Keeping cool without sweat

Note for new readers: this blog presents a rather technical approach to the subject of living comfortably in the tropics. It aims to teach you about tropical comfort and discomfort. Don't read this blog unless you got a "B" or better in high school physics or are an architect/builder who needs to understand heat transfer! If you're more interested in tropical lifestyle matters, the ups and downs of living in paradise, you can check out the blog by my brother Jack at:

http://www.nationmultimedia.com/webblog/home.php?uid=361

One of our readers, reflecting on the pleasures of keeping cool using one of those "mist fan" machines while struggling to enjoy a good cigar, raises the following question:

There has to be some balance point here, where body cooling by external evaporation is such that the body does not need to sweat heavily and so is happy in a high humidity atmosphere. These machines do work, the only problem I have with them is keeping my cigar alight...........

Yao Bar at sundown, your author making the sketch that you see here

Appropriate meditiation on this matter requires a proper setting, and I write here from one of my favorite streetside venues, the Yao Bar off Ekamai, where the sun is setting in a tawny sky at the end of a day warm enough to raise a slight sweat as I was walking here. Now the waitress is serving a large (22 oz) bottle of cold Chang beer, 70 baht, over a large glass of ice cubes (3 degrees C, but my radiant thermometer is probably mistaken here because it's mosquito brain does not really know the exact emssivivity of ice, or as you can see, of the sky which of course is not 6 degrees but closer to 30 degrees, only emitting like a 6 degree object). Here, carressed by a gentle breeze, watching life, much of it casually and lightly clothed as tends to be on these sois, I am comfortable enough, not sweating at all, though I am continuing to produce, as we all do while engaged in these indolent activities -- and subsisting on a healthy Thai diet of fish and rice, not fatty British pub food simultaneously engaged in heated discusion of such matters as cricket and motor racing-- heat. And my heat production is likely to be around 50 watts.

Now sitting here I perceive that I am comfortable but on the verge of sweating. (At this point I can only appeal to the reader's intuition when I say comfortable, since I can neither define that term or conversely define its opposite, uncomfortable, though in the future we will necessarily have to face this definitional task, not as simple as it might first appear).

Let us now proceed to make some estimates of the transfer of heat that is occurring at my bodily envelope (having disposed of the problem of dealing with transfer of moisture, since I'm not sweating, though almost sweating).

Since I am warmer than the air around me by about 3 degrees, and the transfer of heat being a linear function of temperature difference with a constant of proportionality of about 10 (for a pleasant breeze) my transfer via convection to the air will be in watts

W=10*(34-31)=30 watts

I am also radiating heat directly to both the "cold" sky at 6 degrees and my other surroundings, and without going through l the tiresome arithmetic, Stefan Boltzmann constants and so forth (unless I later add an appendix to this chapter), my estimate for radiant loss is a minor 10-15 watts.

The 22 ounce bottle of Chang beer which by ingesting I am raising from 18 to 37 degrees over a time period of 20 minutes provides further cooling of about 5 watts, not counting the vasodilative impact of alcohol on improving the blood supply to my nose. I neglect my cigar, and I am now too weary with these exertions to account for respiratory loss, either sensible or latent, though I think it might be as much as a few watts.

Observation of surroundings: the waitress is hacking at a mango in the course of making herself som tam dinner; though lightly dressed her skinny brown legs to which my eyes are drawn are doubtless more effective in transferring heat to the air than my ham-like euro limbs. The bartender, who has a large belly, is engaged in mild activities, and has elected for his comfort to be shirtless. Altogether, we represent a sample of three varied non-sweating bodies, with heat production approximately equal to the rate of heat being lost to our environment. Therefore we meet a working definition of comfort with sweating:

Energy produced sweatlessly=energy lost without need for sweat.

On this basis, I propose that for a typical fahlang, sweating will not be necessary for sedentary activity with surrounding temperature (e.g. air temperature but also consider surrounding radiant temperatures) of about 30 degrees or a little less. Under these or cooler conditions, we do not call on any heat dissipative mechanism that is sensitive to humidity, so we may say in a gross sense that humidity is not a factor in our comfort. If however our level of effort should increase to say 100 watts, we will not be able to throw off enough heat to the surroundings and after a few minutes our body and skin temperature will begin to rise and our sweat system will activate, with the rate of cooling being very sensitive to humidity (more on this later). And we will begin to say that we are too warm (suggesting that sweat = thermal discomfort.)

So my answer is this: humidity doesn't matter too much (except in relation to such issues as keeping cigars lit and beers cool) as long as we are not sweating, ie at a temprature of roughly 30 degrees in sedentary mode.

