[Danny]

My forecast rules of thumb for good bouldering conditions go something like: no rain (tick); some wind (tick); <10C, >-2C (tick); dry road (tick).

I've never paid much attention to humidity forecasts. Does anyone? If so, what's generally indicative of good/bad? Last night I noticed it was gopping wet in general and the humidity was at ~98%, which seemed fair enough. This eve, however, I'd say it was pretty mint and the forecast was sitting at about 70%, which still seems quite high to me? Do really good conditions correlate cleanly with humidity? Seems like they should.

[dave]

It's been mentioned a lot recently on other threads, but you've got to read humidity percentages (i.e. relative humidity) alongside temperature and dew point. Warmer air can hold more water, so air at 4deg at 90% humidity has a lot less moisture in it than air at 24deg at 90% humidity.

In cool temps I don't think high relative humidity is necessarily a bad thing (it is quite often on days of typical decent cloudy gritstone weather), but if the air temp approaches the dew point you're going to get stuff condensing, which ain't good.

In summer high humidity makes it feel a lot hotter than it is as it reduces the effectiveness of your body's evaporative cooling system - i.e. sweating.

[Danny]

Interesting. I imagine that 100% RH (saturation) would mean that you're going to get condensation regardless of temperature (and regardless of the vapour density) no? Re: dew point, always fun when this happens (usually during a night session) because it's so acute. Is forecasting it a napkin calc job, or is it hiding on some forecast site somewhere?

[Luke Owens]

https://www.wunderground.com/

This gives dew point and other useful info in graph form.

[tomtom]

Condensation also depends upon the temperature of the surface water condenses on to. The classic is your glasses getting steamed up when you walk into a warm house/pub etc.. Warmer air (able to hold more moisture) hits cold glass and condenses..

So it depends on the relationship between rock temp, air temp and humidity.

Mist and fog - they're different beasties... I've climbed at Almscliff in freezing fog (a few times) and conditions have been amazing, but the roads/walls were damp/wet.. crag was mint..

[Danny]

https://www.wunderground.com/

This gives dew point and other useful info in graph form.

Great site! Might have to dump yr.no if there's a good app.

[Danny]

Condensation also depends upon the temperature of the surface water condenses on to. The classic is your glasses getting steamed up when you walk into a warm house/pub etc.. Warmer air (able to hold more moisture) hits cold glass and condenses..

So it depends on the relationship between rock temp, air temp and humidity.

Mist and fog - they're different beasties... I've climbed at Almscliff in freezing fog (a few times) and conditions have been amazing, but the roads/walls were damp/wet.. crag was mint..

I reckon this might be most acute relatively close to the sea? Land cools fast, and warm moisture laden air from the sea spooges the rock up. Maybe.

[SA Chris]

Definitely worse at the sea. Wind direction by the sea id a big factor too; onshore brings moisture in off the sea, offshore usually means better conditions, except if you are at the base of a cliff where the wind will miss you. Cross shore is often best, but so local conditiosn dependant.

[Nibile]

I resurrect this just for a clarification: I've read ina another topic that warm air can hold more humidity, so 50% humidity at 30 degrees is worse (in climbing terms) than 50% at 20 degrees, right?
In cooler temps we can get away with higher humidity percentages, right?

[jwi]

yes.