Washing dries because water passes into the air. The speed of that process depends neither on temperature alone nor on relative humidity alone: it depends on the gap between the vapour the air contains and the vapour it could contain, known as the saturation deficit.
Why this indicator matters
On a cool but dry day, with a low dew point, the deficit stays large and washing dries well, even at 8 °C. On a hot, heavy day, when the dew point is close to the temperature, the air is almost saturated and washing stays damp, even at 25 °C. Wind plays a supporting role: it carries away the layer of damp air that forms against the fabric.
Knowing in advance whether a load of cotton washing will dry before evening avoids two mistakes: hanging it out on a day that is lost from the start and ending up with cold, damp washing, or missing a fine day of free drying, which a tumble dryer would charge you for in electricity.
How to read it
The site adds up, hour after hour, a drying power equal to the saturation deficit multiplied by a wind factor. When the total reaches the load of a cotton wash, the washing is dry. The slot given is the shortest of those that fit between 8:00 and 19:00 without rain.
The time given is an order of magnitude for towels or sheets hung in the open air; thin clothes dry faster, a duvet far more slowly. If no slot is long enough, the sentence says so and gives the reason: rain, or air that is too cold and too damp.
The thresholds used on this site
Every sentence in the “Today” box comes from a numerical rule. Here are the rules for this indicator, exactly as they are applied in the code.
- Drying load of a cotton wash: 90 hPa·h of accumulated drying power.
- Drying power in one hour: saturation deficit in hPa multiplied by (1 + wind in km/h ÷ 10).
- Hours taken into account: from 8:00 to 19:00, without rain (less than 0.1 mm) and with a probability of rain below 40 %.
- Below 1 °C, washing freezes instead of drying: the hour does not count.
- Good washing day: dry in 4 h or less; average day up to 8 h; beyond that, it is a close call before evening.
A worked example: Toronto
In Toronto, with the forecasts of 2026-11-02 08:13: Washing outside: only just possible. Even hung out between 10:00 and 19:00, it will take about 9 h; bring it in before the air turns cooler.
Its limits
The drying factor is a rule of thumb based on everyday experience, not a laboratory measurement: a balcony sheltered from the wind or very thick washing will depart from the estimate. Direct sun speeds up drying but is not part of the calculation, which makes the estimate cautious in midsummer.
Frequently asked questions
Does washing dry outside in winter?
Yes, if it does not freeze and the air is dry, but slowly: a saturation deficit of 2 hPa needs more than twenty hours in the open air for a load of cotton washing, which is why it cannot manage it on a short day.
Should I hang washing in the sun or in the shade?
In the sun, drying is faster, but dark colours fade and delicate fabrics wear. Wind matters as much as sun: washing in the shade but in a breeze dries well.
Why no washing outside when it is drizzling?
Drizzle does not wet things much, but it saturates the air against the fabric: the saturation deficit falls to almost zero and the washing takes back the water it has just lost.