Thin Roofs...and foil

Even now at the winter solstice, with the noon solar radiation reduced some 20 percent, any thin darkish thin surface (such as a metal roof or a tile roof) heats up to 50 degrees C or more, with the heat being almost immediately transferred undiminished to the underside, as you can see from the temp reading of the underside of this rubberized canvas tent which is supposed to provide cooling shade but really provides heating shade which will radiate on the order of (52-32)*5=100 watts per square meter to my balding head, plus perhaps as much again convected to the air below. Not as bad as the 700 watt sun, you might say, but not comfortable either – and suffocating if the underside of the roof is not radically ventilated. So next time you are sitting under one of these tents at a Thai wedding or funeral, be warned -- it's the heat, not your emotions, that is causing you to feel like you are going to faint.

Here is an excerpt from the manual of the Australian company Bluescope Steel who make (and promote) steel roofing in Thailand. This indicates a good reduction in heat transmission through thin roofs (steel, cement tile, etc) with foil insulation, properly installed (air space above and below foil, and foil with shiny side down.) For a dark roof, the thermal transmission is said to drop from 165 to 22 watts, almost 90 percent.

Foil sales folks must be delighted with this conclusion. But be aware that it contains at least one hidden but important assumption -- very low convective transfer from the roof to the room below because the air beneath the foil is taken to be perfectly still. But this is not true in any case for many reasons. Convective cells develop even in sealed spaces, desirable fans move air, and required ventilation to replace that air which will otherwise heat at about 2 degrees per minute (ventilation rate of approx one room air change per minute--that's right, not one or two air changes per hour, as in most attic spaces even with vents) so there is in fact much more downward convective transfer from the roof than this chart suggests.

My estimates do not yield quite such promising results – my 1935 Eshbach Engineers Manual (college edition) indicates an R value bonus of only about 2 or so for a single layer of foil. And a direct measure of the efficacy of shiny vs. black spray paint surface on the foil which can be seen in the simple room simulation which I performed last month yields a reduction of about 50 percent in heat transmission. This implies that the heat flux from downward radiation and convection are about the same. Th foil brigade will try to tell you that here is not convective transfer downward at all, and this just ain't true.

Want to know the exact ruth? The fact is that the physics of downward heat flow in roofs is extremely complex, the stuff of learned papers by mathematically oriented professors.

But I don't question that properly installed foil will improve matters-- from very bad to not as bad--in the case of a thin tile, cement, or metal roof. Studies such as those done in Florida suggest aircon energy savings of perhaps 15 percent with the addition of foil. My caveat is that in most practical situations one cannot rely on foil alone to prevent uncomfortable heat buildup from a foil-insulated roof.

Some reasonably unbiased web sites describing use of foil insulation are:

Florida studies indicating reduction of energy 25 percent and temp 6 deg:

http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1231-01/index.htm

Wikipedia:

http://en.wikipedia.org/wiki/Building_insulation

Q&A on foil insulation by experienced researchers:

http://www.fsec.ucf.edu/en/publications/html/FSEC-EN-15/index.htm

Night before Christmas

Some think of Santa. I dream of Planck's Constant.

I put a steel bar outside on the roof at night, facing upward into the relatively clear sky (stars visible), compare its temp with the temp of a shielded sensor; you see, on December 23-24, 2007, a temp suppression of the bar of only about one degree. This is with a sky radiant temp (Radioshack sensor) of about 5 degrees and relatively high dew point of about 21 degrees. Notice the wave of humidity at about 2 to 4 am, dew point reaches close to air temp, so RH is close to 100 percent and the temp suppression becomes much less (in part because water begins to condense on the steel surface, creating heat.). Therefore we cannot get much radiant cooling under these circumstances of high air saturation.

It happens that I did not in my bed dreaming of sugar plums notice any suffocating wave of humidity at 2-4 am, perhaps it did not arise in the indoor environment. Why? Or was it the low temp, not over 23 degrees, that kept my body well below the point of perspiration? All worth investigating as the hot season approaches and the night temps hover up at around 30 rather than this month's more comfortable mid twenties.

Shortest day

For millenia we northern folk have been mighty nervous about these shortening days of winter and have invented all kinds of potions and ceremonies to ward off the fear that the days were going to get shorter…and shorter.. and God know what then. So we have mulled wine and moldy grain (which is what Tacitus described as the favored beverage of the barbarians of the north) and those big stone henges in Ireland and even Peru where the priests coax the sun back from wherever it has wandered off to, and nowadays Prozac for Seasonal Affective Disorder and doubled up holiday AA meetings for those who overdid it in Christmases past. Here in Thailand it was April, not Decmber, that was the big deal in the old days, but this hasn’t prevented the Thai from going mad with joy over Christmas and New Years as you can see from this little video clip I took last night in the Emporium. Notice at the end that it is against the law to photograph Santa Claus here in the land of smiles, leaving poor Santa looking like some kind of frightened and much overdressed hostage imprisoned here far from his native latitudes.

