Condition Analysis
AI-powered assessment using site data and 14-day weather history
After a very wet spell culminating in 40.5mm on April 11th and 5.5mm on April 12th, today is the first fully dry day with moderate winds and dropping humidity. The rhyolite surfaces may have dried sufficiently given the exposed aspect and wind, but residual dampness in sheltered spots and seepage from the saturated hillside above is likely — climbers should visually check conditions on arrival.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit directly below Dinas Cromlech and the steep hillside above acts as a catchment — after 60mm in the last 7 days, seepage and runoff from the slope above may keep certain boulder faces damp even when rain has stopped.
- The roadside location in the Llanberis Pass creates a wind-tunnel effect that aids drying on exposed faces, but the pass orientation means some boulder faces (especially north-facing sides and overhanging undersides) can remain damp long after rain ceases.
- Jerry's Roof and other steep/overhanging problems may actually retain less surface moisture than slabby or vertical faces, but condensation can collect on roof features — test friction carefully before committing to committing moves.
- April is within the bird nesting restriction window — check BMC RAD for any current peregrine or raven nesting restrictions on Dinas Cromlech and surrounding cliffs before visiting.
Warnings
2
- Do not climb on any visibly damp surfaces — wet rhyolite is extremely slippery and bouldering falls from even moderate height onto pads can cause serious injury.
- Check BMC RAD for bird nesting restrictions at Dinas Cromlech and adjacent crags — April is within the peregrine nesting season.
Reasoning
The rock received 5.5mm of rain on April 12th (mostly in the early hours) followed by roughly 18 hours of dry conditions with moderate winds, but 60mm over the last 7 days and 156mm over 28 days means the surrounding ground and hillside are thoroughly saturated, increasing the risk of seepage onto boulder surfaces.
The SW/W aspect and exposed position with moderate winds (18–25 km/h) through the afternoon of April 12th and into today should have cleared direct surface water from exposed faces, but humidity remained high (77–91%) overnight, limiting evaporative drying and potentially allowing condensation.
Rhyolite is non-porous and does not suffer structural weakening when wet — there is no risk of hold breakage from moisture, and the rock integrity is not a concern.
Early April in Snowdonia is a transitionally wet period with limited solar intensity at 250m altitude; the sun angle is still relatively low and day length is moderate, providing some but not optimal drying conditions.
Contributing Factors
6
40.5mm fell on April 11th and 5.5mm on April 12th, with 60mm total in the last 7 days, thoroughly saturating the surrounding hillside and increasing seepage risk onto the boulders.
Today (April 13th) is forecast completely dry with 0mm precipitation, giving the rock a full day of drying time since the last meaningful rain.
The boulders' exposed position in the Llanberis Pass with moderate SW winds (18 km/h today) helps clear surface moisture from rhyolite relatively quickly.
Average humidity of 79% today and 78% over the last 7 days limits evaporative drying efficiency and may allow condensation to form on cooler rock surfaces, especially overnight.
156mm of rain over 28 days has thoroughly saturated the hillside above the boulders, meaning seepage and drainage onto certain boulder faces is very likely even during dry spells.
12.9mm is forecast for tomorrow (April 14th) with 18.9mm on April 15th, meaning today represents a narrow window that will close quickly and the rock will be re-wetted imminently.
Recommendations
3
- If visiting today, physically test friction on lower, easier problems before attempting anything committing — the rock may feel greasy despite appearing dry.
- Focus on well-exposed SW/W-facing vertical and overhanging surfaces which will have dried fastest; avoid slabs and any faces showing visible dampness or dark water streaks.
- Bring a towel and consider that conditions will likely deteriorate from tomorrow onwards — today is the only viable window this week if conditions check out on arrival.
