Yesterday we arrived back in Cambridge after a few days holiday in Auckland, being tourists. We sampled such delights as the unheated hotel swimming pool, the complicated and expensive process of getting on a bus (basically having to find somewhere from which to buy a HOP card, for a non-refundable $10 a card), the completely non-social distanced pedestrian rugby scrum while navigating the building site outside Britomart, and the incessant foggy-drizzle of Tuesday that rendered the beautiful view of the city from Devonport utterly invisible.
Returning on the now-open Huntly bypass gave me a view of some Waikato countryside that I hadn’t seen before. And passing wetland-after-wetland in the pouring rain reminded me that much of this wonderful region of Aotearoa is swamp. In winter, one cannot escape the fact that Waikato is damp.
We then face the usual dilemma for any family with young boys returning to Waikato from a winter holiday – how to dry your washing. We would normally hang it outside on the line, but with the rain being interspersed only with periods of 98% humidity, it will start growing lichen before it gets dry. I am loathed to unnecessarily add to NZ’s climate change disaster by getting a clothes dryer, and besides, the internal laundry in the house, which has a space for a dryer, has no vent for it, so all we’d do is fill the laundry and garage with moisture. Waiting for the weather to improve before doing washing risks accumulating a volcano of dirty laundry the size of Mt Eden. That leaves drying clothing on a rack inside, which is not recommended either, since it adds to the humidity inside the house and encourages growth of mould.
But knowing some physics does help. When it comes to heating, the moisture content of the air makes a significant difference. Basically, the water molecules, being made of three atoms, have more modes of vibration and rotation available to them than the simpler diatomic nitrogen and oxygen molecules of dry air. That means water molecules require more energy to warm up, and so moist air takes more energy to heat than dry air (its heat capacity is higher). When it comes to dehumidifying, we can note that the process of taking water out of air (by condensing it on a surface as in a dehumidifier) is much more efficient when the air is warmer, because a litre of warm air holds more moisture than a litre of cold air. This second effect is very significant – for example at 15 Celsius air will hold 0.011 g of water per g of dry air, whereas at 20 Celsius is will hold around 0.015 g, an increase of about 35%. The overall interplay of heat and moisture is captured on a psychrometric chart – a complicated looking series of lines that shows how the energy content, moisture content, relative humidity and temperature are related.
ArthurOgawa / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
This means that heating and dehumidifying together is a good idea – warming the air helps the dehumidifying process, and dehumidifying helps the heating process. And both help remove the moisture from the damp washing.
So, out comes the dehumidifier from the garage, and off we go. One day we’ll be back to hanging clothes outside, but it’s not going to be today. Tomorrow isn’t looking too good either.
And one day, Auckland might have a public transport system that actually encourages people to leave their cars at home. (And so, I should add, might Cambridge/Hamilton).