Saturday, 24 May 2014

Humidity makes it hotter ... or colder

When the scientific theory doesn't match your observation of reality, you know you've got one of them wrong. So it's been bothering me for a while that the theory suggets higher humidity makes it feel hotter while the evidence suggests the opposite at colder temperatures.

Finally I've realised the cause of this anomaly.

There are actually two effects of higher humidity on the body losing heat. One reduces the ability of the air to remove heat from the body, and the other increases it.

As we know, the body mainly loses heat through evaporation of the body's perspiration. The ability of this perspiration to evaporate is hindered by high humidity. Humid air just has less carrying capacity for those water droplets and will push up
The other effect is on the air's heat capacity. Water is a very effective carrier of heat, with a kilogramme of the stuff able to hold almost twice as much heat as a kilogramme of air. Adding water to air is going to increase this heat capacity. More heat capacity means a greater ability to take away heat.

Perhaps as it gets hotter, and the difference between body temperature and ambient temperature becomes smaller, the evaporation effect is larger, so humid air makes us feel hotter. Meanwhile, when it gets colder and the difference in temperature is larger, the higher heat capacity effect is larger.

Perhaps, but probably not.

Water certainly does have twice the heat capacity of air, but absolute humidity is measured in grammes of water moisture per kilogramme of air, so the increased heat capacity may only be one percent, comparing dry air with dripping wet air at 10 degrees centigrade. It's difficult to imagine this making the kind of two or three degree differences that humidity makes when it's hot.

Clothes are another matter though. The amount of water they can hold does not depend on temperature and absolute humidity, but on relative humidity and the related vapour pressure. Cotton can hold up to 15% of its weight in water, wool can hold up to 35%. Both of these textiles are hydrophilic and will try to reach an equilibrium with the atmosphere around them steadily releasing or absorbing moisture. That all takes energy, and any water content in the clothes needs to be kept warm.

At last this seems to make sense, although I'm not completely sure it's correct. I know the hiker's adage that cotton kills, but also I've heard that if you are stuck somewhere cold and damp, the best thing you can do is wrap yourself in a woollen blanket, since wool is exothermic.


Notes and references

This article is only tangentially relevant, but has a good explanation of hydrophilic textiles seeking equilibrium with their environment: Iqbal, M., Sohail, M., Ahmed, A., Ahmed, K., Moiz, A. and Ahmed, K. (2012) Textile environmental conditioning: Effect of relative humidity variation on the tensile properties of different fabrics Journal of Analytical Sciences, Methods and Instrumentation 2(2), 92-97

In fact there is much older work on moisture in textiles, for example, Albert C. Walker's Moisture in Textiles (1937, in Bell system technical journal, 16, pp 228-246). This goes into some detail on exactly where the moisture goes within cotton hairs.

And here is some propaganda from New Zealand wool industry: New Zealand Merino Company Limited (no date) Heat and moisture regulation.

You may want to compare this with the moisture content of wood. In textiles terms, of course, wood is just raw rayon.

A Q and A session on physics.stackexchange.com was helpful in debunking my first hypothesis, giving the formula cs = 1.005 + 1.82H, where 1.005 kJ/kg°C is the heat capacity of dry air, 1.82 kJ/kg°C the heat capacity of water vapour, and H the specific humidity in kg water vapour per kg dry air in the mixture.