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Where is the hotter substance? (Dew condensing out of the air onto a lawn)

  1. Jan 9, 2019 #1
    1. Let us postulate a warm, muggy, summer’s night on your front lawn. The air is gently settling onto the cooler grass, generating a sparkling coat of dew.

    2. Conventional condensation theory tells us that as the liquid dew condenses out of the humid air, a quantity of latent heat is released to that surrounding air. This heat either warms that air or causes it to cool at a lesser rate[1].

    3. Thermodynamics tells us that heat can only flow from a hotter substance to a cooler one. No other form of heat flow is possible[2].

    4. Where, then, is that hotter substance that is warming the air? The overlying atmosphere is definitely cooler. We have postulated that the grass is cooler. The droplets of dew are most likely at the same temperature as the grass. In any case, if the dew droplets were warmer than the air, we would have net vaporization and not net condensation.

    5. I repeat, where is the hotter substance that is warming the air?


    [1] R. R. Rogers, M. K. Yau; A Short Course in Cloud Physics; Third Edition; Elsevier, New York, 1989.
    [2] Charles Kittel; Thermal Physics; John Wiley & Sons, New York, 1969.
     
    Last edited by a moderator: Jan 10, 2019
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  3. Jan 9, 2019 #2

    russ_watters

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    No, the latent heat is released onto the grass, heating the grass/causing it to cool at a lesser rate.
    The latent heat of condensation heats - or rather slows the rate of cooling of - the grass, which then slows the rate of cooling of the air. Through this process, the grass is always cooler than the air.
    As you said, the air is not being warmed it is being cooled.

    This isn't fundamentally different from what happens on the surface of cold glass of water, it's just that the transience of the situation - the continuously changing temperature - makes it harder to follow.
     
    Last edited: Jan 9, 2019
  4. Jan 9, 2019 #3
    Russ: Could you please provide a citation supporting your statement. My source says the latent heat warms the air (page 21). In any case, your response begs the question. Where is the hotter substance that is the source of this heat?
     
  5. Jan 9, 2019 #4

    russ_watters

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    The example of steam burns was given to you in another thread:
    https://www.nature.com/articles/s41598-018-24647-x

    But I really think it should be self evident that the thing that the condensation forms on is warmed by the condensation. The cause-effect relationship is pretty obvious.
    Could you provide the whole quote and context please; I don't have access to the source. Perhaps you are misinterpreting it.
    The moist air and liquid water are the hotter substances.
     
  6. Jan 10, 2019 #5
    Russ: Could you please provide a citation supporting your statement.

    <The example of steam burns was given to you in another thread:
    https://www.nature.com/articles/s41598-018-24647-x>


    The statement in question was your assertion that the latent heat was transferred to the grass. Your reference to the hazards of live steam is completely irrelevant to that statement.

    I am still waiting for a citation.

    _________

    My source says the latent heat warms the air (page 21).

    <Could you provide the whole quote and context please; I don't have access to the source. Perhaps you are misinterpreting it.>

    “Pseudoadiabatic process

    If expansion continues after the isentropic condensation point is reached, condensation occurs and the released latent heat will tend to warm the air.” Rogers & Yau, p. 21

    I might add that the release of latent heat to the atmosphere during condensation is the basic principle behind the moist adiabatic lapse rate.
     
  7. Jan 10, 2019 #6

    CWatters

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    The latent heat can be lost to both the air and the ground/grass.

    On a still evening it's possible for water vapour to condense out in mid air forming a thin layer of mist close to the ground. If it's windy stirring prevents this stable layer forming.
     
  8. Jan 10, 2019 #7

    CWatters

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    I can't find a citation that says dew warms grass but it seems obvious. For example Its common practice for farmers to spray water on fragile crops to prevent cold weather damage. When the water freezes it releases heat which keeps the plant warmer than it would have been without the water. The ice also helps insulate the crop from colder air.

    It seems reasonable to me that condensing water vapour would have a similar effect.
     
  9. Jan 10, 2019 #8

    russ_watters

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    No, it's the same phenomena. An atmospheric pressure saturated steam burn does the burning because the steam condenses on your hand, transferring heat to your hand. Conversely, a person can easily handle exposure to a dry 100C gas probably for several minutes without injury.

    In any case, your request is so specific and obscure - condensation on grass - that it is unlikely there is a specific citation either way. But here's a generalized discussion of the heat transfer at the surface - any surface - on which condensation is occurring:
    http://www.thermopedia.com/content/652/

    A brief summary is that the heat transferred to the surface is equal to the mass of water condensed onto the surface times the heat of vaporization/condensation. This link is for the details of how to predict the rates from scratch (calculating the coefficients, mass transfer rate).
    Similar to your criticism of my citation, yours doesn't mention grass and near as I can tell isn't about condensation on a surface at all. Your citation is about condensation inside a cloud, where there is no surface - nothing but the air - on which the water can condense. It's a very different situation than condensation on a surface.
    Agreed. But again; there is no surface on which the vapor condenses, so it is very different from the situation we are discussing.
     
    Last edited: Jan 10, 2019
  10. Jan 10, 2019 #9

    russ_watters

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    Yes, it attacks the problem from both directions. The key issue for crops is you don't want air below 0C. The driver for a foggy morning is that the moisture in the air limits how fast the air can cool down. So liquid water on crops is at least 0C. And evaporating that water into the air, while it cools the water (but not below 0C), it limits how cold the air can get.

    A little tip for weather forecasting: deserts get very cold at night because they are dry and the moisture in the air doesn't inhibit the temperature from dropping. For more humid climates, the night's low temperature is likely to be roughly equal to the day's dewpoint. This of course assumes a consistent weather system.
     
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