[andy moles]

Apologies if this has been discussed already, maybe someone can point me in the right direction (when I searched the word 'condensed' I got an unreadable number of hits...)

Does anyone have any good rules of thumb for judging, based on observed and forecast weather conditions, whether rock will be condensed?

I guess there is always going to be local topography to consider here, whether it's by the sea, enclosed or exposed, steep or slabby... but I just want to wrap my head around it a little better in general terms.

Case in point - yesterday at 6pm, it was 6 Celsius and 69% humidity. Rock was dry. By 3am, 9 hours later, it had risen to a peak of 10 Celsius and 97% humidity. Venue is sheltered from current light winds, and steep. Humidity now dropping again, but not to below 80ish%, temps are stable. This sounds to me very likely to have caused condensation, with a fairly steep rise in temps alongside fairly steep rise in humidity.

Any thoughts?
 

[Fiend]

@petejh your time 

Otherwise your reasoning sounds reasonable.

[andy_e]

I think wind speed is also something to factor in, humidity can be low but can rise very rapidly in a place if there's a human breathing out hot moist air and putting clammy hands on holds where there's no air movement!

[Johnny Brown]

Does anyone have any good rules of thumb for judging, based on observed and forecast weather conditions, whether rock will be condensed?

Locally, yes. But while the broad picture is simple - and you've obviously grasped that - I would hesitate to extrapolate it to other venues. And drying is less easy to predict than condensation but just as important to the result. The venues I'm interested in (grit edges) aren't that prone either, and very quick drying.

On the face of what you've posted I wouldn't be too worried about a 4 degree rise in temps, even given the humidity. But I'd want to know about the temps in the 36hrs before the 6 degree reading - it's been baltic here.

Definitely be interesting to see some proper science done - sensors at LPT, Masters at Bango Uni anyone? Just need to convince the moneymen that it has a significant impact on visitor numbers or rare lichen erosion...

[andy moles]

But I'd want to know about the temps in the 36hrs before the 6 degree reading - it's been baltic here.

Because a longer period of cold brings the temperature of the rock itself down?

It was actually slightly less cold in the previous days and condensed AF after the cold weekend.

[andy moles]

I think wind speed is also something to factor in

Definitely. I'm sometimes amazed how the different the 'feels like' humidity can be with the presence/absence of wind.

[remus]

Definitely be interesting to see some proper science done - sensors at LPT, Masters at Bango Uni anyone? Just need to convince the moneymen that it has a significant impact on visitor numbers or rare lichen erosion...

A project I've dreamt about for a while is having hundreds of small, low cost weather stations at crags around the country. They'd have some sort of remote power source and would communicate current conditions back to the mothership via LTE or something. You could then get live updates on conditions at your crag of choice.

Stage 2 would be moisture sensors in often-damp breaks to send you an alert when your project starts drying out.

Stage 3 is a matrix of soil saturation sensors embedded in the land surrounding the cornice, 10 years of data collection and some AI to predict precisely when Nemesis will dry out.

[andy moles]

I'll chip in to your crowdfund Remus, provided we can get some in the Celtic fringes   

[andy_e]

Stage 3 is a matrix of soil saturation sensors embedded in the land surrounding the cornice, 10 years of data collection and some AI to predict precisely when Nemesis will dry out.

Just before I started my undergrad Andy F tried to encourage me to specialise in hydrogeology so I cuold accurately predict Kilnsey conditions and save him a drive from Liverpool if it was wet.

[Probes]

Old fuzzy building services knowledge, so with a disclaimer... all you need is to stick this lot in your bag... a psychometric chart, relative humidity sensor, air pressure sensor and the thermometer for rock temp (drybulb temp). Wind I would think, just improves drying of the rock surface once the surface is below saturation point. Simples 
https://www.wikihow.com/Read-a-Psychrometric-Chart
 

[WillRobertson]

Condensation will occur when the surface temperature of the rock falls below the dew point (or the dew point rises above the rock temperature).

