Condition Analysis
AI-powered assessment using site data and 14-day weather history
The Cromlech Boulders have had a damp week with light but frequent precipitation, and today sees only trace amounts; the SW/W-facing rhyolite should be largely surface-dry by late morning but high humidity (77%) and overcast skies limit confidence. A good window exists midday through early evening, but climbers should check holds for residual dampness on arrival.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside below Dinas Cromlech and catch drainage and seepage from the hillside above, meaning even after light rain some problems — particularly overhanging faces and roofs like Jerry's Roof — can have persistent drips from above.
- The SW/W aspect receives afternoon sun when skies clear, which helps surface drying significantly, but today's near-total cloud cover negates most solar benefit.
- At 250m in the Llanberis Pass, the boulders can sit in low cloud or mist that keeps surfaces damp even without measurable rainfall — the 77–89% humidity range today makes this a real possibility.
- The site is extremely popular on dry weekends; midweek sessions like today avoid crowds but the recent unsettled spell means polish on popular problems may be holding moisture in textural pockets.
Warnings
2
- Rhyolite becomes very slippery when wet — do not trust friction-dependent moves if any dampness is detected on the surface.
- Heavy rain (10.8mm) is forecast tomorrow; conditions will deteriorate significantly and remain poor through mid-next-week.
Reasoning
Light rain (1.4mm on May 5 and 6, 0.5mm today with only trace amounts in the hourly forecast) means surfaces have received repeated light wettings, though nothing heavy; the rock is likely damp in sheltered spots but drying on exposed faces.
The SW/W aspect would normally dry quickly, but persistent cloud cover (70–100%) and moderate humidity (67–77% during the day) are slowing evaporation; moderate southerly winds (13–18 km/h) help but cannot fully compensate without solar radiation.
Rhyolite is non-porous and does not suffer structural weakening when wet — there is no risk of hold breakage from moisture.
Early May in Snowdonia is transitional; days are lengthening and temperatures are mild (10–12°C), but the recent unsettled pattern with high humidity is typical of spring and limits reliable drying windows.
Contributing Factors
6
Repeated light precipitation over the past week (1.4mm on May 5, 1.4mm on May 6, 0.5mm today) keeps surfaces periodically re-wetted despite no heavy downpours.
Humidity averaging 80% over the past seven days and 67–77% during today's daytime hours slows evaporation and can leave a moisture film on the rock.
Moderate southerly winds of 13–18 km/h across the exposed roadside boulders promote surface drying of the non-porous rhyolite.
Cloud cover is 70–100% throughout the day, severely limiting solar drying on the SW/W-facing surfaces.
Rhyolite does not absorb water, so surface moisture is the only concern and it can dry within hours given the right conditions.
10.8mm forecast for May 8 means today may be the last usable window before another prolonged wet spell.
Recommendations
3
- Visit today between late morning and early evening (roughly 11:00–18:00) for the best chance of dry surfaces, and physically test friction on easier problems before committing to harder lines.
- Bring a towel and soft brush to dry and clean holds — the rhyolite's textured surface can trap surface water in micro-pockets even when it looks dry.
- Avoid the next few days if possible; 10.8mm is forecast for May 8 followed by continued unsettled weather through May 12, so the next reliable window may not arrive until May 13 or later.
Previous Analyses
Marginal — Assess Conditions
45%
2 days ago
Light rain is falling today (1.6mm forecast) following several days of intermittent showers, so the boulders are likely damp right now. There may be a brief window this morning before the afternoon showers, but conditions are marginal and climbers should visually check the rock before committing.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The Cromlech Boulders sit roadside in the Llanberis Pass and can catch seepage and runoff from the hillside above, meaning even light rain can leave persistent damp patches on certain problems.
- Jerry's Roof and the overhanging problems can stay drier underneath during light rain, but heel hooks and slab starts may remain greasy long after showers pass.
