Physics in action: keeping cool with a wet unicorn · 2:27pm Jul 24th, 2022
Last week we saw the first pics from the James Webb Space Telescope. The stunning images of early galaxies were amazing, although as an instrument nerd I find myself paying more attention to the camera specs. It’s an impressive bit of kit: 6.5m mirror and IR camera operating under 50K, requiring a five-layer sun shield, which had to unfold in space after being launched, with no chance of fixing things if it went wrong. Got to respect the team doing the QA/QC.
Then this week, we have experienced the predicable (and predicted) consequences of global heating as Western Europe was hit by a record scorching heatwave, leading to raging wildfires in Spain and France and sending temperatures in the UK above 40°C for the first time on record (the average for this month is 23°C). Such a maximum may not seem so remarkable if you live in Seville, Phoenix, or Timbuktu. The trouble is that buildings here are not designed to take this sort of thing. Few have air conditioning.
The huge glass windows on my office building were probably thought very stylish when it was built in 1970. It was a Nuclear Physics Laboratory built for the nuclear age, when electricity was going to be too cheap to meter, so no need for insulation. It used to get very cold in winter, then a few years ago, the management looked at the metered heating bills and calculated it was sensible to fit double glazing. It now doesn’t get so cold in winter. Instead it gets unpleasantly hot in summer. I knew from experience that on Monday morning solar radiation would have heated the interior of my office to an unpleasant level even before I could get in and open a window. I decided to work from home.
In these high temperatures, we see lots of physicists coming out on social media to tell you what you need to do to stay cool. Then we see more physicists coming along to explain why they are wrong. The discussions get a bit get heated. After all, when it’s too hot to work, you may as well get into an online argument. The trouble is, the best practice for keeping cooling varies enormously depending on the local climate and design of your house or apartment.
One piece of good advice is to not leave windows open in the middle of the day. Let the warm air in and it just gets hotter. This is very true, but only as long as it is colder inside—valid enough if you live in a large building with enough heat capacity that it takes time to warm up. When I lived in the South of France, the houses had thick stone walls, and heavy outside shutters on the windows. In the middle of the day, you would just retreat inside and enjoy the cool, then come out in evening for a pastis on the terrace before dinner.
However, when bright sun shines on the bare unshuttered windows in a small house like my English home, it very quickly warms the interior. You learn that when going out on a hot day, it’s best to leave windows open, otherwise you will return in late afternoon to find the house is still hot even after it has cooled down outside.
But what if you’re at home in the middle of the day? There is a logical case to close windows once the outside temperature rises about that inside. Well this week I had the opportunity to do the experiment. With temperatures rising to 36°C outside, with my windows shut, the inside temperature reached 29°C. On opening the windows, it rose to 31°C. However, it was much more pleasant being in 31°C with a hot breeze blowing than a 29°C stuffy space, so a decided to keep them open.
But that was definitely a bit too hot. I needed some to use some further physics to let me properly chillax. A very useful principle to apply is latent heat of evaporation – the energy required by a liquid to transform into a gas. This is a not inconsiderable amount. The practical consequence is that if you wrap a wet towel around your head, it will cool you nicely as the water turns to vapour.
That is assuming that you are in a dry environment with enough ventilation to disperse the vapour. It won’t work very well in the humid heat you get in summer in Tokyo or New York. Or in a sealed room. However while my house was scorching, it was dry and well-ventilated. A wet towel would do the trick, but I had another idea: I would give my unicorn a bath.
The unicorn I refer to is a 90cm tall plush Twilight Sparkle who shares my house. She is not the sort of quality custom made piece that committed fans with good taste would pay significant sums of money for, proudly show off on their Instagram feed, and then keep in a glass case. She is instead the sort of random merch that a fan like me might impulse buy on noticing that the local department store had big ponies in stock. She was an alicorn once, but the wings came off.
Despite her less-than-royal appearance, Twilight was a comfort when I was stuck at home during the lockdowns. And she makes a good foot stool. However, she had recently started to smell a bit like sweaty feet and she was in need of a good clean. I had been aware of this for some time, but it was a job that I was inclined to put off. She is too big to fit in the washing machine, so would need to lie down in the bathtub, then, as the volume of unicorn filled with water as those plush fibres wick up all the liquid, she would become too massive to peg to a clothes line. Yet waiting for her to dry by evaporation could take a while. Except on a day like Monday.
