UKH

Just thinking about breathability, directionality, vapour transmission and condensation with respect to hilly and cycling rain gear, and thought I'd check this basic assumption. If (capital IF, since that's the question here) atmospheric humidity is necessarily100% during rain, how can moisture escape through a breathable jacket even if it doesn't wet out? (By 'wet out' I mean the surface get covered by water rather than forcing discrete droplet formation by physical DWR etc..)

Just thinking out loud, but I guess it would be possible to have 100% RH where the rain drops form, but that the ground level conditions are different.

But I guess you're thinking of UK conditions...

Not necessarily. Air is poorly mixed, so you need to consider different parcels with different properties. In the clouds, where raindrops from, air is at 100 % relative humidity (guide to types here: https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/h...). At near to 100 % RH, water vapour condenses into clouds. Water droplets then coalesce into larger drops which become heavy enough to fall as rain. However, once in motion, its difficult to get these large drops to switch back to vapour without some large energy input. Therefore they can (and do) fall through areas of lower (i.e. < 100 % humidity)  at the lower levels of the atmosphere where humans tend to hang out. Note that precipition which is ultimately liquid at the surface is often frozen in clouds, but we'll keep things simple.

Moreover, you need to consider other gradients beyond humidity. For example, there's a temperature gradient around the membrane - it's much warmer under your jacket than outside it. This will drive air movement, and is where a lot of the breathability of jackets come from - movement of warm air away from the body through the membrane. This air will include water as vapour within it, from your sweat - which is one mechanism through which water is wicked away. 

Edit: typos

Isnt that fairly normal since humidity is dependant on temperature.

No. Can be very much lower

Thanks for that Ian,

So real world non-equilibrium conditions mean water vapour pressure isn't reaching saturation just because there's locally lots of free water surface during rain?

I'd been thinking air movement inside a jacket, whether bulk movement by pumping due to body and fabric movement (which I'd expect to dominate) or micro-climate currents due to temperature or humidity gradients, would result in drawing in 'replacement' air at ambient (wet) values. I was thinking this would work against the gradient drivers no evaporation could take place

If new air still has some capacity for more vapour that answers my query. But it also led to the realisation how many complications there will be...

Just to complicate it further, Gore-Tex does actually breathe when used under water. 

They do a "glove" test when marketing this (for fishing waders etc). One hand in a Gore-Tex glove, one in a non-breathable similar weight fabric, stick both in a bucket for a few minutes. Only the latter gets clammy. 

I don't know, but I found out recently that when it rains, the Forth rail bridge becomes 10,000 tons heavier. 

=10,000,000 litres of water.

if that were  2467m x 12m (plan area of the FB)  area it would be ~30cm deep. Given the verticals and tubulars and run-off, it's proabably a wee bit high,

It's 37m wide, 2529 long. 

Thats better... call it 10cm deep then

Plus it's one of the most amazing things ever made by humans. Possibly one of the most amazing things ever. I live nearby and look at it often, and I find it as awesome and beautiful as my favourite mountains. 

That makes sense as the wather vapour in the glove is a lot warmer so will 'condense' into the colder water on the other side of the barrier. 

Interesting, thanks, though to be honest that sounds too high to me. Do you have a source or reference for that number? I'd be interested to read it, thanks a lot 

It's on an information board about the bridge, at South Queensferry. 

Humidity can be measured in a number of ways RH (relative humidity) and Absolute Humidity.

It can be fun trying to control, and it's quite interesting when you look into it, if you like that sort of thing.

From the Internet.

Absolute humidity is the measure of water vapor (moisture) in the air, regardless of temperature. It is expressed as grams of moisture per cubic meter of air (g/m3).

The maximum absolute humidity of warm air at 30°C/86°F is approximately 30g of water vapor – 30g/m3. The maximum absolute humidity of cold air at 0°C/32°F is approximately 5g of water vapor – 5g/m3.

Relative humidity also measures water vapor but RELATIVE to the temperature of the air. It is expressed as the amount of water vapor in the air as a percentage of the total amount that could be held at its current temperature.

Warm air can hold far more moisture than cold air meaning that the relative humidity of cold air would be far higher than warm air if their absolute humidity levels were equal.

Relative humidity is cited in weather forecasts as it affects how we “feel” temperature.

I regularly checked humidity when I was interested in how it affected climbing conditions. 

After ten minutes of rain, with wet surfaces the humidity in my back yard in Sheffield is always over 90%. Once it's properly set in, its usually 95%

It’s not 100% when it’s raining. 
 

At 100% the air cannot hold anymore water so the air becomes foggy. At 100% you wouldn’t be able to see your hand in front of your face for the water vapour condensing. 
 

Rain is droplets that have previously condensed from air that is 100%. eg clouds. 
 

Thanks CH, in the OP I was thinking of 100% Humidity as RH, the saturation capacity of ambient air, during a rain shower. In that context I was wondering what would drive vapour transmission out of a higher temperature environment inside your jacket.

I see now that we can have, say, a situation like  80% RH at 30C going out into to 60% RH at 10C (arbitrary numbers, not calculated!).

The Goretex underwater example above still puzzles me though- I'd expect condensation against the cool inner surface, not the outer one that's in contact with liquid water. Relevant to wetted out jackets as well as gloves in water.

Not strictly true, it would depend on the temperature and also on how pure the air is; if there are no impurities, as in high atmosphere, the water vapour will not condense out, because there's nothing for it to condense on to. This is why you can get  greater than 100% humidity.

It's also (the impurity bit) why you can have liquid water at -40C under normal pressures if it pure enough.

Thanks. 
 

The OP was asking about hill walking and cycling. 

I've just come in from drizzly rain and the humidity reading on my little weather centre outside the door is showing 88%.

Did you come to a conclusion?

Yes. For hand grip it makes hardly any difference in comparison to temperature.

Temperature is key. 

20C and 60pc humidity is far worse conditions than 15C and 90pc humidity. 

Hi, thanks a lot for your reply

Does that essentially boil down to 'hot, sweaty hands are the factor' then?

Yes, - for me at least. And wind makes a huge difference too. 

High temps are ok if windy enough. 

Sorry I read this "If (capital IF, since that's the question here) atmospheric humidity is necessarily100% during rain" and must have misinterpreted it.

Goretex is good but it doesn’t always work. There are situations where this happens. 

I can't decide which is the more interesting/bizarre debate here - whether goretex works underwater, or how much heavier the Forth Bridge gets when it rains!

I think the debate was how much lighter it is when it's dry.