- The SW/W aspect means the boulders receive afternoon sun when skies clear, which can dry surface moisture quickly — but today's heavy cloud cover (96–100%) will severely limit solar drying.
- The pass can funnel wind effectively, but today's light winds (4–14 km/h) offer limited drying assistance compared to the breezy days seen last week.
Warnings
2
- Rhyolite becomes extremely slippery when wet — do not attempt highball or friction-dependent problems if there is any surface moisture.
- The forecast shows significant rain on May 9–10 (22mm+), so plan sessions around the brief dry windows of May 7–8.
Reasoning
The rock has received light but repeated precipitation (1.4mm yesterday, 2.7mm on May 3, 2.5mm on May 2) with only a single dry day (May 4) in the last five, so surface moisture is likely present, compounded by today's 1.6mm of rain forecast in the afternoon.
The SW/W aspect would normally aid drying, but persistent overcast skies (96–100% cloud) and light winds (under 15 km/h) today are severely limiting evaporation, meaning the brief dry spell of May 4 was insufficient to fully dry the boulders before further rain arrived.
Rhyolite is non-porous and does not suffer structural weakening when wet, so hold breakage and rock damage are not concerns here.
Early May in Snowdonia is transitional — days are lengthening and temperatures are mild (~10°C), but persistent Atlantic weather systems continue to deliver frequent showers, and high humidity (77% average over the last week) slows drying.
Contributing Factors
7
Light but frequent showers over the past five days (total ~8.6mm since May 1) with only one dry day mean the rock has had little chance to fully dry.
Approximately 1.5mm of rain is forecast between 14:00–16:00 today, which will wet exposed surfaces during the prime climbing window.
Cloud cover is 96–100% for most of the day, virtually eliminating solar drying on the SW/W-facing boulders.
Winds are only 4–14 km/h today — well below the 25–37 km/h seen on recent drier days — reducing wind-assisted evaporation.
Humidity ranges from 70–92% today, keeping the air close to saturation and slowing surface drying significantly.
The volcanic rock does not absorb water, so once surface moisture evaporates the rock returns to full friction — no extended internal drying period is needed.
No rain is forecast between roughly 07:00 and 13:00, offering a potential window if the rock dried overnight or in the early morning.
Recommendations
3
- If visiting this morning, physically touch the rock and test friction on an easy problem before attempting anything committing — the morning dry window (07:00–13:00) may yield climbable conditions on sheltered faces.
- Avoid the afternoon entirely as showers are forecast from 14:00–16:00 and the rock will be wet.
- Consider waiting until May 7, which is forecast dry with slightly lower humidity and moderate winds — a better prospect for dry rock.
Marginal — Assess Conditions
70%
3 days ago
The Cromlech Boulders have had two full dry days following light rain on May 3rd (2.7mm), with the rock likely surface-dry by this afternoon as humidity drops and clouds clear. However, the preceding weeks were very wet (116mm in 28 days) and morning humidity remains high, so a visual check on arrival is warranted — the afternoon window from ~15:00 onwards looks best.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside in the Llanberis Pass and are partially sheltered by the steep valley sides, meaning the N-facing aspects of some problems can retain seepage longer than the SW/W-facing surfaces.
- Cromlech Boulders are famously friction-dependent — classic problems like Jerry's Roof require excellent skin conditions and bone-dry rock; even slight residual moisture dramatically affects the hardest lines.
- The pass funnels wind effectively, and the exposed position means that even moderate breezes aid surface drying significantly despite the high ambient humidity.
- Being roadside at 250m in the pass, low cloud and mist are common in spring and can re-dampen surfaces even on nominally dry days — check for cloud base on approach.
Warnings
1
- Low cloud or mist rolling through the pass can re-dampen rock surfaces rapidly — be prepared to stop climbing if visibility drops or the air feels moist.
Reasoning
The last meaningful rain was 2.7mm on May 3rd followed by two dry days; surface moisture on the non-porous rhyolite should have evaporated, though ambient humidity in the 78–90% range through this morning may have slowed full drying.