This can happen for a variety of reasons including, but not limited to:
- Cooling of the rock - typically overnight, and occurs quickest when winds are calm and skies are clear
- Mass increase in dew points - typically a synoptic type change in the weather e.g. a warm front
- A change in wind direction/strength - e.g. an offshore wind switching to onshore (this could both lower temperature and increase dewpoints depending on sea temperature/temperature over land/time of year etc.)
- All sorts of impact from latent heat release/recovery when water transitions between gas/liquid/solid, particularly on a local scale

All complicated by a huge number of local conditions which will only really be found out by experience. For example I'd imagine a crag such as Raven Tor will see the rock cooling slower, due to the mass of earth behind it, when compared to stand-alone boulders that you see in some coastal locations (not taking rock type into account). However there will be less residual heat in the ground in late winter than autumn so it'll likely take less for condensation to occur it all other conditions are identical.

Essentially a basic awareness of weather patterns and the changes on a synoptic scale will be useful but local knowledge/experience is vital in accurately forecasting condensation.

[m.cooke.1421]

It is also worth considering that chalk covered holds will take longer to dry out than clean rock.

[Offwidth]

Ditto for lichen. The key bit missing so far upthread is aspect and cloud/tree cover, as the sun makes a massive difference to evaporation.

[SA Chris]

The key bit missing so far upthread is aspect and cloud/tree cover, as the sun makes a massive difference to evaporation.

Was going to say the same, conditions always seem to be better if the sun has been on the rock for a bit, even after it goes into shade (either sun moving or cloud cover).

[Bonjoy]

This can happen for a variety of reasons including, but not limited to:
- Cooling of the rock - typically overnight, and occurs quickest when winds are calm and skies are clear

This seems a misleading way to phrase it. Condensation due to the temperature of the rock is caused by the lag between how fast air temperature changes versus rock temperature.
So whilst a rock is actively "cooling" you will not get condensation because this cooling is the result of the air temperature being lower than the rock temperature. The heat exchange between the rock and air means the air around the rock can carry more water than the surrounding air, so drying takes place as opposed to condensation.
Eventually the surrounding air temp will go up and exceed the rock temp, at which point air around the rock will be cooled by heat exchange and it's capacity to hold moisture will drop, the excess moisture will form condensation on the rock surface, but at this point the rock may be cold but is actively heating up not cooling.

[Johnny Brown]

There's also the fact that despite apparently similar ambient conditions, recently dried rock can offer better conditions than long-term dry rock due to the lowering of the surface temperature by evaporation.

[m.cooke.1421]

Is a fan always going to improve conditions or are there scenarios where you will be making the rock wetter?

[WillRobertson]

This can happen for a variety of reasons including, but not limited to:
- Cooling of the rock - typically overnight, and occurs quickest when winds are calm and skies are clear

This seems a misleading way to phrase it. Condensation due to the temperature of the rock is caused by the lag between how fast air temperature changes versus rock temperature.
So whilst a rock is actively "cooling" you will not get condensation because this cooling is the result of the air temperature being lower than the rock temperature. The heat exchange between the rock and air means the air around the rock can carry more water than the surrounding air, so drying takes place as opposed to condensation.
Eventually the surrounding air temp will go up and exceed the rock temp, at which point air around the rock will be cooled by heat exchange and it's capacity to hold moisture will drop, the excess moisture will form condensation on the rock surface, but at this point the rock may be cold but is actively heating up not cooling.

I'm not sure I agree. The rock - similar to grass, road surfaces, and vehicles - will typically cool quicker than the surrounding air - this may differ depending on rock type, structure and exposure, but is generally true. This is why ice can form on surfaces despite the air temperature being above zero Celsius (and hence why cars typically alert of a risk of ice when the air temp falls below 3 C).

And condensation doesn't require the rock temperature being lower than the air temp (dry bulb), but just lower than the dewpoint of the surrounding air. If, as you suggest, it requires "excess moisture" to form condensation then condensation would only form when the air is fully saturated (~100% RH - and hence forming fog or cloud). We see condensation form when this isn't the case so we can rule this out.