With the sun vacationing somewhere down in Australia some 36 to the south, solar radiation, as you can see from the readings of a solar radiometer at Bnagkok's Mahidol University is now peaking at about 750 watts per square meter.

http://nanotech.sc.mahidol.ac.th/weather/Weather_Soil_Experiment.htm

As always you correspondent finds the season an emotional challenge, taking comfort that though the days are a little shorter they aren’t a lot shorter. This morning, in fact, the sun rose over Bangkok at 6:30 am, and tonight setting at 6:00, as I make my way to the pub for what I hope will be a modest pint or two, leaving the sky over Sukhumvit washed with a smoky plum, when I take this photo thirteen minutes later.

But where are those refreshing cold snaps that are supposed to be blowing down from the north? This year you hear Thai complaining of the heat and muttering about global warming, with the wind even now backing around to the south and the temps creeping up into the low thirties, a preview of the coming hot season.

Red wine in them hills

Jack and Dah had me over for dinner, quite a nice pasta with little sea clams, plus a red wine that Jack said was a local Thai wine, 600 baht a bottle.

It was certainly drinkable. We did a second bottle. I thought about this. "Don't wine grapes have to grow in a cool place?"

"That's it, this stuff is only 100 clicks from Bangkok, it's a kind of hill station. Look at the bottle."


"Fourteen degrees latitude, that's not far."

Jack said "About three hundred meters above sea level. Cool nights, not far from our village. We're at two hundred." Jack and Dah were building another house in this little Isan village they had discovered, four hours from Bangkok.



Now after the second bottle Jack was mad to grow wine grapes. He wanted to know all about the climate there, he wanted me to compare Bangkok, and the PB wine growing place near Khao Yai, and his new village at the foot of the same hills a little to the west of Khao Yai. The next week, mid December, we took the bus to a little place south of Korat called Pak Tong Chai, near where Jack and Dah were building their house. An uncle picked us up and drove us around, we stayed a couple of days. The mornings were delightful, dew on the grass and nice little jungly tropical mists over the patches of rice paddy.


I took temp measurements with my roving Hobo, compared them with Bangkok. Here they are.




Cool nights indeed, you can see the difference at night, I shaded it blue. Sundown the temp dropped a lot up there, much more than Bangkok. You could se stars at night.

Sunday roast

This Sunday afternoon I was alone and I took to the streets walking, no more than a slight film of sweat on the brow on this fine California-like afternoon, then sitting in the park for a while then consumed a giant roast pork dinner and the Financial Times in the Robin Hood.

The steady graphed points are a recording of the temp and dew point on my deck at home, beneath a large mango tree. The irregular traces are a recorder that I carried with me, you can see every time I go into an air conditioned space where the temp and dew point both drop a few degrees. Consultation with Carrier's chart indicates that each cubic meter of conditioned air has been subjected to about 10 KJ of enthalpy reduction--at considerable energy cost.

Sunday afternoons bring a few families out and into the pubs, you can see this sixty-something guy with his thirty-something Thai wife and "half child", the dream of many a Thai girl. Some of the foreign-Thai relationships are disastrous for the man, but this one looks pretty pleasant for all, arguably more interesting than playing golf and visiting prospective nursing homes back in the old country (or obsessively recording temps in the new), and certainly a step up in comfort and respectability for the girl from an poor Isan village.

Interesting stuff blows in from Isan

Sometimes for relaxation in the cooler evenings now I like to go down to Soi Cowboy and watch the world go by, just sitting at a little bar by the side of the road, watching tourists and bar girls, I try to figure the girls that are fresh from upcountry from the ones who say they are fresh from upcountry. They blow into Bangkok, the prettiest girls from the villages, dozens a day, on buses to Mo Chit and trains to Hua Long Pong, and a lot of them drift over to this street and the other two or three venues that cater to the tourists looking to see Bangkok's notorious sex scene. There they wait by the side of the road, in front of their clubs, like trout waiting under the banks of a stream for passing grubs.






This time of year the wind is from the north to northeast too, and along with the girls comes the cool air of the north, equally delightful, the air warming like the girls themselves as it blows out of Udorn and Korat and Surin, down from the hills, across the plain, and into the wicked concrete alleys of the town. The journey takes about a day, and you can see from the twin pictures here of parallel temp and dew point of November air in Bangkok and Korat, 165 to the north. The air moisture stays pretty much the same, but you can see the arrival of the air in the big city heats it up by about 4 degrees C. Even so, most Bangkokians will be complaining of the cold, and if they venure to the northeast, they will make special purchases in fahlang thrift shops or expensive department store, depending on their situation in life, of "winter" clothes.





Oh, I should note that not all of them are girls.