Previous Analyses
Do Not Climb
92%
2 days ago
The Cromlech Boulders are currently wet and unsuitable for climbing. Today has seen 5.1mm of rain on top of 40.5mm yesterday and nearly 60mm over the past week, with high humidity and no meaningful drying window — the rhyolite will be slippery and dangerous.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside at the base of Dinas Cromlech and receive runoff and seepage from the hillside above, meaning they stay wet longer than their aspect alone would suggest after heavy rain.
- The SW/W aspect is currently receiving the prevailing SW wind and rain directly, offering no shelter from the current weather pattern.
- Jerry's Roof and other overhanging problems may appear dry underneath but surrounding landings and topouts will be saturated and treacherous.
- At 250m altitude in the Llanberis Pass, low cloud and mist are common in these conditions and can keep surfaces damp even between rain showers.
Warnings
2
- Wet rhyolite is extremely slippery — do not attempt to climb on damp surfaces as falls from even low bouldering heights can cause serious injury.
- The forecast shows another very heavy rain event on April 15th (34.4mm) which will reset any drying progress made in the interim.
Reasoning
The rock is currently wet: 5.1mm of rain today follows 40.5mm yesterday and a cumulative 60mm over the past week, with zero consecutive dry days and 83% humidity.
Despite the exposed position and moderate SW wind (30 km/h), there has been no meaningful dry window since the heavy rain on April 11th — the rock surface has had no opportunity to dry.
Rhyolite is non-porous and does not suffer structural damage when wet, so there is no risk of hold breakage or rock degradation.
Early spring in Snowdonia brings persistent wet weather fronts from the Atlantic; the current pattern of repeated heavy rainfall with short breaks is typical and conditions are unlikely to improve until a sustained high-pressure system establishes.
Contributing Factors
6
40.5mm fell yesterday and 5.1mm today, with nearly 60mm in the past 7 days — the boulders are thoroughly wet.
There has been no dry day since the last significant rainfall, meaning no drying window has occurred.
Humidity at 83% today and averaging 77% over the past week significantly slows surface evaporation on the rock.
The prevailing SW wind direction means the boulders are fully exposed to the incoming weather rather than sheltered from it.
Rhyolite does not absorb water, so once a genuine dry window arrives the surface will dry relatively quickly — but that window has not yet occurred.
Wind at 30 km/h would normally assist drying, but with ongoing rain and high humidity it cannot overcome the continuous wetting.
Recommendations
3
- Do not visit today — the boulders will be wet and dangerously slippery.
- Monitor conditions for April 13th which shows only 0.4mm forecast; if it stays dry and wind picks up, a late-afternoon assessment might find some surfaces drying, but caution is still warranted.
- If visiting on a marginal day, prioritise steep or overhanging problems that shed water fastest and test friction carefully on easy moves before committing to anything harder.
Do Not Climb
95%
3 days ago
The Cromlech Boulders are currently soaked from 31mm of rain today on top of a very wet preceding week (46mm in 7 days). The rhyolite will be extremely slippery and dangerous for bouldering, with no dry window in sight for several days.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The Cromlech Boulders sit directly below the steep hillside of Dinas Cromlech, meaning surface runoff and seepage from above can keep the boulders wet well after rain stops, particularly on overhanging faces and cracks.
- Despite the SW/W aspect providing reasonable solar exposure, the Llanberis Pass is a narrow valley that limits direct sunlight hours, slowing drying compared to open hillside crags.
- The roadside location at 250m in the Pass acts as a natural funnel for wet south-westerly winds, which while aiding evaporation also bring persistent moisture-laden air from the Atlantic.
- Some of the classic problems (e.g. Jerry's Roof) involve steep or roof terrain where water pools and seeps persist long after the main surfaces appear dry — these require extra drying time.
Warnings
3
- Rhyolite becomes extremely slippery when wet — bouldering on wet Cromlech problems with hard landings poses a serious injury risk.
- Hillside runoff from Dinas Cromlech above will keep seepage active on the boulders for hours after rain stops.
- The forecast shows no reliable dry window for at least 5 days; do not assume a brief gap between showers is sufficient.