Water condensing on rock is the same process as when dew is deposited on surfaces, briefly described here:
https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/temperature/dew

[Bonjoy]

I'm not sure I agree. The rock - similar to grass, road surfaces, and vehicles - will typically cool quicker than the surrounding air - this may differ depending on rock type, structure and exposure, but is generally true. This is why ice can form on surfaces despite the air temperature being above zero Celsius (and hence why cars typically alert of a risk of ice when the air temp falls below 3 C).

How do you figure that? What is cooling these masses if the air around them is warmer? Loss of heat through conduction/radiation? This will not drop below air temp (unless there is some internal cooling mechanism within the mass e.g ice in a glass of water). What you go on to describe is freezing due to thermal inertia, which will only happen if the mass has previously been at below zero (during which period the air will also have been sub zero). I.e this is due to the mass warming up slower than the air, not cooling down quicker than the air.

And condensation doesn't require the rock temperature being lower than the air temp (dry bulb), but just lower than the dewpoint of the surrounding air. If, as you suggest, it requires "excess moisture" to form condensation then condensation would only form when the air is fully saturated (~100% RH - and hence forming fog or cloud). We see condensation form when this isn't the case so we can rule this out.

Humidity is relative to temperature. Fully saturated at one temperature = below saturation at a higher temp. Hence condensation when a given air mass drops in temp below a threshold (the dewpoint).
Try walking into a hot pub with cold glasses on. The glasses stop being steamed up once they reach thermal equilibrium with the air. In what scenario would the glasses (or the rock) be cooling whilst in warm air? Or put another way, in what scenario would you say your glasses are steaming up due to cooling down?

[Will Hunt]

Sometimes I pine for those halcyon days before conditions were invented (for me, about 2010) when you would just look out of the window and, if it wasn't raining, go rock climbing.

[Banana finger]

What we need is a phone app where people can easily log, where they are date and time, and record the connies....with enough data we could be able to get a bit of a model of where might be in good nick....linking to weather reports etc

Or we could check if someone was at a crag we want to go to earlier in the day and know connies....basically this could be like magic seaweed for connies. Needs a good name though. 'Red tips news' perhaps

...Who wants to dev an app with me?

[WillRobertson]

I'm not sure I agree. The rock - similar to grass, road surfaces, and vehicles - will typically cool quicker than the surrounding air - this may differ depending on rock type, structure and exposure, but is generally true. This is why ice can form on surfaces despite the air temperature being above zero Celsius (and hence why cars typically alert of a risk of ice when the air temp falls below 3 C).

How do you figure that? What is cooling these masses if the air around them is warmer? Loss of heat through conduction/radiation? This will not drop below air temp (unless there is some internal cooling mechanism within the mass e.g ice in a glass of water). What you go on to describe is freezing due to thermal inertia, which will only happen if the mass has previously been at below zero (during which period the air will also have been sub zero). I.e this is due to the mass warming up slower than the air, not cooling down quicker than the air.

Hmm, yes apologies I think I made assumptions that I didn't state.

 I was assuming that the temperature within the rock (e.g. 20-100 CM below the surface) is lower than the max temperature the surface had been at during the day. This is assuming a typical diurnal heating/cooling pattern - and I guess may not be reliable in a north facing limestone cave on an overcast autumn/winter day. I imagine it would also be markedly different when comparing a crag like Raven Tor to somewhere with stand alone exposed boulders - hence my comment about local knowledge/experience being key.

Also a drop of say 5 C in surface temperature is unlikely to result in a similar drop of air temp due to the fact that air is generally mixed much more freely than the molecules in the surface, hence the loss in energy is spread over a larger area. The outgoing radiation is also not necessarily absorbed by the air immediate to the surface - this is why we see the effect of nights being warmer when there is cloud present as this cloud reflects some of the outgoing radiation and it effectively becomes trapped in the lower layers of the troposphere.