The SW/W aspect and exposed position with moderate winds (~15–17 km/h) through today should ensure surface drying, particularly from mid-afternoon when humidity drops to 63–66% and cloud cover breaks significantly.
Rhyolite is non-porous and extremely hard — there is no risk of structural damage or hold breakage from moisture at this venue.
Early May in Snowdonia is transitional; shorter drying windows, residual ground moisture from a very wet April (116mm), and low cloud in the pass are all typical spring concerns that limit confidence.
Contributing Factors
6
Only 2.7mm fell on May 3rd and the last two days have been completely dry, giving adequate time for surface moisture to clear from non-porous rhyolite.
Humidity has been 80–89% through the morning with full cloud cover, which can leave a film of condensation on boulder surfaces and slow evaporation.
From 15:00 onwards cloud drops to 12–44%, humidity falls to 63–66%, and moderate northerly winds of ~17 km/h should provide excellent drying and friction conditions.
The 28-day total of 116mm means the surrounding ground is saturated, increasing the risk of seepage lines and residual dampness on lower boulder faces.
Rhyolite does not absorb water, so once surface moisture evaporates the rock offers full friction with no risk of internal dampness or structural weakening.
Steady winds of 14–17 km/h channelled through the pass assist surface drying throughout the day.
Recommendations
3
- Target the afternoon session from approximately 15:00 onwards when humidity drops below 66% and cloud cover breaks — this is the best friction window today.
- Physically touch holds on arrival and test friction on easy problems before committing to harder lines; any greasy or damp feel means conditions are not ready.
- Check sheltered and north-facing faces of the boulders carefully, as these may retain dampness from morning humidity and the saturated ground below.
Marginal — Assess Conditions
65%
4 days ago
Light rain fell yesterday (2.7mm) in very humid conditions (91%), and the rock has had only one dry day with moderate humidity and limited wind to dry. The SW/W-facing rhyolite should be surface-dry by midday today given the clearing skies forecast this afternoon, but residual dampness in sheltered spots is plausible — check on arrival.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The Cromlech Boulders sit in the floor of Llanberis Pass where cold air pools overnight, meaning dew and condensation can persist on rock surfaces well into the morning even on nominally dry days.
- Several classic problems (notably Jerry's Roof and the overhanging faces) have sheltered undercuts and north-facing holds that can retain moisture longer than the sun-catching slabs.
- The boulders are roadside at ~250m and the Pass funnels wind effectively — when the breeze picks up from mid-morning today (forecast 10–17 km/h NE), exposed faces should dry quickly.
- Recent weeks have seen very heavy cumulative rainfall (116mm in 28 days) which can lead to seepage from the hillside above trickling down onto some boulder tops and backs.
Warnings
2
- Rhyolite becomes very slippery when damp — if holds feel greasy on touch-test, do not climb as a sudden foot slip on these boulders can cause serious injury.
- Weekend crowds are likely on a Sunday; if conditions are marginal, polished popular problems will feel greasier than less-trafficked ones.
Reasoning
Yesterday's 2.7mm of rain in 91% humidity left surface moisture on the boulders, and overnight humidity remained high (87–92%), so some residual dampness is likely on sheltered faces this morning.
Today is forecast completely dry with humidity dropping to 73–75% by late afternoon and a NE breeze building to 17 km/h, which combined with the SW/W aspect receiving afternoon sun should dry exposed surfaces by midday, though sheltered areas may take longer.
Rhyolite is non-porous and does not suffer structural damage when wet — the only risk is reduced friction on damp surfaces.
Early May in Snowdonia is a transitional period with lengthening days aiding drying, but the recent prolonged wet spell (116mm in 28 days) means background moisture levels are elevated.
Contributing Factors
7
2.7mm fell on May 3rd with 91% humidity, leaving surface moisture that has had limited drying time.
