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.

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.

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

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.

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!)