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Thread: Hot, humid air

  1. #1
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    Hot, humid air

    What percentage of air with heat and humidity both at 100 (American) would be water vapor? And BTW, would it be survivable?
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  2. #2
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    There's a calculator here. Plug in your numbers, and you get a partial pressure of water of 1.93 inches of mercury in a standard atmosphere of 29.92 inches of mercury. I'll leave you to do the division.

    A wet-bulb temperature of 35 degrees Celsius (95 in US numbers) is generally considered to be the maximum survivable combination of heat and humidity, though of course it would take you a while to die.

    Grant Hutchison

  3. #3
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    From a table in Wiki I find about 6.5% by volume, which is about 4% by mass. I would worry about long term survival, because you would be unable to cool your body by evaporating sweat. It would be like having a fever all the time.

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    Has this ever happened in RL? I suppose it would be a disaster but I’d there such a historic disaster?
    I read once that somewhere at sometime (I am tempted to say Portugal in the 1930s but don’t quote me) there was a freak heat wave of two minutes at 158F/70C at some village. Is this an urban legend?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

  5. #5
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    See my link. The wet-bulb physiological limit has been exceeded several times, but for periods of only 1-2 hours, which (it seems) is survivable. But nothing as severe as you seem to want.
    There are a number of claimed "heat burst" events, including the one in Portugal you mention, but these are associated with a fall, rather than a rise, in humidity.

    Grant Hutchison

  6. #6
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    It could certainly happen in certain industrial situations, but I don't know about under natural conditions.
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  7. #7
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    Quote Originally Posted by grant hutchison View Post
    There's a calculator here. Plug in your numbers, and you get a partial pressure of water of 1.93 inches of mercury in a standard atmosphere of 29.92 inches of mercury. I'll leave you to do the division.

    A wet-bulb temperature of 35 degrees Celsius (95 in US numbers) is generally considered to be the maximum survivable combination of heat and humidity, though of course it would take you a while to die.

    Grant Hutchison
    It may, of course, take somewhat less time for you to wish you were dead.
    Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.

  8. #8
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    Been There, Done That

    Was stationed at Subic Bay, P.I., in1969. One pleasant evening it was 98 F (36.67 C) and raining. Wet bulb temperature unknown. As mentioned above, evaporative cooling does not work. So . . . you are going to be wet no matter if you are under roof or not Just stand in the rain works. From experience, it works.

    Big Don may have had similar experiences, along with others who have lived in the tropics. Remember to change clothes at first opportunity. Chafing is worse than the heat.

  9. #9
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    The day I arrived in Darwin Australia it was low 40s C and that afternoon and night there was a big thunderstorm. Luckily the hotel had good AC.

  10. #10
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    Is foehn wind a heat burst?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

  11. #11
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    Quote Originally Posted by Tom Mazanec View Post
    Is foehn wind a heat burst?
    No. They're kind of opposites, in some ways.
    In the Föhn Effect, the wind moves wet air up a mountainside. It cools slowly with increasing altitude, because it's warmed by the latent heat of condensing water vapour. The water vapour rains out, the now-dry air crosses the mountain ridge and descends the other side, and (with its water content much reduced) warms much more quickly with decreasing altitude. (Bit of a simplification--there are other contributing factors to the process.)
    Whereas a heat burst involves rain falling into cool dry air aloft. The rain evaporates in the dry air, cooling it further as it extracts latent heat. The cold dense air falls out of the sky, warming rapidly with decreasing altitude (because all its water is already vapourized), and ends up moving so rapidly it shoots below its equilibrium height, warms even more, and then hits the ground.

    Grant Hutchison

  12. #12
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    I researched comfort in combinations of temperature and humidity for people floating supine and established a curve where higher relative humidity needs lower temperature. In looking at the prior art there has been work on discomfort and loss of performance in, for example pilots suffering hot humid conditions. When you cannot get rid of your metabolic heat , you are in danger, obviously. We can be comfortable in 100% RH at say 90 F, doing nothing, but physical activity would drive down the acceptable temperature.
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