heliography/Stockimo/Alamy
I would like to know, scientifically speaking, is it more efficient to dry my towels with them hung up portrait or landscape? Which towel will dry quickest?
Chris Daniel
Glan Conwy, Conwy Bay, UK
When a towel dries, it absorbs heat from the surrounding air. The temperature of the water in the towel is based on the average of the range of energies of its molecules, represented by an asymmetric, bell-shaped curve called the Maxwell-Boltzmann distribution. At the high end of the curve, the water molecules have enough energy to break free from the surface to become water vapour.
This results in a slight drop in the average temperature of the remaining water, which then absorbs more heat energy from the surrounding air equal to the latent heat of evaporation. This is a continuous process while the towel is drying and means that the towel is always slightly cooler than the surrounding air.
If there is a breeze, the vapour will be carried away and the towel will dry quickly. In still air, the rate of drying will depend on the diffusion speed – that is, the rate at which the water vapour can disperse laterally into the surrounding air. This, in turn, depends on the air’s humidity.
“
On an airing rack, towels hung landscape will dry faster. The downside is the towel takes up more rack space
“
At the surface of the wet towel, the relative humidity of the air is 100 per cent, and this decreases with distance away from it. The higher the general humidity of the air, the slower the diffusion speed of the vapour. The towel will therefore take much longer to dry than if the air is moving.
An additional factor is that water vapour has a density relative to dry air of about 0.6, so water molecules will rise as they escape the moisture in the towel. If this happens from a fibre within the towel, or perhaps just along its outer surface, the vapour may be cooled again by coming into contact with another damp fibre and be recaptured as liquid water.
As this pattern will be repeated over the whole towel, it would be better to hang it in landscape orientation to minimise the vertical distance water vapour has to travel before clearing the top edge. It will also minimise the opportunity for mould or mildew spores in the air to grow on the fabric.
Better still, hang the towel in any orientation in a warm room with good ventilation, or outside on a dry day.
Hillary Shaw
Newport, Shropshire, UK
With clothes hung inside on an airing rack, the items on the top rungs dry noticeably faster than those 1 metre lower. The room air must be appreciably warmer at this level. So, towels hung landscape will dry faster, on the same rung, as more of the towel is higher up. The downside is the towel takes up more rack space.
If you are short of rung space, hang the items portrait and, when the upper items are dry, move the lower ones up. Portrait is better for some clothes, as gravity will help to remove creases, so there is less ironing, but that probably doesn’t apply to your towels.
Mary Hall
Auckland, New Zealand
I have found that my towels dry quicker if pegged in landscape, because they dry from the top down. Therefore, it is a shorter distance than if pegged in portrait.
Matthew Stevens
Sydney, Australia
In practical terms, you will be able to fit more towels if you hang them vertically, and therefore be able to dry more washing in the same space.
To answer this question – or ask a new one – email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
Â鶹´«Ã½ retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.