Reasoning
The rock is saturated: 31.1mm fell today, following 8.4mm and 5.7mm on the two preceding days, with 46mm total in the last week — the boulders will be streaming wet.
Despite strong SW winds (39 km/h), there has been zero consecutive dry hours today to begin any meaningful drying, and the SW aspect receives limited late-afternoon sun in early April at this valley location.
Rhyolite is non-porous and structurally unaffected by water, so there is no risk of hold breakage or rock damage from the wet conditions.
Early April in Snowdonia is a notoriously wet period with frequent Atlantic fronts; temperatures around 7°C and high humidity (85%) significantly slow surface evaporation.
Contributing Factors
7
31.1mm of precipitation today is a substantial downpour that will leave all boulder surfaces streaming wet.
46mm over the past 7 days with only brief dry interludes means the ground is saturated and hillside seepage onto the boulders will be persistent.
There have been no consecutive dry days, so no meaningful drying window has occurred since the recent wet spell began.
Humidity at 85% today severely limits evaporative drying even with strong wind.
39 km/h winds aid surface evaporation once rain stops, but today's ongoing heavy rain negates any drying benefit.
Rhyolite does not absorb water, so once rain stops and surfaces drain, it will dry faster than porous rock types.
Temperatures of 7°C provide minimal thermal energy for evaporation, extending drying times.
Recommendations
3
- Do not visit today — the boulders will be dangerously slippery with active rainfall and no prospect of drying.
- Monitor conditions closely; the earliest realistic window may not arrive until April 17 if the forecast dry day materialises, but even then verify on arrival.
- If visiting the Pass during this wet spell, consider indoor alternatives such as the Beacon Climbing Centre in Caernarfon.
Do Not Climb
90%
4 days ago
The Cromlech Boulders are currently wet from today's rain (2.9mm) following a very unsettled period with 22.4mm in the last 7 days and zero consecutive dry days. With high humidity, ongoing precipitation today, and a major 28mm+ storm forecast tomorrow, conditions are clearly unsuitable for climbing on friction-dependent rhyolite.
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 fully exposed to the prevailing SW weather systems that funnel up the pass, meaning they receive the brunt of incoming Atlantic rain.
- Despite the SW/W aspect offering good solar exposure, the pass orientation channels moist air directly over the boulders, and seepage from the hillside above (below Dinas Cromlech) can keep certain problems damp long after rain stops.
- At 250m in the pass, low cloud and mist are common in unsettled spring weather, maintaining surface moisture on the rock even between rain events.
- Jerry's Roof and other steep overhung problems may appear dry underneath but surrounding landing zones and slab starts will be slippery and hazardous in current conditions.
Warnings
2
- Wet rhyolite at the Cromlech Boulders is extremely slippery — friction loss on slab and vertical problems creates a serious fall risk.
- A major storm (28mm+) is forecast for tomorrow (April 11th); do not plan climbing for the next two days at minimum.
Reasoning
The rock is currently wet — 2.9mm of rain today with zero consecutive dry days, following 8.4mm yesterday and a prolonged unsettled spell totalling 22.4mm over the past week.
Although the exposed position and moderate winds (33 km/h from the south) would normally aid drying on this non-porous rhyolite, there has been no meaningful dry window since April 5th to allow surfaces to fully dry.
Rhyolite is non-porous and does not suffer structural damage when wet; the risk here is entirely friction loss and climber safety on slippery surfaces.
Early April in Snowdonia is still a high-rainfall period with cool temperatures averaging 9°C, limiting evaporation; the recent pattern of repeated rain events with only brief dry spells is typical for spring in the Llanberis Pass.
Contributing Factors
6
2.9mm of rain recorded today means the rock surfaces are currently wet.
There has been no sustained dry period since the last significant rain on April 9th (8.4mm), keeping surfaces persistently damp.
A major 28.4mm rainfall event is forecast for April 11th, ensuring conditions will worsen before they improve.
