it a few degrees in the sun and its capacity climbs steeply while its content does not — so the headroom opens right up. Meanwhile a 70°F day at 85% is carrying around 15.6 g/m³ and has almost nowhere left to put anything.

Which is why the forecast percentage tells you so little on its own, and the dew point tells you a lot.

Wind matters as much as warmth

Evaporation saturates the thin boundary layer of air against the fabric almost immediately. Without movement that layer stays saturated and drying effectively stops, however dry the rest of the garden is.

Wind replaces it continuously. A breezy 50°F day will routinely beat a still 65°F one.

The working rule

Check the dew point, not the humidity percentage.
Hang out once the temperature has risen well clear of it — usually mid-morning.
Bring in before the late-afternoon drop, not at the end of the day.
On a day where the forecast shows the temperature and dew point close together all day, the line is not going to work. That is an indoor day.

The same rule everywhere else

An unheated garage or outdoor workshop is the same problem with the door shut. So is a hay crop waiting to be baled, a freshly laid coat of paint, and a lawn that will not dry out.

All of them are asking the same question: does the air have anywhere to put this water?

Common questions

What is the best time to hang washing outside?

Mid-morning to mid-afternoon, when the temperature is highest relative to the dew point. That is when the air has the most headroom and evaporation is fastest. Bring things in before the late-afternoon temperature drop rather than at the end of the day.

Why does washing dry faster on a cold, clear day than a mild, damp one?

Because cold air holds very little water in absolute terms, so even at a high percentage it has room to accept more once slightly warmed by the sun. A mild, humid day can be at 90% capacity and take almost nothing.

Does wind help washing dry?

Considerably. Evaporation saturates the thin layer of air against the fabric, and without movement that layer stays saturated and drying stalls. Wind replaces it continuously, which is often worth more than a few degrees of warmth.

Will washing dry if the humidity is 90%?

Barely, and not at all if the temperature is also falling. At 90% the air is nearly full, so there is very little room for water to go. Whether it dries at all depends on whether the day is still warming.

Can washing freeze-dry outside in winter?

Yes — it is called sublimation, and it genuinely works. The water freezes and then passes directly from ice to vapour. It is slow, but very cold air is extremely dry in absolute terms, so the headroom is there.

Sources

NOAA / National Weather Service — Dew point vs humidity

Published 12 August 2026