Question of the Week

How cold can it be before evaporation stops?

Sun, 31st Jan 2010

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Question

David, North Wales asked:

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

Answer

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

John -   Even when itís very cold, washing will still dry, but it may dry so slowly that it really just isnít 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.  Snowy trees on mount Brocken, Harz, GermanyEventually, 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 itís 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 itís warmer than when itís cold.  But even when itís quite cold, as long as the air isnít 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.

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This is a very good question. Evaporation has a lot less to do with temperature than with relative humidity. Florida tends to be very warm and very wet. A bit a washing hung out to dry will take a long time to dry if the humidity is very high (say 96%) even if it's very warm. On the other hand, if you in say Flagstaff, Arizona in January it will be quite cold but also very dry. Even if it's very cold out your washing will dry faster than in warm Florida. As long as it's above freezing your stuff will dry faster in dry climate than a humid one regardless of temperature.

  Freezing does slow the drying time but doesn't stop it.

  I was living by Lake Tahoe one winter when I left some wet clothes outside all night by accident. It was very dry but also very cold. About 10 below F. In the morning I found the clothes frozen and quite stiff. I took them inside and warmed them up. They were totally dry in less than 5 minutes. (they'd been soaked before).

  You can see this if the seals in your freezer aren't very good. Ice cubes shrink due to sublimation. The water goes directly from ice to vapor without becoming liquid water. mountaineirc1969, Wed, 27th Jan 2010

I put my washing out on the line early today and the temperature was around zero. It rose to +2 degrees C at midday. However, the wind was blowing reasonably well so everything got dry in 3 hours.

No wind; no dry. FuzzyUK, Mon, 1st Feb 2010

In principle you can still dry something even if it's close to absolute zero as long as the partial vapour pressure of water in the space round it is less than the vapour pressure of the water at that temperature.
On the other hand it will be slow. Bored chemist, Mon, 1st Feb 2010

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