Humidity at 75% today and averaging 77% over the past week significantly slows surface evaporation.
Winds at 33 km/h from the south across this exposed site would aid drying if rain ceased, but cannot overcome active precipitation.
128.8mm over the past 28 days reflects a saturated local environment with waterlogged ground contributing to ambient moisture.
Recommendations
3
- Do not climb today — the rock is wet and rhyolite becomes dangerously slippery when damp, making friction-dependent problems unsafe.
- Monitor conditions for April 14th, which is the first forecast dry day with low humidity and a preceding drying window from April 13th, though verify on arrival.
- If visiting the Llanberis Pass area, use the time to scout problems and plan sessions for the next dry spell rather than risking a slip on greasy rock.
Do Not Climb
45%
5 days ago
Today has seen 1.2mm of rain with high humidity (82%) and the rock is likely damp; the preceding dry spell (Apr 6–8) was promising but today's moisture and overnight near-saturation humidity have re-wetted surfaces. With rhyolite's friction-dependent nature and the current dampness, conditions are unsuitable for confident climbing today.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The Cromlech Boulders sit in the Llanberis Pass corridor which funnels prevailing SW/W winds, aiding drying but also channelling moisture-laden Atlantic air that can keep surfaces damp even without direct rainfall.
- Several of the classic problems (notably Jerry's Roof and the overhanging lines) have sheltered undersides that can remain dry when lighter rain falls, but seepage from above can create greasy patches on vertical and slab faces.
- The roadside location at ~250m means the boulders can sit in low cloud and mist common in the Pass during spring, depositing surface moisture even on nominally 'dry' days.
- High overnight humidity (94–96%) through the early hours of Apr 9 strongly suggests condensation formed on the rock surfaces, which requires solar heating and wind to clear — today's overcast and damp conditions may not provide this.
Warnings
2
- Rhyolite becomes extremely slippery when wet — falling risk is significantly elevated on damp slab and vertical problems.
- The Llanberis Pass can sit in cloud at 250m depositing invisible moisture even when it appears dry at road level; always touch-test the rock.
Reasoning
Today's 1.2mm precipitation combined with overnight humidity peaking at 96% means the rock surfaces are very likely damp from both rain and condensation, despite the good drying window of Apr 6–8.
The SW/W aspect and exposed position normally provide efficient drying, but today's westerly wind at 30 km/h is carrying moist air (82% humidity) and the 1.2mm rain negates the previous dry spell's benefit for surface conditions.
Rhyolite is non-porous and does not suffer structural weakening when wet; there is no hold-breakage or rock-damage concern, only friction loss.
Early April in Snowdonia is transitional with frequent Atlantic frontal systems; the pattern of multi-day wet spells punctuated by brief dry windows (as seen Mar 18–22 and Apr 6–8) is typical and conditions can change rapidly.
Contributing Factors
6
1.2mm of rain today with 82% average humidity means the rock is actively receiving moisture and unlikely to be dry.
Humidity reached 94–96% overnight into Apr 9 with 100% cloud cover, making condensation on cold rock surfaces very likely.
Three fully dry days (Apr 6–8) with warm temperatures up to 19.9°C and moderate wind would have thoroughly dried the rock before today's re-wetting.
151.7mm over the past 28 days indicates a persistently wet period, and while rhyolite doesn't absorb water, surrounding ground saturation and general ambient moisture levels remain high.
The exposed position and 30 km/h westerly wind would clear surface moisture relatively quickly once precipitation stops, but today's rain is ongoing.
Rhyolite does not absorb water, so once rain ceases and surfaces dry, the rock will perform well — no extended internal drying period is needed.
Recommendations
3
- Wait for the rock to dry — if rain stops and wind persists through the afternoon, conditions may improve later today but check surfaces carefully by touch before committing to any problem.
- If visiting today, consider only well-sheltered overhanging problems where rain hasn't directly wetted the holds, and test friction on easy terrain first.
