My washing is hanging to dry outside and I need to bring it in before dew forms on it. How do I know at what time or temperature that will be?
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Peter Meehan London, UK
Hang a fir cone on the washing line, in the shade. It will close up in humid air.
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Mike Follows Sutton Coldfield, West Midlands, UK
Water vapour will condense onto a surface when the air temperature falls below the dew point – the temperature at which the air is saturated and can hold no more water vapour. As the temperature falls, more water vapour will condense.
Predicting when to bring the washing indoors is tricky as there are many variables, including the material of the clothes, whether it is sunny, the temperature and humidity of the surrounding air and how much the clothes are flapping around in any breeze.
As well as dripping from the washing, water will evaporate from wet clothes, taking with it the latent heat of vaporisation, at least until the surrounding air becomes saturated. This evaporation will raise the humidity and dew point of the air in direct contact with the clothes. At the same time, the temperature of the clothes is lowered, which can be exacerbated by heat transfer via convection and radiation. Additionally, the temperature of the air in contact with the clothes will fall as heat is transferred down the temperature gradient from the warmer air to the cooler clothes.
If the air temperature falls below the dew point, unless there is a heat source (generally the sun) or a breeze to replace the humid air next to the clothes with drier air, an equilibrium between evaporation and condensation is reached, which prevents any further drying.
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Chris Szymonski Waupaca, Wisconsin, US
The temperature of the wet laundry being dried outside stays close to the dew point temperature of the air in the area.
As the air temperature goes up throughout the day, the relative humidity of the air (expressed as per cent saturation of the air with water vapour) decreases. This also means that the ability of the air to hold moisture increases. In other words, closer to noon, the air is capable of holding more moisture.
In the early evening, when the air temperature drops, the opposite effect takes place, thus the relative humidity gradient between clothes being dried and the surrounding air drops towards zero, and the drying effect stops.
A handheld psychrometer to measure the relative humidity of the air would do the trick. Just remove the laundry when this comes close to 100 per cent.
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Stephen Johnson Eugene, Oregon, US
Typically, the combination of temperature and humidity required for dew to form is most likely in the early morning when few people would attempt to dry laundry outside, if for no other reason than there is little to no sun.
Under rare or extreme conditions, it is possible for dew to form in the middle of the day – for instance, just before a severe storm driven by a cold front. As an extreme example, the highest dew point ever recorded is 35°C in Saudi Arabia during a day with an outside temperature of 42°C. The lowest dew point was -29°C on a 34°C day in Coober Pedy, Australia.
If you want to know the dew point, it can be calculated from temperature and relative humidity, or measured by many inexpensive hand instruments.
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@steve_tees via Twitter
It will always be just before you decide it is time to bring the washing in.
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