Today is forecast entirely dry with 0.0mm precipitation, allowing progressive drying through the day.
Humidity remained 87–92% overnight and through early morning, likely sustaining surface dampness and possible condensation.
Cloud cover drops to near 0% by 18:00 with humidity falling to 73% and a 15–17 km/h NE breeze, creating good drying conditions from mid-afternoon.
116mm over 28 days means the hillside above is saturated, increasing the chance of seepage onto boulder tops and backs.
Rhyolite does not absorb water so only surface moisture matters, and it dries relatively quickly once conditions improve.
The south-west and westerly aspect will receive direct afternoon and evening sun, accelerating surface drying from early-to-mid afternoon.
Recommendations
3
- Target an afternoon session (after 13:00) when sun hits the SW/W faces and humidity has dropped — the rock is most likely to be fully dry from mid-afternoon onwards.
- Physically touch holds and test friction before committing to harder problems, especially on sheltered or north-facing surfaces and undercuts.
- Avoid boulder tops and backs near the hillside where seepage from the saturated ground above may persist even when main faces are dry.
Do Not Climb
65%
4 days ago
Today has seen 2.7mm of rain with 90% humidity and light winds, meaning the boulders are likely wet or damp right now. Although rhyolite dries relatively quickly, the combination of ongoing moisture, high humidity, and modest wind means reliable friction cannot be expected today.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The Cromlech Boulders sit roadside at the base of the pass and can channel damp air, keeping humidity high even when the wider forecast improves.
- The SW/W aspect means the boulders receive afternoon and evening sun, but today's overcast and humid conditions will limit effective solar drying.
- Many classic problems (e.g. Jerry's Roof) involve slopey, friction-dependent holds on polished rhyolite that become treacherous with even a thin film of moisture.
- The boulders are low-lying and surrounded by grass and boggy ground, which can contribute to localised dampness and slow ambient drying in humid conditions.
Warnings
2
- Rhyolite becomes very slippery when wet — do not attempt friction-dependent problems if the rock feels greasy.
- The recent run of damp days (May 1–3) means moss and lichen on lesser-trafficked problems may be especially slick.
Reasoning
With 2.7mm of rain today on top of 2.0mm and 2.5mm on the preceding two days, and current humidity at 90%, the rock surfaces are almost certainly wet or carrying a film of moisture.
Wind is only 14.8 km/h from the north (not the favourable SW/W aspect direction), and high humidity severely limits evaporative drying — surface moisture will persist through much of the day.
Rhyolite is non-porous and structurally unaffected by moisture; there is no risk of hold breakage or rock damage from climbing when wet, but friction loss makes climbing dangerous.
Early May in Snowdonia is transitional — days are lengthening and temperatures are moderate, but prolonged wet spells with high humidity are common and can keep conditions marginal for days at a time.
Contributing Factors
6
2.7mm today following 2.0mm and 2.5mm on May 1st and 2nd means the rock has had no meaningful dry window in three days.
Humidity at 90% today drastically slows surface evaporation and may cause condensation on cool rock surfaces.
Wind at only 14.8 km/h from the north does not effectively reach the SW/W-facing boulder surfaces, reducing drying benefit.
Rhyolite does not absorb water so will dry quickly once conditions improve, and there is no structural risk from climbing on damp rock.
Temperatures around 12–13°C are adequate for drying but not particularly warm, limiting the pace of evaporation.
A strong dry spell from April 21–30 means there is no deep-seated residual moisture in seepage lines — once the current surface moisture clears, conditions should recover quickly.
Recommendations
3
- Wait for tomorrow (May 4th) which is forecast dry with lower humidity — assess conditions on arrival as the rock should be drying through the day.
- If visiting today, bring a towel to test surfaces but be prepared to walk away; friction-dependent problems like Jerry's Roof should be avoided entirely.
- Check conditions in the afternoon when any residual moisture from today's rain has the best chance of clearing, though high humidity may limit this.