- Monitor conditions for the next proper dry window; Apr 13–14 looks more promising with minimal precipitation forecast and declining wind.
Marginal — Assess Conditions
62%
8 days ago
After a very wet recent period, today is the first fully dry day with improving conditions — warmer temperatures, dropping humidity, and moderate wind should dry the rhyolite surfaces reasonably well through the day. However, with only one dry day following heavy rain on April 3rd–4th (23mm combined) and light showers on the 5th, some sheltered or shaded surfaces may still hold residual moisture, so on-site assessment 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 funnels wind effectively, aiding drying, but the pass can also trap low cloud and mist that re-wets surfaces unexpectedly.
- Several classic problems (including Jerry's Roof) involve overhanging or roof features that are sheltered from direct rain but can accumulate seepage and condensation — these may dry more slowly than vertical faces.
- The SW/W aspect means the boulders receive afternoon and evening sun, which in early April provides useful but limited solar heating; morning sessions may encounter damper rock than afternoon sessions.
- The boulders sit at the base of a steep hillside and runoff from Dinas Cromlech above can cause persistent dampness at the base of certain blocks even after rain has stopped.
Warnings
2
- Rhyolite becomes extremely slippery when wet — even a thin film of moisture on polished holds can cause unexpected slips on highball problems.
- Hillside runoff from Dinas Cromlech above may keep the base of some boulders damp even when exposed faces appear dry.
Reasoning
The last significant rain was 10.3mm on April 4th followed by 0.8mm on the 5th, with today (April 6th) being fully dry at 0.0mm — surface moisture on exposed rhyolite faces is likely evaporating but some sheltered areas may retain dampness given only ~24 hours of proper drying.
The SW/W aspect and exposed wind position (23.8 km/h southerly today) combined with a warm 13.8°C max and dropping humidity (73%) provide good drying conditions, though overnight humidity was high (88–90%) meaning dew or condensation likely formed and needs burning off through the day.
Rhyolite is non-porous and extremely hard; there is no risk of structural damage or hold breakage from recent wet conditions.
Early April in Snowdonia is transitional — days are lengthening and today's warmth (13.8°C) is above average, but the predominantly wet March/April pattern (180.8mm in 28 days) means the general environment remains saturated and ambient moisture levels are high.
Contributing Factors
6
Over 35mm fell in the last 7 days including 10.3mm on April 4th, with the broader 28-day total of 180.8mm indicating a thoroughly saturated environment.
Today is forecast completely dry with 13.8°C, 73% humidity, and 23.8 km/h southerly wind — strong drying conditions for exposed rhyolite.
With just one consecutive dry day after prolonged wet weather, there has been limited cumulative drying time and some surfaces will retain moisture.
Overnight humidity reached 88–90% with temperatures around 4°C, likely producing dew or condensation on rock surfaces that needs daytime warmth to clear.
The Llanberis Pass funnels wind across the boulders, significantly aiding surface evaporation on this moderately windy day.
The SW/W facing boulders will receive direct sun from midday onwards, warming the rock and accelerating surface drying through the afternoon.
Recommendations
3
- Wait until mid-afternoon for best conditions — the combination of solar warming and wind should have dried most exposed surfaces by then.
- Physically touch-test all holds before committing to a problem, especially on lower or sheltered faces that receive less direct sun and wind.
- Bring a towel and soft brush for light surface moisture; if holds feel greasy or polished surfaces lack friction, back off and wait for further drying.
Do Not Climb
35%
9 days ago
The Cromlech Boulders have been subjected to persistent heavy rainfall over recent weeks (181mm in 28 days, 36mm in the last 7 days), with significant rain yesterday (10.3mm) and light precipitation today (0.9mm). Despite the exposed aspect and strong westerly winds aiding surface drying, zero consecutive dry days and high ambient humidity mean the rhyolite surfaces are very likely still damp and slippery.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside at the base of the Llanberis Pass where moisture funnels down the valley, and runoff from the hillside above (Dinas Cromlech) can keep the tops and backs of boulders wet long after rain stops.
