How to peg washing out
Two lists. Things the model can justify, and things that are folklore as far as it is concerned. Keeping them apart is the whole point — everything here either has a reason or is labelled as not having one.
What the physics actually says
Hang towels and sheets at full length, not folded over the line
This is the big one, and it falls straight out of the model rather than being asserted. Drying is governed by the water held per square metre of exposed fabric. Fold a towel over the line and the same water now has half the surface to leave through, so it takes roughly twice as long.
On a good day that is the difference between four hours and eight. On a marginal day it is the difference between finishing and not.
Space things out — but only up to a point
Crowding does not scale with how many things are on the line. Ten shirts take about as long as one until they start to touch; packed tight, they dry into each other's damp air. So the gain from spreading out is large at first and then stops mattering.
Measured in the model, roomy against packed is worth about 25% of the drying time — more than a 10% error in the wind forecast, which is why the app asks.
Thickness matters, size does not
A double bed sheet is enormous and dries faster than a pair of jeans, because what counts is water per unit area rather than total water. Jeans are heavy cloth with doubled seams and a waistband; a sheet is one thin layer.
Wind does more than sun, and people badly underrate it
A cold breezy day beats a warm still one by a wide margin — in the model's own reference test, about twenty to one on the part of the calculation that wind drives. If you can only choose one thing, choose the breeze.
Dark clothes do dry faster in direct sun — and only in direct sun
Dark fabric absorbs more of the sun's beam, so on a bright day it runs warmer and dries quicker. Under cloud there is almost no beam to absorb and the effect nearly vanishes. It is real, and it is much smaller than it is usually given credit for.
Which way the line runs changes the day's total
A line facing across the sun intercepts more of it than one facing along. It changes how much you get, though it never changes whether wind helps.
What we cannot justify, and will not pretend to
All of the following are plausible, widely repeated, and not something this model can support. It treats a garment as a flat sheet with a mass per unit area; it has no notion of a garment being open or closed, inside out, or hung by one end rather than the other.
- Hanging shirts by the hem rather than the shoulders
- Turning things inside out so the pockets are exposed
- Unbuttoning collars and cuffs to let air through
- Hanging trousers by the waistband rather than the ankles
They may well all be true. Adding the physics to find out would mean modelling airflow through a garment rather than over it, which is a considerably harder problem than the one solved so far. Until then, these stay on this list.
The whole credibility of this page rests on the two lists staying separate. Padding the first one with confident-sounding guesses would be the easiest way to make all of it worthless.
Fewer wrinkles, which is a different question
Mostly not a drying-rate problem, so mostly not something the model owns:
- Shake each item out before pegging. The single biggest factor, and free.
- Peg at seams and hems, not across shoulders, so the peg marks land where nobody looks.
- Do not crowd, for creases as well as for speed — fabric pressed against fabric sets a fold.
The one part that is ours is over-drying. Cotton presses best at around 14% moisture, and a good drying day takes it straight past that down to 7 or 8%. Washing left out all afternoon on a bright day ends up harder to iron than it needed to be — so if something is going on the board, bring it in when it is done rather than at teatime.
Pegday will tell you when that window is, on the days it is wide enough to be worth naming.
Frozen washing
It dries. The ice sublimates straight to vapour and the load comes in stiff but drier than it went out — slowly, because cold air holds very little water, but genuinely. A crisp dry January day with a breeze is a better drying day than a mild muggy one in May.
Everything in the first list is checked by a test that runs on every change, so if the physics moves, this page fails rather than quietly becoming wrong. How it works.
Weather from Open-Meteo, licensed CC BY 4.0.