Do Not Climb
45%
4 days ago
The Cromlech Boulders have received 2.7mm of rain today on top of light rain over the past two days (7.2mm in the last week), and humidity is high at 88% with light winds — the rock is likely still damp on the surface. While rhyolite dries relatively quickly, the combination of ongoing moisture input and poor drying conditions today means friction will be compromised.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside at the base of the Pass and can channel damp air flow, meaning humidity often lingers longer than the aspect alone would suggest.
- Jerry's Roof and the steeper overhanging problems may retain seepage and drips from the hillside above, especially after a prolonged wet period like the one from mid-April.
- The SW/W aspect provides good afternoon sun when skies clear, but today's conditions (overcast, high humidity, northerly wind) mean the aspect advantage is largely negated.
- Being roadside boulders rather than a cliff face, the tops of some blocks can pool water and seep down faces, keeping certain problems damp even when adjacent lines appear dry.
Warnings
2
- Rhyolite becomes very slippery when wet — a fall from even a low boulder problem can cause serious injury on the rocky landing zone.
- The heavy rainfall in the past month (117mm) may cause seepage from the hillside above, keeping some boulder faces damp even after surface rain has evaporated.
Reasoning
With 2.7mm of rain today, 2.5mm yesterday, and 2.0mm the day before, the rock surface is almost certainly still damp in many areas despite the non-porous nature of rhyolite.
Light northerly winds at 14.4 km/h and 88% humidity provide minimal drying potential; the SW/W aspect receives no benefit from a northerly wind direction, and overcast skies limit solar warming.
Rhyolite is non-porous and does not suffer structural damage from moisture — there is no hold-breakage concern here.
Early May in Snowdonia is transitional; days are lengthening and temperatures are mild (~13°C), but the recent pattern of frequent showers and high humidity is typical of a Welsh spring and limits drying windows.
Contributing Factors
6
2.7mm today following 2.5mm yesterday and 2.0mm the day before means the rock has had no meaningful dry window.
Very high humidity severely inhibits evaporation of surface moisture from the rock.
Wind at only 14.4 km/h from the north provides poor drying for these SW/W-facing boulders, which are somewhat sheltered from northerlies by the Pass walls.
Rhyolite does not absorb water so no structural damage risk and surface moisture can evaporate quickly once conditions improve.
117mm of rain in the last 28 days means the ground and surroundings are saturated, contributing ambient moisture and potential hillside seepage onto boulder tops.
Temperatures around 13°C are adequate for drying but not exceptional; no freeze-thaw concern at this altitude in May.
Recommendations
3
- Wait for tomorrow (May 4th) which is forecast dry — assess conditions on arrival as the rock may have dried sufficiently by afternoon with any sun or breeze.
- If visiting today, limit yourself to overhanging or sheltered problems that may have stayed dry, and test friction carefully on low moves before committing.
- Avoid slab and friction-dependent problems until you have confirmed the rock is completely dry to the touch — rhyolite loses grip dramatically when even slightly damp.
Do Not Climb
40%
4 days ago
Today has seen 2.4mm of rain with 87% humidity and light winds, meaning the boulders are likely still damp right now. Although rhyolite dries relatively quickly, the combination of today's precipitation, high humidity, and light northerly winds (sheltering the SW/W-facing boulders from drying airflow) means surface moisture is unlikely to have cleared.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The Cromlech Boulders sit at the base of Dinas Cromlech in the Llanberis Pass, which can funnel and redirect wind — a northerly wind today means the SW/W-facing boulder surfaces are largely sheltered from direct airflow, slowing drying significantly.
- Roadside seepage from the hillside above can feed moisture onto certain problems even after rain stops, particularly on the back walls and lower sections of the main boulders.
- The Pass can trap cloud and mist at 250m, and today's 87% humidity suggests persistent dampness in the air that will inhibit surface evaporation even on exposed faces.