- Jerry's Roof and other overhanging problems may feel drier underneath but landing zones can be saturated and muddy after prolonged wet spells, affecting pad placement and safety.
- The SW/W aspect means afternoon sun can warm and dry faces quickly when it appears, but in early April daylight hours and sun angle are still limited — check conditions improve through the afternoon.
- Crowding at weekends can churn up wet ground around the boulders, making approaches and landings increasingly slippery through the day.
Warnings
2
- Rhyolite becomes extremely slippery when wet — even a thin film of moisture dramatically reduces friction and increases the risk of unexpected slips on slab and friction-dependent problems.
- Landing zones around the boulders will be saturated and muddy after weeks of heavy rain; ensure pads are placed carefully and be aware of slippery ground.
Reasoning
With 10.3mm yesterday, 0.9mm today, and zero consecutive dry days, the rhyolite surfaces are almost certainly still carrying surface moisture and will be slippery.
Strong westerly winds (36.7 km/h today) and the exposed position will strip surface water relatively quickly, but humidity remains high (76%) and there has been no meaningful dry window to fully clear moisture from all faces.
Rhyolite is non-porous and does not suffer structural weakening when wet — there is no risk of hold breakage or rock damage from climbing in these conditions.
Early April in Snowdonia is still firmly in the transition period with frequent Atlantic fronts; the 181mm over 28 days is typical of a wet spring pattern, and ground saturation means even light rain lingers on surfaces longer than usual.
Contributing Factors
7
10.3mm fell yesterday and 0.9mm today, with 36mm in the last 7 days and 181mm over 28 days — the boulders have had virtually no opportunity to dry.
There has not been a single fully dry day leading into today, meaning surface moisture has been continuously replenished.
Average humidity over the last 7 days is 88%, severely limiting evaporative drying even on surfaces not directly rained upon.
Sustained westerly winds of 36.7 km/h today at this exposed roadside venue will accelerate surface drying significantly.
The SW/W-facing boulders receive afternoon sun which aids drying, though early April sun angle and cloud cover limit effectiveness.
Rhyolite does not absorb water, so once surface moisture evaporates the rock returns to full friction — but it must be genuinely dry first.
Early April in Snowdonia brings frequent Atlantic weather systems and the persistent wet pattern shows no sign of breaking until April 7th at the earliest.
Recommendations
3
- Wait for a sustained dry window — April 7th looks like the first realistic opportunity with 0mm forecast, lower humidity (65%), and warmer temperatures (16.3°C).
- If visiting today or tomorrow, bring alternative plans such as indoor climbing at Beacon or The Indy in Llanberis, as conditions are unlikely to be climbable.
- If you do visit on a drier day, test friction carefully on easy problems first before committing to harder lines — residual dampness can lurk on north-facing holds and in cracks.
Do Not Climb
35%
10 days ago
The Cromlech Boulders have received 5mm of rain today on top of a very wet recent period (40mm in the last 7 days, 175mm in 28 days), and with 90% average humidity, surfaces are almost certainly still wet or greasy. Although strong SW winds will aid drying, the rock has had zero consecutive dry days and conditions are not suitable for climbing today.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside at the base of Dinas Cromlech in the Llanberis Pass, where the pass funnels wind effectively — the strong SW/W winds today (43.6 km/h) will help strip surface moisture, but rain fell today so drying has barely begun.
- Several classic problems like Jerry's Roof involve steep or overhanging terrain that may shed water faster, but slab and vertical faces in the venue will remain greasy longest.
- The Llanberis Pass is notorious for lingering dampness — cloud and mist can roll through the pass even on nominally dry days, re-wetting surfaces that appear to have dried.
- Seepage from the hillside above can feed moisture onto certain boulders even after rain stops, particularly on problems at the back of the boulder field closer to the slope.
