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    • CommentAuthorEd Davies
    • CommentTimeJan 29th 2015
     
    In another thread:

    http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=13038

    Posted By: cjard: “Warm air holds more moisture, so heat the air and the relative humidity will fall. As others note, cold spaces act like dehumidifiers. A surface below twelve degrees c will be liable to condensation, and single figures a virtual certainty if air that is "comfortable to live in" comes into contact ith it.”

    While I sort of agree with the practical conclusions one might draw from thinking like this it seems to me to be a potentially very confusing approach and also from a pure physics point of view just wrong.

    Air doesn't “hold” moisture. Water vapour is a gas like any other gas and coexists with the others in the normal way. Air no more holds moisture than nitrogen holds oxygen, argon, carbon dioxide, ozone, methane or any of the other components of the atmosphere.

    The amount of water vapour (WV) present in a volume is largely independent of the other gases present. It only makes about 0.5% difference to the amount of WV if all the dry air is removed so that it's otherwise a vacuum.

    What does control the WV partial pressure is the temperature, shape of the surface of any exposed water and anything dissolved in it. There's a good explanation of the basics in the middle of this page:

    https://protonsforbreakfast.wordpress.com/2015/01/21/warmer-and-wetter-but-not-as-wet-as-i-thought/

    I prefer the term “equilibrium vapour pressure” rather than “saturated vapour pressure” for this reason: it puts at the front the fact that the amount of WV is controlled by the balance of the processes of evaporation and condensation rather than encouraging an image of the air as a sponge which can only hold so much water in the holes.

    Where the idea of the air holding water breaks down is when the surface of the liquid water isn't flat (e.g, when it's droplets condensed in a cloud with curvature one way or forming in the capillaries of wood or other materials with curvature the other way) or when it isn't pure (e.g., with salt particles forming cloud condensation nuclei dissolving in the droplets and becoming more dilute as the droplet grows).

    More relevantly to the original thread, it also mattes that the air and surface temperatures may be different. When you think about it as the surface temperatures controlling the water vapour content of the air the second part of cjard's quote above follows naturally, rather than being a special case.
  1.  
    <blockquote><cite>Posted By: cjard</cite>
    heat the air and the relative humidity will fall. </blockquote>

    <blockquote><cite>Posted By: Ed Davies</cite>
    What does control the WV partial pressure is the temperature</blockquote>



    Ed is correct in the case that the air is in contact with a large surface of liquid water such as the sea, such that it is at equilibrium ("saturated") so at 100% RH. Then when temp rises, more kilogrammes of liquid water will evaporate into the air, increasing the conc and partial pressure of the water vapour so remaining at 100% RH.

    Cjard is correct in the case that the air is not in equilibrium contact with a large surface of liquid water, such as inside a house. This air is often less than 100% RH. When temp rises, the actual concentration of water vapour doesn't change, but the [equilibrium concentration] increases, so the RH reduces.



    Concentration = mass of water vapour / mass of air x 1.6

    Partial pressure = concentration x barometric air pressure.

    Equilibrium concentration = concentration if the air were at equilibrium with liquid water - this increases with temperature

    RH = [actual concentration] / [equilibrium concentration]
    • CommentAuthorEd Davies
    • CommentTimeJan 29th 2015
     
    Posted By: WillInAberdeenEd is correct in the case that the air is in contact with a large surface of liquid water such as the sea, such that it is at equilibrium ("saturated") so at 100% RH.
    I think what I said is much more broadly correct than just in those conditions. Please tell me what I wrote that only applies with them.

    Whatever - air doesn't “hold” water vapour.
    • CommentAuthorrhamdu
    • CommentTimeJan 29th 2015
     
    Posted By: Ed DaviesWarm air holds more moisture, so heat the air and the relative humidity will fal


    I can live with air 'holding' moisture because the loose terminology doesn't lead to any false conclusions.

    My problem with that sentence is slightly different. People may very easily ignore the word 'relative', and conclude that air can be dried by heating it, in the same way as a wet towel can be dried in the sun. This has practical consequences: people throw heat at their condensation problems, not realising that the only place where more heat can help is at the surface where the moisture condenses.

    Put that another way. Heat air, and other things being equal, the RH drops. But all the moisture is still there.
    • CommentAuthorcjard
    • CommentTimeJan 30th 2015 edited
     
    Indeed that's why I include the word relative.. While I'm sure all here are correct in thei lengthy posts it's not something that could easily be written on a mobile device, nor absorbed by the OP. Within the context of his discussion (and there is a wider context that further reinforces the relativity of humidity discussed elsewhere within it) it's an acceptable simplification to assert that air is like a sponge that holds water, and the warmer the air the greater the potential mass of water floating around in it non condensing. It also then stands to reason that the greater the air temp the more likely a large delta t with surrounding surfaces (in a housing context) and consequent likelihood that the air adjacent the surface will cool, losing "water storage capacity" and giving rise to condensation on the surface

    Overall, I think the air like a sponge,higher temps are a stretch and lower temps a compression works quit well as an analogy. I'll be more explicit in highlightng the notion of relativity if it helps in future
    • CommentAuthormike7
    • CommentTimeJan 30th 2015 edited
     
    Quoting Ed:-
    " Water vapour is a gas like any other gas and coexists with the others in the normal way. Air no more holds moisture than nitrogen holds oxygen, argon, carbon dioxide, ozone, methane or any of the other components of the atmosphere."