- Spring conditions in the Llanberis Pass are notoriously changeable — the heavy rain period from mid-April (over 90mm in 9 days from Apr 9–17) may still be contributing to hillside runoff and seepage onto some boulder faces.
Warnings
2
- Rhyolite becomes very slippery when wet — a slip on highball problems like Jerry's Roof area could result in serious injury; do not attempt committing problems unless surfaces are confirmed dry.
- Three consecutive days of rain mean landing zones may be muddy and slippery — take extra care with pad placement and spotting.
Reasoning
With 2.4mm of rain today, 2.5mm yesterday, and 2.0mm the day before, the boulder surfaces are almost certainly carrying surface moisture, compounded by 87% humidity.
Light northerly winds at 14.4 km/h offer minimal drying benefit to SW/W-facing surfaces; the boulders are effectively sheltered from today's airflow, and high humidity prevents effective evaporation.
Rhyolite is non-porous and does not suffer structural damage from moisture — there is no risk of hold breakage from climbing on damp rock, but friction loss makes falls much more likely.
Early May in the Llanberis Pass is transitional — days are lengthening but temperatures remain modest (13°C max) and the recent wet spell from mid-April has kept the environment saturated.
Contributing Factors
6
2.4mm today, 2.5mm yesterday, and 2.0mm the day before means three consecutive days of light rain with zero consecutive dry days.
87% humidity today severely inhibits surface evaporation, keeping the rock damp even where rain hasn't directly fallen.
Northerly wind at only 14.4 km/h provides poor drying for SW/W-facing boulder surfaces which are sheltered from this wind direction.
Rhyolite sheds water quickly once conditions improve and does not absorb moisture internally, so drying can be rapid when wind and humidity cooperate.
A max of 13.1°C provides some thermal energy for drying but is not warm enough to drive rapid evaporation in humid, still conditions.
116.9mm over the last 28 days indicates a very wet month, meaning hillside seepage and ambient ground moisture are elevated.
Recommendations
3
- Wait for tomorrow (May 4th) which is forecast dry — assess conditions on arrival as the boulders may have dried sufficiently by afternoon if humidity drops.
- If visiting today, bring a towel and be prepared to walk away — test friction on easy low problems before committing to anything committing or highball.
- Check for seepage on the back walls and lower sections of the main boulders, as hillside drainage after the recent wet period may persist for days.
Marginal — Assess Conditions
55%
5 days ago
Today saw 2.5mm of rain with high humidity (84%) and light winds, so the boulders are likely still damp right now. However, the preceding week was largely dry with a good drying spell from April 21–30, and the rhyolite surface should dry quickly once the drizzle stops — a visual check on arrival is essential before committing to climb.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The Cromlech Boulders sit roadside at the base of Dinas Cromlech and can collect runoff and seepage from the hillside above, meaning even light rain can leave persistent damp patches on overhanging faces and in cracks.
- The SW/W aspect means the boulders receive afternoon and evening sun which will help dry today's moisture, but the Pass itself is a narrow valley that can remain shaded and humid in the mornings.
- Jerry's Roof and other steep/overhanging problems may retain moisture on undercut holds and lip sequences even when vertical faces appear dry — test friction carefully before committing to crux moves.
- Being roadside and extremely accessible, the boulders see heavy traffic; polished holds on popular problems like Jerry's Roof become especially slippery with even a trace of residual moisture.
Warnings
2
- The rhyolite becomes very slippery when wet — do not attempt highball or committing problems unless you have verified the rock is completely dry and friction is good.
- Seepage from the hillside above can keep specific faces damp even when it hasn't rained recently; check all holds before pulling on.
Reasoning
Today's 2.5mm of rain on top of 2.0mm yesterday means the rock surface is currently damp, though the total recent precipitation is modest and the rock had an extended dry spell (April 21–30) prior to this.
Light SW winds at only 14.8 km/h and 84% humidity today provide limited drying potential, but rhyolite is non-porous and the SW/W aspect should receive some afternoon sun to assist surface evaporation if skies clear.