And condensation doesn't require the rock temperature being lower than the air temp (dry bulb), but just lower than the dewpoint of the surrounding air. If, as you suggest, it requires "excess moisture" to form condensation then condensation would only form when the air is fully saturated (~100% RH - and hence forming fog or cloud). We see condensation form when this isn't the case so we can rule this out.

Humidity is relative to temperature. Fully saturated at one temperature = below saturation at a higher temp. Hence condensation when a given air mass drops in temp below a threshold (the dewpoint).
Try walking into a hot pub with cold glasses on. The glasses stop being steamed up once they reach thermal equilibrium with the air. In what scenario would the glasses (or the rock) be cooling whilst in warm air? Or put another way, in what scenario would you say your glasses are steaming up due to cooling down?

This scenario would be covered by my original post where I talk about an increase in dew points (e.g. due to an airmass change or a change to an onshore wind), to above the temperature of the surface.

*Edited to sort quote format error.

[remus]

What we need is a phone app where people can easily log, where they are date and time, and record the connies....with enough data we could be able to get a bit of a model of where might be in good nick....linking to weather reports etc

Or we could check if someone was at a crag we want to go to earlier in the day and know connies....basically this could be like magic seaweed for connies. Needs a good name though. 'Red tips news' perhaps

...Who wants to dev an app with me?

This was gonna be another branch of the 'weather stations at every crag' business idea. The app would show you connies around the UK, then when you got to your crag of choice you could provide feedback in way too much detail about which holds were dry, whether it was condensed etc. Then, once you'd collected enough data you could apply some maths-magic and have a perfect conditions calculating machine.

[Fultonius]

What we need is a phone app where people can easily log, where they are date and time, and record the connies....with enough data we could be able to get a bit of a model of where might be in good nick....linking to weather reports etc

Or we could check if someone was at a crag we want to go to earlier in the day and know connies....basically this could be like magic seaweed for connies. Needs a good name though. 'Red tips news' perhaps

...Who wants to dev an app with me?

We had #isitgreasy for connies reports at Dumby on twitter for a while.

[petejh]

I got interested in this around 2010 after we'd started regularly climbing at the Diamond and noticed conditions (i.e. condensation on the rock) could be frustratingly difficult to predict. I thought there must be some local weather data I could observe and see if there were associations to be made on the days when connies were good or bad. I found some local amateur weather stations on a website called weather underground where people could buy their own DIY weather station and link it to the website to show to the world the dew point, RH, wind direction and air temp in their back garden. Fascinating stuff!

On the days when conditions on the crag were good there was always an increase in the gap between local dew-point temp and local air-temp. On days when conditions were poor, the gap was clearly smaller. Pretty obvious in hindsight, but it was useful to make the link because it could be forecasted a day ahead. With the website I'd found there were actually DIY weather stations handily located just a few hundred metres away from the Diamond at barely higher altitude. So you could look up the current dew-point and air-temp online from home and predict connies on the diamond before going down there.
Relative humidity not so useful as it's too.. relative.. as a predictor for conditions as 75% RH means different things at different temps. Wind always a benefit (except if it means sea spray).

Obviously this is reliant on people setting up their DIY weather stations properly and local oddities with siting in the sun/shade/under a tree or next to a river  etc. etc. Here are two local stations in Llandudno, I think I used to use the first one:
https://www.wunderground.com/weather/gb/llandudno/ICONWYLL2
https://www.wunderground.com/weather/gb/llandudno/ILLANDUD7

You can search the map for your local amateur weather stations and get real-time dewpoint/air temp data. Here's one near cragx: https://www.wunderground.com/weather/gb/buxton/IBUXTO22

Obviously each crag will have it's own set of factors, but the principle of linking the magnitude of gap between dew-point temp and air-temp to the likelihood of good conditions on a crag seems to be legit at least from the Diamond experience.

The below is my theory for the Diamond and LPT, which we put in the guidebook.