Warnings
2
- Rhyolite becomes extremely slippery when wet — even a thin moisture film can dramatically reduce friction, making falls likely on friction-dependent moves.
- Do not rely on visual appearance alone; rhyolite can look dry while retaining a greasy surface film, especially in high humidity.
Reasoning
With 5mm of rain today, 40mm over the past week, and persistently high humidity (90%), the rock surfaces are currently wet and greasy.
Strong SW winds (43.6 km/h) and the exposed SW/W aspect will promote rapid surface drying once rain stops, but with rain falling today and no consecutive dry days, meaningful drying has not yet occurred.
Rhyolite is non-porous and does not suffer structural damage when wet, so hold breakage and rock conservation are not concerns here.
Early April in Snowdonia is a transitional period with frequent Atlantic fronts; the recent pattern of repeated wet spells is typical, and temperatures around 10°C provide only moderate drying potential.
Contributing Factors
7
5mm fell today following 12.7mm yesterday and 8.7mm the day before, meaning surfaces are actively wet with no drying window.
Average humidity of 90% over the past week severely limits evaporative drying even between rain events.
43.6 km/h SW winds at this exposed roadside venue will accelerate surface drying once precipitation ceases.
The south-westerly aspect receives reasonable afternoon sun in early April, aiding drying when skies clear.
175mm of rain over 28 days with very few dry spells means the ground and surrounding hillside are saturated, promoting seepage and lingering dampness.
Rhyolite does not absorb water, so once surface moisture evaporates, friction recovers quickly without needing extended internal drying.
There have been no consecutive dry days leading into today, meaning the rock has had no sustained drying opportunity.
Recommendations
3
- Wait for the forecast dry spell beginning April 6th — by the afternoon of April 6th or morning of April 7th, conditions should be significantly better.
- If you visit tomorrow (April 5th), expect residual dampness from today's rain; only climb if you can confirm surfaces are fully dry by touch-testing holds.
- Focus on overhanging or well-sheltered problems first as these dry fastest, and avoid slabs and vertical faces which retain a moisture film longest.
Do Not Climb
92%
11 days ago
The Cromlech Boulders are currently wet and unsuitable for climbing. Today has seen 17.2mm of rain with 93% humidity, following a persistently wet period with nearly 40mm in the last 7 days and no consecutive dry days — the rhyolite surfaces will be thoroughly wet and dangerously slippery.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside at the base of the Llanberis Pass and receive runoff and spray from the hillside above, meaning surface water lingers even after rain stops.
- The SW/W aspect is directly exposed to the prevailing SW weather systems funnelling through the Pass, so these boulders bear the full brunt of Atlantic fronts.
- Jerry's Roof and other overhanging problems may appear dry underneath but surrounding landings and lower holds will be saturated and greasy.
- Seepage from the hillside above Dinas Cromlech can keep certain boulder faces damp well after surface rain has dried, particularly on the uphill sides of the blocks.
Warnings
2
- Rhyolite becomes extremely slippery when wet — attempting boulder problems in these conditions risks serious falls and injury.
- The Llanberis Pass is experiencing a prolonged wet spell; even surfaces that look dry may have a thin film of moisture reducing friction significantly.
Reasoning
With 17.2mm of rain today, 93% humidity, and zero consecutive dry days, the rock surfaces are actively wet and will have no friction.
Despite strong SW winds (34 km/h) which aid surface evaporation, the continuous rainfall and very high humidity (88% average over the last week) mean no meaningful drying has occurred.
Rhyolite is non-porous and does not suffer structural damage when wet — there is no hold breakage or rock conservation concern here.
Early April in Snowdonia is a notoriously wet period; the persistent Atlantic frontal systems are delivering frequent rain, and temperatures around 7–12°C combined with high humidity slow any drying between showers.
Contributing Factors
6
17.2mm of precipitation today means the rock is actively wet right now.
Nearly 40mm in the past 7 days with zero consecutive dry days has kept the boulders perpetually damp.