Rhyolite is non-porous and extremely hard — there is no risk of structural weakening or hold breakage from the recent moisture, so the concern is purely friction loss on wet surfaces.
Early May in Snowdonia is transitional with improving daylight hours but still prone to Atlantic weather systems; humidity remains elevated and temperatures are moderate, giving decent but not rapid drying conditions.
Contributing Factors
6
2.5mm of precipitation today with high humidity (84%) means the boulder surfaces are likely still wet or damp this afternoon.
The extended dry period from April 21–30 (10 consecutive dry days, low humidity, warm temps up to 20.5°C) thoroughly dried the rock before the recent light showers.
Today's wind is only 14.8 km/h with 84% humidity, limiting the rate of surface evaporation on the rhyolite.
The south-west and westerly aspect means the boulders receive direct afternoon sunlight which will accelerate drying of surface moisture if cloud breaks.
Rhyolite does not absorb water internally, so only surface moisture needs to evaporate — drying can be rapid once rain stops and conditions improve.
The exposed roadside setting in Llanberis Pass channels wind across the boulders, aiding drying once conditions improve, though today's winds are relatively light.
Recommendations
3
- Visit later in the afternoon when any afternoon sun on the SW/W aspect has had time to dry surface moisture, and physically touch the rock before climbing.
- Avoid polished or overhanging problems (especially Jerry's Roof) until you have confirmed excellent friction on easier warm-up problems.
- Bring a towel to dry holds if only residual dampness remains, but do not climb if friction feels compromised — conditions should improve significantly by tomorrow or the day after.
Marginal — Assess Conditions
60%
6 days ago
After an extended dry spell from April 21–30, today saw 2.6mm of rain breaking the dry streak. The rhyolite should dry quickly given its non-porous nature and the exposed aspect, but with today's rain, elevated humidity (70%), and moderate winds, surfaces may still carry residual moisture — a visual check on arrival is essential.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The Cromlech Boulders sit roadside in the Llanberis Pass where the valley can funnel moisture-laden SW winds, meaning today's southerly airflow may keep surfaces damp longer than open hillside crags.
- The SW/W aspect means afternoon sun will hit the boulders well, but the pass topography can shade sections earlier in the day, so morning sessions may find lingering dampness in sheltered nooks.
- Jerry's Roof and other overhung problems may actually be drier than vertical faces since overhangs shed direct rain, but seepage from above can track down and pool on lip holds — inspect carefully.
- The boulders had an excellent prolonged drying window (April 21–30, nine consecutive dry days with low humidity and strong easterly winds) which would have thoroughly dried any deep seepage from the heavy mid-April rain.
Warnings
2
- Rhyolite becomes very slippery when wet — even a thin film of moisture dramatically reduces friction on slab and vertical problems; do not attempt friction-dependent lines if any dampness is detected.
- The forecast shows no significant dry window in the next 5 days; plan sessions carefully around breaks in precipitation and be ready to bail.
Reasoning
Today's 2.6mm of rain has broken a 9-day dry spell, leaving surface moisture on the rhyolite; at 70% humidity and moderate 22 km/h southerly winds, the rock surface is likely damp but not saturated.
The SW/W aspect will receive afternoon sun on May 1st, and the exposed position aids wind-drying, so surface moisture from today's light rain could clear within a few hours of dry weather — but the day is not over and conditions remain unsettled.
Rhyolite is non-porous and extremely hard; there is no risk of structural damage or hold breakage from today's moisture.
Early May in Snowdonia is transitional — days are lengthening with decent solar angles, but Atlantic fronts still bring frequent showers; the forecast shows continued unsettled weather through the coming week.
Contributing Factors
6
2.6mm of precipitation today has wetted the rock surface, breaking a prolonged dry spell and resetting drying time.
