UKH

Hi All,

What does 100% humidity mean?

What is a low and high humidity percentage?

Where I plan to climb tomorrow has predicted 90% humidity... does that mean it will be ming?

What if its windy, surely that makes a difference, is that built into the percentage?

I've had a trawl of past threads and although there are some, none seem to really explain how humidity works.  Also, as a geography teacher I'm sure at some point I may get asked about it by a student but its not an area I have ever studied.

Thanks for your potentially wise and non sarcastic responses!

SSB

As a geography teacher you're probably aware of 'Geography, an integrated approach' by David Waugh.

Invaluable for staying one page ahead of those inquisitive students and has a useful section on humidity, etc.

I do indeed.  I understand the basic idea that its a measure of how much moisture the air is holding compared to how much water it could hold.  I'll take a look in Waugh this pm and see what it says.

Absolute humidity is the amount of water vapour in the air. More useful though, and almost universally quoted, is relative humididty. That takes the temperature into account too.

Basically, it's a measure of how likely the water vapour in the air is to condense so 100% humidity has the vapour forming droplets and 0% has no water vapour at all.

You'll understand that a certain physical quantity of water vapour is more likely to condense in cold air than it is in warm air (similar to the fogging of a cold camera lens when it comes into a warm room).

90% is fairly likely to keep you sweating, regardless of temperature bacause the extra vapour from your skin will push the relative humidity next to your skin up to 100%. As I implied, temperature is effectively irrelevant: what's important is how close the number is to 100% so depending on temperature 90% will either be clammy or muggy. It's probably high enough to make the rock feel a tad greasy too.

D

Wikipedia is your friend.

https://en.m.wikipedia.org/wiki/Humidity

Good questions and I look forward to hearing from someone knowledgable too.

I seem to remember reading that for climbers the dew point was a more useful measure of humidity but I can't remember why.

Humidity always seems to be the enemy. When it's cold high humidity makes you feel even colder, and when it's hot it makes you feel hotter. Presumably there must be some middle temperature where it doesn't make any difference.

I think everyone must have heard of Wikipedia by now.

not Ot if you want to climb sea cliffs... ~70% humidity at 20C+ is guaranteed to be greasy, while 70%@15C is likely great friction!

If the air has 100% (relative) humidity it means that it can't hold any more evaporated moisture at its current temperature.  If that air is cooled slightly (for example by moving higher in the atmosphere when blown against the side of a hill) some of the water vapour it contains will condense and form cloud.  On the other hand, if that air is warmed up, it will be able to hold more evaporated moisture and its relative humidity will drop below 100%.

Not sure how to answer the low vs high question.

90% relative humidity means that the air doesn't have a whole lot more capacity to hold moisture so it doesn't have much capacity to cause drying of the rock.  In addition, if it does come into contact with something slightly colder, condensation will occur and the cold object will get damp.

Wind makes no difference to relative humidity - it's simply a property of a given parcel of air no matter how fast it's moving.  At a given pressure, relative humidity is only a function of temperature and water vapour content.  However if you blow a larger quantity of humid air onto a cold surface you're providing more water to condense on it.  Try blowing on a cold window - the longer you blow for, the more water condenses (until the window warms up!)

In reply to stp, high humidity at high temperatures makes you feel hotter because the air doesn't have much capacity to evaporate more water.  So it doesn't evaporate your sweat which therefore can't cool you down.  I can think of a couple of reasons why high humidity might make cold feel colder:  1.  Your clothing will be damper and conduct more heat outwards and 2.  The specific heat capacity of humid air is higher than that of dry air which means that any humid air that contacts your skin will pull more heat from you before it reaches body temperature.

Sorry - I missed your question about dew-point.

