Science Questions

How cold can it be before evaporation stops?

When does it make sense to hang washing out on the line? Will it still dry even in low temperatures? David, North Wales

We put this question to John King, from the British Antarctic Survey in Cambridge:

John -   Even when it’s very cold, washing will still dry, but it may dry so slowly that it really just isn’t worth it.  The reason washing dries is because water evaporates from it.  If a wet surface is in contact with the air, some molecules of water will leave the surface and go into the air, but at the same time, molecules of water vapour from the air will be coming into the surface.  Eventually, it will reach some kind of equilibrium where the amount of water leaving the surface is the same as the amount coming in.  We then say that the air is saturated with water, and once the air is saturated, no more [net] evaporation can take place.  Now, if we look at the basic physics underlying this, we find that the amount of water that air can hold when it’s saturated depends very strongly on temperature, and the warmer the air is, the more water it can hold.  So, evaporation tends to proceed much more quickly when it’s warmer than when it’s cold.  But even when it’s quite cold, as long as the air isn’t saturated, your washing will dry, but it may dry very, very slowly, and it may rain before it gets dry!  In general, we don't hang washing out to dry in the Antarctic because it is so cold that things would take such a long time to dry.  Maybe on a really nice sunny day in the middle of summer, you might get the tea towels dry, or something like that.

Diana -   Evaporation does require energy and the warmer the air, the more energy there is to remove dampness from your washing.  But as our forum goer, Eric Taylor said, it has more to do with the relative humidity than temperature.  So, if you live in a dry but cold area, you might be better off hanging out your washing than if you were in a hot but humid country.  Something similar can happen in the Antarctic where, in a region called the Dry Valleys, there is no ice or snow on the ground because what does land there is sublimated directly into vapour.

January 2010