Nine consecutive dry days (April 21–30) with low humidity and strong easterly winds thoroughly dried the boulders, meaning no deep-seated moisture issues exist.
Moderate humidity slows evaporation from rock surfaces, though it is not as problematic as the 90%+ levels seen in mid-April.
The exposed site with 22 km/h southerly winds promotes faster surface drying of the non-porous rhyolite.
The south-west and westerly aspect receives direct afternoon sun in early May, aiding surface evaporation during the second half of the day.
The coming 5 days show persistent light precipitation (0.4–2.5mm daily) and high humidity (80–95%), suggesting conditions will not significantly improve.
Recommendations
3
- Visit in the afternoon when the SW-facing boulders have had maximum sun and wind exposure to dry today's rain, and physically touch key holds before committing to problems.
- Prioritise overhung problems like Jerry's Roof which are more sheltered from direct rain, but check lip holds carefully for seepage tracking from above.
- Bring a towel and soft boar's hair brush to dry individual holds if surfaces are only lightly damp, and be prepared to walk away if friction feels compromised.
Likely Safe to Climb
90%
9 days ago
The Cromlech Boulders should be in excellent condition today. Three consecutive dry days with strong easterly winds and dropping humidity have thoroughly dried the non-porous rhyolite, and today's forecast of 16°C, 52% humidity, and 38 km/h winds will maintain prime friction conditions.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside in the Llanberis Pass and are exposed to the prevailing wind, which has been a sustained easterly at 25–38 km/h over the past two days — highly effective at stripping any residual surface moisture.
- Some problems on the north-facing or sheltered sides of boulders (away from the SW/W aspect) may retain dampness in cracks or undercuts, so check these features individually.
- The heavy rainfall earlier in April (40.5mm on April 11, plus repeated wet spells) means seepage from the hillside above could still be active on certain boulder faces — look for dark streaks near the base of Dinas Cromlech.
- With the recent dry spell and an easterly wind direction, the main SW/W-facing problems will have had optimal drying conditions and should offer superb friction.
Warnings
1
- Some seepage lines fed by the hillside above may still be active despite surface dryness — avoid problems directly below obvious wet streaks.
Reasoning
Only 0.4mm of rain in the last 7 days (a trace on April 26), with three fully dry days and humidity averaging 57% — the non-porous rhyolite surface will be thoroughly dry.
Strong winds (25–38 km/h) from the east over the past two days, combined with daytime temperatures reaching 12–16°C, have provided excellent evaporative drying conditions for this exposed site.
Rhyolite is non-porous and extremely hard; there is no risk of structural damage or hold breakage from the recent wet weather.
Late April in North Wales brings lengthening days and reasonable solar angles for the SW/W aspect; temperatures are mild and conditions are entering the prime spring climbing window.
Contributing Factors
6
No meaningful precipitation since April 26 (0.4mm trace), giving ample time for all surface moisture to evaporate from the non-porous rhyolite.
Winds of 25–38 km/h over the past 48 hours at this exposed site have been highly effective at drying surface moisture.
Today's humidity of 52% is well below the condensation threshold and significantly lower than the 80–95% range seen during the wet spells, promoting excellent friction.
Temperatures of 16°C today aid evaporation and indicate no risk of frost or condensation on the rock surface.
144mm over the last 28 days is substantial, but the non-porous rhyolite does not absorb water, so the main concern is localised seepage from the hillside which has likely subsided after 3 dry days.
The easterly wind is relatively dry (rain shadow from Snowdonia's western slopes) and blows across the SW/W-facing boulders, enhancing surface drying.
Recommendations
3
- Conditions are excellent — head out and enjoy the friction, but briefly check any shaded north-facing features or undercuts for residual dampness before committing.
- Bring a brush to clean any mud or grit tracked onto holds from the previously wet hillside approaches.
- Check BMC RAD for any active bird nesting restrictions at Dinas Cromlech and nearby crags before visiting, as this is peak nesting season.
Climbing Outlook