Humidity averaging 88% over the past week and 93% today severely inhibits surface drying.
Winds at 34 km/h would normally aid drying but are currently delivering moisture-laden air from the southwest.
The exposed position of the boulders means that once rain stops and humidity drops, surface moisture can clear relatively quickly.
The SW/W aspect receives afternoon sun which will help once dry weather arrives, though spring solar angles are still moderate.
Recommendations
3
- Do not climb today — the boulders are wet and rhyolite is dangerously slippery when damp, with a high risk of injury on technical problems.
- The best window in the coming days is April 6th, which is forecast dry with lower humidity (76%) following a drier April 5th — arrive in the afternoon to maximise drying time.
- Check the BMC RAD for any bird nesting restrictions at Dinas Cromlech and surrounding crags before visiting, as the peregrine nesting season is underway.
Do Not Climb
55%
12 days ago
The Cromlech Boulders have received 2.7mm of rain yesterday evening and 2.0mm is forecast today, following an extremely wet period (175mm in 28 days). While rhyolite dries quickly in good conditions, persistent high humidity (85–92%), limited sunshine, and today's ongoing light rain mean surfaces are very likely damp and greasy — conditions deteriorate further over the next two days.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside at the base of the Llanberis Pass, which funnels prevailing SW/W winds effectively, but also channels mist and low cloud that can keep surfaces damp even without direct rainfall.
- Cromlech Boulders' SW/W aspect means they catch afternoon sun when available, but in early April with heavy cloud cover (near 100% today) there is minimal solar drying benefit.
- Several classic problems (notably Jerry's Roof and the overhanging faces) have seepage from the hillside above after prolonged wet spells — 175mm in 28 days will keep seepage active for days.
- The polished nature of some of the most popular problems (particularly lower-grade slabs) makes them especially friction-dependent and dangerously slippery when even slightly damp.
Warnings
2
- Do not attempt slab or friction-dependent problems — rhyolite becomes extremely slippery when wet and the risk of injury from unexpected slips is high.
- Hillside seepage after 175mm of rain in 28 days may keep certain boulder faces wet even during dry spells; inspect carefully before climbing.
Reasoning
Rain fell yesterday evening (2.7mm total) and today brings another 2.0mm, with humidity at 85% and near-total cloud cover, so rock surfaces are almost certainly wet or at minimum damp.
Despite the exposed position and moderate westerly winds (28 km/h), the lack of sunshine, high humidity, and ongoing precipitation mean effective drying has not occurred since last evening's rain.
Rhyolite is non-porous and does not suffer structural damage when wet — the risk here is purely friction loss and climber safety on slippery surfaces.
Early April in the Llanberis Pass typically sees unsettled Atlantic weather; temperatures averaging 7°C with high humidity are typical and unfavourable for rapid drying at 250m altitude.
Contributing Factors
6
2.0mm of rain forecast today on top of 2.7mm yesterday evening means surfaces are wet right now.
Humidity has been consistently 85–98% over the last 24 hours, preventing effective evaporative drying.
175mm of rain over the past 28 days keeps the ground saturated and hillside seepage active, wetting boulder bases and some faces.
Westerly winds at 28 km/h provide good air circulation that will aid drying once rain stops.
Near 100% cloud cover eliminates solar warming that would otherwise help the SW/W-facing boulders dry.
Rhyolite does not absorb water, so once rain stops and humidity drops, surface drying can occur within hours.
Recommendations
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- Wait for a sustained dry window — conditions are unsuitable today and will worsen tomorrow and Saturday; the earliest viable opportunity is Sunday 5th April if the forecast holds.
- If visiting on Sunday or later, test friction carefully on lower-grade problems before committing to anything steep or technical, as seepage from the hillside above may persist.
- Check the BMC RAD for any seasonal bird nesting restrictions at Dinas Cromlech and nearby crags before travelling, as peregrine closures are common in the Llanberis Pass from February–June.
Climbing Outlook