    I don't think so - at earthly temperatures, water can exist as a liquid or a solid - the other gases you mention are only gaseous. At 20C you couldn't blow a balloon up with just water vapour.

    Not sufficiently engaged to know if this matters - just sayin'....
    • CommentAuthorsquirrel
    • CommentTimeJan 30th 2015
     
    Posted By: rhamdu
    Posted By: Ed DaviesWarm air holds more moisture, so heat the air and the relative humidity will fal


    I can live with air 'holding' moisture because the loose terminology doesn't lead to any false conclusions.

    My problem with that sentence is slightly different. People may very easily ignore the word 'relative', and conclude that air can be dried by heating it, in the same way as a wet towel can be dried in the sun. This has practical consequences: people throw heat at their condensation problems, not realising that the only place where more heat can help is at the surface where the moisture condenses.

    Put that another way. Heat air, and other things being equal, the RH drops. But all the moisture is still there.

    Or maybe heating the air up actually increases the condensation problem - the RH will increase with the next boiling kettle and still not feel 'humid', but the walls/windows stay colder.
    • CommentAuthorEd Davies
    • CommentTimeJan 30th 2015 edited
     
    Posted By: rhamduI can live with air 'holding' moisture because the loose terminology doesn't lead to any false conclusions.
    As I explained, I think it could lead to false conclusions.

    Posted By: rhamduPeople may very easily ignore the word 'relative', and conclude that air can be dried by heating it, … Heat air, and other things being equal, the RH drops. But all the moisture is still there.
    Yes, easy to imagine somebody being confused in that way.

    Posted By: cjardWhile I'm sure all here are correct in thei lengthy posts it's not something that could easily be written on a mobile device, …
    Yep, it's difficult to know how to describe what's actually happening concisely - I'm sure that's at least part of the reason for the persistence of the sponge analogy from those who (should) know better. Still, I think it's better to avoid giving an impression which is actively misleading in all but the simplest cases.

    Posted By: mike7I don't think so - at earthly temperatures, water can exist as a liquid or a solid - the other gases you mention are only gaseous.
    But it doesn't stop WV acting as a normal gas at the partial pressures that it does exist at. It's the wall (or condensation nuclei or whatever) that suck the vapour out of the air, not the nitrogen and oxygen which squeeze it out.

    Oxygen isn't only gaseous at normal Earthly temperatures. It can be part of Fe₂O₃, for example. Oxygen couldn't exist in the atmosphere for long in the quantities it does without complex entities actively replenishing the supply - it'd quickly oxidise away. Still, nobody says that the air can only “hold” 21% oxygen - even though that's the current equilibrium level.

    https://en.wikipedia.org/wiki/Great_Oxygenation_Event

    Posted By: squirrelOr maybe heating the air up actually increases the condensation problem - the RH will increase with the next boiling kettle and still not feel 'humid', but the walls/windows stay colder.
    Indeed, warmer air could easily increase evaporation from other sources.
    • CommentAuthorrhamdu
    • CommentTimeFeb 3rd 2015
     
    Posted By: Ed DaviesWhere the idea of the air holding water breaks down is when the surface of the liquid water isn't flat (e.g, when it's droplets condensed in a cloud with curvature one way or forming in the capillaries of wood or other materials with curvature the other way) or when it isn't pure (e.g., with salt particles forming cloud condensation nuclei dissolving in the droplets and becoming more dilute as the droplet grows).

    OK Ed, maybe there are practical consequences - though most of those examples are more relevant to the outdoor environment.

    Indoors, one particularly important equilibrium is that between water vapour and the cytoplasm of moulds. Moulds can absorb enough water from the vapour phase even when it is not condensing on neighbouring surfaces. I have seen it said that mould can grow at "x% RH" (can't remember x). But, does anyone know, is x even remotely independent of temperature? If so, then relative humidity really means something, because mould growth is the main reason for limiting humidity in UK homes.

    (In hotter climates, and occasionally in the British summer, human comfort also becomes an issue. May discuss that, and the relevance or not of RH, in another post!)
    • CommentAuthorEd Davies
    • CommentTimeFeb 5th 2015
     
    Sorry, no idea about that mould and temperature/RH thing.

    However, the water content of wood is very dependent on RH and is not much affected by temperature. That's an important indoor example of the curvature of the surface being relevant. It's the meniscus in the small capillaries in the wood that changes the equilibrium vapour pressure as the concave surface allows the surface tension to hold on the water molecules better.

    One could speculate (somewhat wildly) that mould has similar effects, whether actual curvature or something analogous in the cell membranes.

    Posted By: rhamduIf so, then relative humidity really means something, …
    I don't think anybody doubts that, as the wood example shows. Having significant RHs other than 100% (e.g., about 95% for wood absorption, some lower value (70ish %?) for mould growth) reinforces, rather than contradicts, my point that it's better to think about the equilibrium on the surface rather than how much the air can “hold”.

    RH basically expresses the way the absorption and evaporation processes scale with temperature relative to those over a flat surface of pure water so it's not terribly surprising that other surface conditions are more dependent on RH than they are on temperature.
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