The dew-point of a particular clump of air is the temperature at which the water in it will start to condense.  If you took a few handfuls of the air in front of you now and cooled it down, it's relative humidity would rise.  That doesn't mean it's gaining water vapour - just that it's ability to contain evaporated water is reducing as the temperature falls. As you kept cooling it there would be a temperature at which you started to see cloud forming - that temperature is the dew point & it's also the temperature at which the relative humidity is 100 %.  (Try opening the freezer door in a warm kitchen if you want to see this in action.)  Keep cooling it and you'll condense out more and more of the water until you get precipitation.

The all-too-common practical application of this, of course, is relatively warm, moist air blowing off the sea onto Scotland and being forced to rise by the hills.  As it rises it cools and when its temperature falls to the dew-point it forms clouds, then precipitation.  And we go to the pub.

Humidity is as many have said water vapour in the air, and it gets to the point where the air is so saturated with water vapour that it can't hold any more. 

90% is not far off 100%, but what does that mean in the real world?

When a person exercises their muscles generate heat during that process.  Its part of aerobic respiration.  Now the body has a few ways to cool itself down.  One of those ways is for the body to sweat.  The sweat then evaporates from the body and during that process heat is lost from the body.

Now for sweat to evaporate from the body there has to be a low amount of humidity in the surrounding air for this evaporation to take place.  Now if the humidity is very high then very little if any of the sweat can evaporate.  This means that the skin stays wet because the sweat will not evaporate.  And if this happens then the body can not cool down through the evaporation process.  The body just gets hotter and hotter because it can not cool down during excretion/exercise

The next fun fact about breathing, is that when we breath out especially during exercise, we breath out air containing saturated water.  When the humidity is low then the surrounding air can cope with this.  However if the humidity is very high then the body is breathing in already air saturated with water making it harder for the lungs to do their job of getting oxygen to all those muscles screaming for oxygen.  That pesky air saturated with water vapour is making that gaseous exchange all the more tricky.

Put all that together and exercising in high humidity makes for an awful experience.  Just staying still can be an effort in itself especially with very high temperatures making life hard for the body to cool down.

Something similar happens while walking on the hills in a goretex jacket in humid weather.  The surrounding humidity is just as saturated with water vapour as inside that jacket meaning that your water vapour can not pass through the gortex membrane into the surrounding air.  You then get all sweaty and think about buying a new jacket because the one you are wearing is broken. 

I may have got my science terminology all messed up but I hope you get the jist of why exercising on a hot humid day isn't much fun.

Humidity is also imortant at night.  It's to do with latent heat of vaporisation,  As the air temperature cools, it eventually gets to the dew point. i.e.  the temperature where the air can't hold any more water in gaseous form.  So the excess water starts to liquify.  This gives off the aforementioned latent heat which warms the air/gaseous water/water droplets.  The consequence is that the air temperature at night does not drop much below the dew point.  A knock-on effect is that anu further reduction in temperature causes more and more water vapour to condense into water droplets - we see this as dew on the grass, or on your sleeping bay (if you're camping).

I doubt if Brideshead Revisited will help you much. 

Don't trust weather forecasts! In my experience crag conditions are often different, a little thought with respect to wind, crag attitude and altitude will often find a climbable location. Besides, you'll never know or learn unless you commit!

If on a sea cliff, the weather often changes with the tide turning, just to keep you on ya toes.

And limestone soapyness often changes with distance from the ground.

I also know that if the walk in is in any way midgy, then on the crag (unless really exposed and a wind) the midges will be above my threshold for the little darlings. 

I've seen people climb 8bs on humid days at over 40 degrees in Asia.  Some days, I guess you just have to try harder for the tick?

In summary, get out there and try! If you fail, try harder.

Have a great day out! I'm baby sitting today, I'd like to think someone is having a climb in my absence.

Thanks all.  That all makes a lot of sense.  I'll look into the dew point next.

As regards the conditions - I actually went to a different crag than planned as family etc meant time was pushed but conditions were fine. 

Have you tried phoning the local Tool Hire place for advice?

https://www.ukhillwalking.com/forums/the_pub/i_dont_know_im_a_girl-678455

LOL! if a girl answers...hang up