DryWindowIs It a Good Drying Day? The Science of Drying Washing Outside
A good drying day is not simply a sunny one. A cool, breezy day with dry air can dry washing faster than a hot, still, muggy one. Here is what actually makes clothes dry, and how to time it.
What drying really depends on
Washing dries when water evaporates from the fabric into the air. The rate of evaporation depends on three things working together:
- How dry the air is. Low relative humidity means the air can take up a lot more moisture. Above about 80%, drying slows to a crawl.
- Wind. A breeze constantly replaces the damp layer of air next to the fabric with drier air. Even a light wind of 5–15 mph makes a huge difference.
- Warmth. Warmer air and sunshine provide the energy needed for evaporation and lower the relative humidity.
Because these combine, a 12°C day with 55% humidity and a steady breeze is usually a better drying day than a 22°C day at 85% humidity with no wind.
The evening problem
As the sun goes down, the temperature falls and relative humidity rises. Once the air cools towards the dew point, drying stops – and washing left out can actually start to get damp again. In spring and autumn this can happen as early as mid-afternoon. Bring washing in while the air is still warm, or plan to hang it in the morning so it has the best part of the day.
How long washing needs
Lightweight cotton and synthetics may dry in two or three hours on a good day. Towels, jeans and bedding often need five hours or more. DryWindow uses a five-hour window for its "Drying washing outside" option, with temperature at least 8°C, humidity below 80%, rain probability no more than 10% and the dew point gap kept above 1°C. It also gives a small bonus to breezier hours, because wind is such an important factor.
Tips for faster drying
- Use a high spin speed – the less water in the fabric, the faster it dries.
- Peg items so air can move through them: hang shirts by the hem, turn pockets out, and leave space between items.
- Put the heaviest items at the ends of the line where they catch the most wind.
- Turn dark clothes inside out to reduce fading in strong sunshine.
- Hang washing out as soon as morning dew has cleared rather than at first light.
Why drying outside is worth it
Tumble dryers are among the most energy-hungry appliances in a typical home, and drying clothes indoors on a rack adds a lot of moisture to the air, which can contribute to condensation and mould in poorly ventilated homes. A few well-timed outdoor drying days each week can make a real difference to both your energy use and indoor air quality.
Plan it
Choose "Drying washing outside" in DryWindow and enter your town. The tool ranks every possible five-hour window in the next week, so you can see at a glance whether today is worth it or whether tomorrow morning is better.