Carol-Ann Dovey of Balclutha asks :-

My mother and grandmother often mention that following washing, white clothes always take longer to dry than non-whites. Are they correct?

Gerry Carrington, a physicist at Otago University with an interest in energy, responded.

During drying liquid-water in the wet clothes is converted into water-vapour, which is taken away by the air. The energy needed to separate the molecules of liquid from each other, called the latent-heat of vaporisation, is particularly large in the case of water. On the clothes line this energy is free, much of it coming from the air, but sunlight helps too. In one hour full sunlight on a large towel can produce enough heat to remove hundreds of grams of water. This should have a bigger effect on non-white than white washing, because darker colours absorb sunlight better.

It’s quite easy to find out if this really happens yourself. Towels are quite convenient if you can find two of about the same age and manufacture, one dark, the other light. First weight the towels dry, using kitchen scales, then wash and spin them and hang them on the clothes line. Weigh them regularly to determine how much water remains in each towel as they dry. I did this on a sunny afternoon with green and cream towels. I swapped their positions after every weighing to make sure they had similar drying conditions.

The figure shows that the green towel lost 270g per hour on average, whereas the cream one lost just 190g. This means the drying rate of the darker one was about 40 pre cent more than the lighter, which is quite a large difference.

So your mother and grandmother certainly seem to be correct when the washing is in the sun. But would you expect this effect when there is no sun? Test your ideas by experiment and do send your results to me, gerry@physics.otago.ac.nz. You could also look at how other factors, such as the air-temperature, wind-speed and relative-humidity, affect the drying rate.