My mother called it "sticky", and it's a subject that we've been avoiding here, hoping that the weather would just stay cool enough. But when the Don Muang temp hits 29.5 as it did the last couple of nights we just can't seem to convect and radiate that heat away, and the dreaded sweat begins to bubble from our pores and we have to get our hard little pillow, however full of mold it may be (according to our faithful correspondent wch). The other day I took a wet washcloth out on the deck in the shade with a nice breeze and let it dry a bit. It was ten inches square and after an hour it had lost 8 grams of moisture, that's about 5 watts of cooling power, equivalent to 80 watts per square meter.
Depends on wind and humidity, you're thinking, so here's a formula to figure it for whatever you like:
| Evaporation in tropics, grams | |||
| E=(25+19*V)*(30*(1-RH)/3.6/1000)*A*T | |||
| E=evaporation, grams | |||
| V=wind, m/s | |||
| RH=relative humidity | |||
| A=area, sm | |||
| T=time, sec | |||
And here is a handy little graph of that formula:
Now you might have the same problem that I do here, because when I try to find a rate of 80 watts per square meter on this grpah, I don't. So maybe I'm a lousy experimentalist, or the formula, which is supposed to be for open pan evaporation, doesn't work for wet washcloths. I'll have to look into it. But what you can see in general here is what you know already, that is, when you begin to sweat to the point where half of your boys is soaked, and you're in a good breeze on a hot dryish day, you get a lot of cooling power, enough to take care of the 200 watts or so that you generate when excercising.
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