Ponies are made of fluff. Fluff attracts water. Water molecules stick on to the fibres and creep over the surface of each one. Each water molecule attracts more water molecules and they pull each other up the fibres, overcoming the downwards pull of gravity. Then they collect in the hollow channels and air pockets deep in the fluff. Do not underestimate the power of fluff.
So, as the outside temperature maxed at 38°C and the warm breeze drifting through my windows sent my thermostat thermometer up to 32°C, I let Twilight soak in the cold tub (with a dose of lavender bath foam), shampooed and rinsed her mane and tail. Then as she relaxed in a towel on my bed in front of a fan, I could lie back with my head on her flank. The water evaporated from the fluffy pockets inside her, vanishing into thin air, taking the heat with it, leaving us both cool.
By Wednesday, the temperature had fallen back to more normal levels and life returned to normal. Maybe I should look into installing a JWST-style thermal shield, or at least fitting some shutters or blinds to my windows, before the next heatwave arrives.
Window film. Worth its weight in gold foil.
A creative homemade evaporative cooler!
I'm sure there's a "how G5 came to be" joke to be had somewhere.
My apartment in California has no A/C but it's shaded by redwood trees so it rarely gets very hot inside even on a 38C day. During midday, I agree the best thing to do is to close the blinds (which function as shutters) and block out sun as much as possible.
I'm always fascinated by how building design can be optimized for passive cooling/heating, which I've researched about a lot in my free time. You mentioned the difficulties of passive cooling in hot humid climates--it's tricky, but can be done! Older homes in the US also had high ceilings to reduce the heat, and transom windows to maximize cross-ventilation. The traditional shophouses of Singapore/Malaysia is an example of adapting the British colonial townhouses to the unique climate. This study showed a small 1-2C reduction in indoor temperature in traditional shophouses. With a more modern understanding of building science I'm sure we can develop homes that are even more effective at passive cooling, such as this one in the Phillippines.
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While window films do block solar heat gain, they're a fairly temporary installation since uneven heating of the glass can cause the window film to lose adhesion and "bubble" off the window. I saw this at one of my projects at work recently.
I always thought Twilight was cool, alicorn or not.
Living in hot climates all my life, I've gotten used to houses that are well-adapted to the climate to the point that I don't give it much thought. Yes, as a Californian, I'm guilty of the unthinking reaction, "104°? That's not bad!" As a kid, living in the Sonoran desert, I recall going out and playing in that sort of weather without thinking about it.
We were scheduled to be in Oxford last week for a summer course, but cancelled. (Not due to the weather, but because of Covid numbers and airline chaos.) Talking with my friend there, and listening to her horror stories, I think now that we possibly had a lucky escape rather than a scheduling misfortune.
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A bit short of gold at the moment. Might have to settle for aluminium.
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It's fascinating learning how different cultures in very hot or humid countries designed their buildings and towns to keep cool. I was reading how in some Arabic countries, when air conditioning arrived, the urban planners suddenly had more flexibility and usually chose to follow western models, but now they are moving back to traditional designs.
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Yes, last week, with the heat and all the Brexit-induced border chaos, on top of covid, it was probably not the best time to visit the UK. Hope you get here at some point in the future.
Got me when theyre going on about thick walls for insulationa nd sash windows for ventilation. The main ventilation needed is reopeneing the eaves in the roof space so the 50C plus roof heated air can be expelled one way or the other instead of the whole roof space acting as a wasted denyed integrated solar thermal panel?
Also helped that old UK victorean terraces had buried cellars for cooling and coal, energy storage. thats an a whole bunch of other design considerations in the old rooms can be tweaked to give all sorts of positives in modern sensibilities, like diffuse lightning, pplenty of reachable power points, etc?
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We're planning on it next summer, but to be honest, we'll probably fly into Paris and then take the Eurostar.
Alternative solution: buy more ponies.
I do not have airconditioning. When it goes above 40C, I have to take a shower, not towel off, but soak a towel, get on top of the bedding and put the wet towel over myself with a fan blowing onto me to sleep. Here is a video about keeping efficiently cool: