Do you use electric clothes dryers?

YoungBlueEyes

That was simply and beautifully put, thank you grumbler.  I've got it now

Bendy_House

grumbler said:

  Yes, beer from the fridge is colder, but not the windscreen that has the same temperature as the surrounding air (unless you switch on the aircon inside).

Interesting.

I'm going to guess that the screen on the inside has condensation on it because the screen IS colder than the surrounding air. The outside surface of the screen will be cooled by evaporation - a bit like if you blow on wet hands, the effect is cooling. The colder window will encourage condensation to form on the inside, even if the RH inside is very low.

Bendy_House

YoungBlueEyes said:

Yep my brain loves science, it's fact-loving and logical

Ok I've got the moisture/humidity thing now, but (and I nearly mentioned this last night) but what about temp + condensation?  If an open window is letting in colder air, wouldn't cond form as the house will be warmer? Even in an unheated room isn't there a natural... seepage..? of heat from other rooms in the house, so incoming external air will be cooler...?

ETA Thanks for taking the time on this guys, I appreciate it

Yup - counter-intuitive! And I had to try it to believe it.

Yes, there will be some seepage from the warm & moist house air into an unheated room, and if that room is both unheated and unventilated, you can expect condensation and mould to occur in there over time. This will be on the usual colder areas, such as window panes (coldest spots) and in the upper and lower corners of rooms (where there is the least natural air circulation). Happens behind free-standing wardrobes too, as there is much less air movement there.

As soon as you ventilate the room, the air flow will bring that cool room down to roughly the RH of the outside, and that's enough to keep the room dry. Cold, but dry. It'll cope with any seepage. It would likely cope with even a lot of warm, moist air getting in, from, say, the room door being left open, but that would obviously be a waste of energy.

Anyone who wakes up to condensation formed on their window panes should only have to try this once to be convinced - turn off the heating in the bedroom (except to the usual safe 'frost' setting), crack open the windows, leave the door shut, and dive under the covers.

Wake up to a cold - but dry - room. Push partner out of bed to shut the window and turn up the rad.

And put on t'kettle.

YoungBlueEyes

I didn’t realise about RH and AH (my fact for today), that explanation allowed my brain to get it.  

It does all sound counter-intuitive! 

Oh and sorry OP for taking your thread off on a tangent. No I don’t have an electric airer. 

grumbler

Bendy_House said:

grumbler said:

  Yes, beer from the fridge is colder, but not the windscreen that has the same temperature as the surrounding air (unless you switch on the aircon inside).

Interesting.

I'm going to guess that the screen on the inside has condensation on it because the screen IS colder than the surrounding air. 

If it's only inside, that's because of higher humidity resulting from breathing - right now or yesterday (or if you leave something wet to dry inside).

The outside surface of the screen will be cooled by evaporation - a bit like if you blow on wet hands, the effect is cooling. The colder window will encourage condensation to form on the inside, even if the RH inside is very low.

Well, inside it's either (a) condensation or (b) evaporation or (c) balance.
(a) - the glass is getting warmer as a result of condensation (opposite to evaporation in your post)
(c) - the glass has the same temperature
(b) - the only reason for evaporation is that the air inside or the glass is getting warmer, e.g. from the sun rising. That's not the condition when condensation outside normally happens.

YoungBlueEyes

I conducted an experiment this morning.  It's humid and breezy here, and dark when I did this (5am) so the sun couldn't complicate matters.

I've a patio/flagstone area at the back door.  It was dry when I got up even though it has been a humid night. I threw a cup of water over it and it was dry when I checked it a few mins ago.

So drying is more of an air moving thing, rather than a heat/humidity thing (as in there was plenty of humidity and no real heat, but it dried anyway.  Not the most rigorous experiment, but them's me findings. 

Bendy_House

grumbler said:

Bendy_House said:

grumbler said:

  Yes, beer from the fridge is colder, but not the windscreen that has the same temperature as the surrounding air (unless you switch on the aircon inside).

Interesting.

I'm going to guess that the screen on the inside has condensation on it because the screen IS colder than the surrounding air. 

If it's only inside, that's because of higher humidity resulting from breathing - right now or yesterday (or if you leave something wet to dry inside).

The outside surface of the screen will be cooled by evaporation - a bit like if you blow on wet hands, the effect is cooling. The colder window will encourage condensation to form on the inside, even if the RH inside is very low.

Well, inside it's either (a) condensation or (b) evaporation or (c) balance.
(a) - the glass is getting warmer as a result of condensation (opposite to evaporation in your post)
(c) - the glass has the same temperature
(b) - the only reason for evaporation is that the air inside or the glass is getting warmer, e.g. from the sun rising. That's not the condition when condensation outside normally happens.

That makes sense.

It doesn't mean, tho', that a misting inside screen indicates 100% humidity inside the car? Only that the screen is either cool enough to act as a condensing surface for the inside air - at whatever humid it's at - or the inside air has increased in humid to the point it condenses out on that temp of screen. 

I don't think - but could be wrong - that a misting screen = 100% humidity? But I guess it could.

In essence, if moisture in the air condenses out, does that MEAN it is effectively saturated? Surely not. Going back to the beer glass, if the inside air is at, say, 60% RH, and feeling 'dry', then the presence of the cold glass doesn't change that RH figure, but cond still forms on the glass?

Confused... :-(

Bendy_House

It's really dreich outside - a heavy drizzle - and seemingly at 92% H.

Still good enough to dry clothes inside in a ventilated room

YoungBlueEyes

Aye it's dreich here too now, 89% humidity says met office weather app.  Blowing a hooley through my garden gate tho, and the ground is dry along the wind's path

YoungBlueEyes

Isn't the cond on the beer glass as a result of the temp difference? Same as the windscreen, it's warm air meeting less warm air....?

(Look at me trying to answer questions when 48 hours ago I was pure ignorant ha haa!)