The Perfect Drying Formula

Vileda has been working with Jim NR Dale, founder of the British Weather Services, who has discovered the perfect formula for quickly drying your clothes outdoors - and we now know the answer.

The Formula = ∆Hvap {or E} (The Heat of Evaporation {or Evaporation}) = W/m2 ≥ 1 hr + (RH <70%) + (T ≥21C) + (ff8-12)

In other words, all you need is 1 hour or more of sun + less than 70% relative humidity + temperature of 21°C or more + 8 to 12mph of sustained wind.

The result?

Perfect drying results in one hour*

TV Weather Presenter Laura Tobin has commented on the formula, "drying clothes outdoors is the most environmentally-friendly way to dry them. When we look to science, we can pinpoint the specific weather elements you need to achieve perfect drying results, all without using any manufactured energy whatsoever."

Laura Tobin continues, "the sunshine increases the direct energy absorbed by your washing and low moisture content in the surrounding means the air can absorb more water. An influx of fresh air to increase agitation creates molecular energy, which allows the moisture from the washing to be released. Choose a warm day because warmer air can hold more of the moisture released, and ensuring it's also a dry day will mean there's no reverse cycle of this moisture (plus your clothes won't get wet again!)."

Follow Laura on her Instagram account

Drying Outdoors

The Perfect Formula

Drying Outdoors

For an environmentally friendly way to dry your laundry items outside, put your clothes on a rotary washing line, a clothes airer, or a retractable washing line in the conditions outlined in the formula above, and you won't need to wait long for fresh, dry clothes.

As well as reducing drying time, putting your laundry outside also reduces ironing time. After you have washed your clothes, give them a good shake and add them to the rotary washing line or clothing clothes airer to minimise creasing, reduce ironing time, and minimise any damage a tumble dryer may do to them. Just make sure you leave enough space between items, so any moisture doesn't transfer between them.


If it's shirts, blouses or dresses you're drying, then you can put them on a clothes hanger before you put them on the rotary clothes line, or an airer to further reduce any creasing. The UV radiation from the sun and a gentle breeze will also kill off any dust mites or bacteria, which is a blessing for any allergy suffers.

And, if it's your clean white sheets or towels that you've put on the washing line, then the sun will act as a natural bleach, keeping your whites white for longer.

And the location of your washing line, rotary dryer or airer is also essential, so, if possible, hang your clothes from west to east to make the most of south-facing irradiation from the sun and avoid any shadowing.


July, August and September are the best months for outdoor drying - they have the highest number of days with optimal drying conditions. But, April to May and late September can also offer some excellent drying weather. Even with the unpredictable British weather, the UK will experience 40 to 70 'perfect drying days', so make the most of them when they happen.

The perfect time of day for drying your laundry is from about 12 pm to 5 pm, with 3 pm being the optimal drying period during summer. Geographically, the southeast areas around Kent and Greater London will offer more 'perfect drying days', but you'll also get some excellent drying conditions during summer in the north around Aberdeen and eastern Scotland.


Avoiding humidity is essential and weather conditions with high water content, such as fog and mist. And, wait until later in the day to hang your washing out.

Drying Indoors


Drying Indoors

However, what about the inevitable not so perfect drying days the UK experiences? In this case, you can still maximise your drying times but using an indoor clothes airer.

As soon as you take our laundry out of the washing machine, shake it and drape it over an indoor airer positioned near an open window or door. If you can, use an airer with thick aluminium wires to avoid creasing and marking on your clean clothes and household laundry. Some indoor airers will have additional hangers for small items, so you won't need to use pegs.

Again, put shirts and blouses on a hanger and make sure trousers, dresses, and other items are straight when you put them on the clothes dryer.

So next time the perfect drying day arrives, you'll know how to get your clothes dry within the hour.

*Full scientific equation, created by the British Weather Services:

Definition of the symbols:

Evaporation = E

(if preferred) The Heat of Evaporation = ∆Hvap

Dry day, there is no symbol.

Sunshine = W/m2

Relative Humidity = RH

Wind = ff

Temperature = T

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