Do you use electric clothes dryers?

YoungBlueEyes

I can’t work out to phrase this without sounding deliberately argumentative; I’m trying to understand the science behind it 🙂

Surely water is 100% humid, so washing after a good spin must be lower. I said 50% but that was just a figure for illustration, I don’t know if it’s accurate. 

Maybe I’m thinking of it acting in the same way as heat and cold. Heat always rushes to cold, so does moisture not settle in/onto something that’s drier than it? So 80% moist air will dampen 50% moist clothes..? 

grumbler

 Forget the 'humidity' of the washing - it is irrelevant. Imagine a droplet of water on a piece of glass. If the relative humidity of the surrounding air is below 100% the droplet will dry gradually. This is called evaporation. The lower the relative humidity and the bigger the flow of the air, the faster the evaporation.

If the humidity reaches 100% the reverse process starts - condensation. The droplet will grow and other droplets can appear.

As simple as that.

grandadgolfer

YoungBlueEyes said:

This'll sound like a troll question, but can I ask - Bendy I've heard you say a few times about leaving windows wide open for the breeze to dry washing indoors. But what I don't understand is how it would dry clothes if/when the outside air is damp. Wouldn't it just be letting damp air into a damp(ening) room?  So how would the washing dry...?

YoungBlueEyes said:

But if the air coming in the window is damper than the washing…. 

How damp are clothes after a good spin? If clothes are 50% damp and the incoming air is 80% humid, wouldn’t the clothes just absorb the humidity from the air til it all evened out? 

I don’t get it 🤨

Your over thinking this

Bendy_House

Not argumentative at all! And science is everything

Humidity - or relative humidity - refers to the air, and the moisture content dissolved in it. Apparently it's the %-age relative to the air being 'saturated', ie incapable of holding any more.

'Liquid' water is just, well, wet. Water. 100% humid?! If wet clothes can be said to be 100% humid, then spun-dried clothes are... ditto - still 100% humid, but there's just less of it. It's still liquid 'water'. It is not less-humid.

So, as Grumb says, if the outdoor air has less than 100% humidity - and it surely always has, in this country at least - then liquid water will evaporate in to it. If the air is moving, then more evaporation will occur. Ditto with heat. But neither are essential for evaporation.

grumbler

Bendy_House said:

if the outdoor air has less than 100% humidity - and it surely always has, in this country at least

Are you saying that it's always less than 100%?

Not always - otherwise we would never see fog or need to wipe condensation from our windscreens. It can reach 100% when the air is moist and temperature drops - usually late at night or early in the morning.

However, usually a moist windscreen dries soon and fog doesn't last long and dissipates.

Bendy_House

Good point. And we'd struggle to dry our clothes in fog

Bendy_House

grumbler said:

 or need to wipe condensation from our windscreens. It can reach 100% when the air is moist and temperature drops - usually late at night or early in the morning.

However, usually a moist windscreen dries soon and fog doesn't last long and dissipates.

I'm pretty sure that condensation on the screen doesn't require 100%, or anything close. It's simply that the screen is colder, and the moisture in the air - at whatever humidity level - will condense out on it. If you make a surface cold enough, it'll almost certainly have some moisture condensing out on it.

A cooool glass of beer straight from the fridge will have condensation forming on the outside, whether you're imbibing inside or outside the house, but that's not because it's an RH of 100%.

grumbler

Bendy_House said:

grumbler said:

 or need to wipe condensation from our windscreens. It can reach 100% when the air is moist and temperature drops - usually late at night or early in the morning.

However, usually a moist windscreen dries soon and fog doesn't last long and dissipates.

I'm pretty sure that condensation on the screen doesn't require 100%, or anything close. It's simply that the screen is colder, and the moisture in the air - at whatever humidity level - will condense out on it. If you make a surface cold enough, it'll almost certainly have some moisture condensing out on it.

A cooool glass of beer straight from the fridge will have condensation forming on the outside, whether you're imbibing inside or outside the house, but that's not because it's an RH of 100%.

  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).

YoungBlueEyes

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

grumbler

YoungBlueEyes said:

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...?

Absolute humidity of air = water content, amount of water in the air.

Relative humidity = how far it's from saturation. 100% = saturation.

Hotter air can hold more moisture than cold. With the same absolute humidity the relative one of hotter air will be lower.

When cold air from outside gets inside its absolute humidity doesn't change. Its temperature either remains the same (and so does the relative humidity) or rises (and the relative humidity drops as a result).