Cold and humid

Saturday was one of those days that the Waikato winter is famous for. Cold and damp – by damp I mean humid as well as raining – in fact the kind of weather that reminds you that you are living in an area that used to be one massive swamp. The sort of dampness that makes you feel it is raining inside the house as well as outside and makes you wonder whether you’d be better off weather-wise living in Wellington. Just yukky, soppy, dampness, everywhere.

At about 2pm I’d had enough of it and drove into town to a well-known homeware store (which, surprise, surprise, was having a sale) and bought a dehumidifier. It spent the rest of the day sucking vast quantities of water out of the internal atmosphere, while the heat pumps heated it.

Dehumidifiers are pretty simple in theory. The idea is that since cold air holds less moisture than warm air, all you have to do is to create a cold surface, pass the air over it, and the moisture will condense out. Just collect the moisture and, hey presto, you have dry air left.  There’s not a great deal of difference between a dehumidifier and a fridge or freezer – in fact, you could use your freezer as a crude dehumidifier if you left the door open. (Try this on a hot summers day, like we did once accidently when we went away for a week, and see how much water has been deposited – in this case as ice). 

Dehumidifiers will also heat the air (not cool it)  This is because to create the cold surface, they need to shift heat elsewhere, and this process, by the second law of thermodynamics, is going to generate more heat. Also, when moisture condenses out of air, it releases heat. (Think of the reverse process, evaporation requires heat). Feel the air leaving the dehumidifier – it should feel somewhat warm.

However, dehumidifiers in some parts of NZ (of which Waikato is one) have a bit of a problem. They are mostly designed to work in places like Singapore, where it is hot and humid, not in badly insulated NZ homes in winter, where it is cold and humid. That means there’s less scope for cooling down the already cold air, and so less moisture can be removed.  It helps to heat the air in the first place (i.e. dehumidifiers are likely to do better when your house is warm). Also, since dry air is easier to heat than moist air, it is easier to heat your house when the air is dry, so heating and dehumidifying in tandem both help each other.

Saturday afternoon certainly felt much more pleasant than Saturday morning.




2 thoughts on “Cold and humid”

  • Is dry air really easier to heat than wet air? Just a priori, I would expect the specific heat of wet air to be smaller based on its lower average molecular weight. Lighter molecules (more 18s among the 28s and 32s) should need less energy to get up to the speed characteristic of a particular temperature. Or maybe I’m misunderstanding something.

  • Marcus Wilson says:

    I think you’re correct. Wet air will be less dense. But its heat capacity will be greater. A tri-atomic water molecule has many more modes of vibration open to it than a diatomic nitrogen or oxygen, and so a litre of water vapour will need more energy to heat it to a given temperature than a litre of nitrogen/oxygen (assuming both are ideal gases).

Leave a Reply