Condition Analysis
AI-powered assessment using site data and 14-day weather history
The Cromlech Boulders are currently soaked from heavy rain today (25.7mm) following a very wet week, and the forecast shows continuous heavy rain for the next four days. The rock will be wet and dangerously slippery with no viable climbing window today or in the coming days.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside in the Llanberis Pass and receive significant run-off from the hillside above (Dinas Cromlech), meaning water pools and seeps persist on overhanging faces and around boulder bases even after rain stops.
- Despite the SW/W aspect that normally aids drying, 100% cloud cover and near-continuous rain negate any solar drying benefit this week.
- Jerry's Roof and other steep/overhanging problems may appear sheltered but still collect seepage and condensation in prolonged wet spells like this one.
- The venue's roadside accessibility means climbers may be tempted to 'just try' in marginal conditions — rhyolite friction drops dramatically when wet, making even moderate problems hazardous.
Warnings
2
- Rhyolite becomes extremely slippery when wet — do not attempt problems even if the surface appears to be drying, as residual moisture drastically reduces friction.
- Significant rainfall and run-off may create muddy, slippery ground around the boulders, increasing the risk of landing injuries.
Reasoning
The rock is thoroughly wet from 25.7mm of rain today on top of 45.6mm in the past week, with humidity consistently above 85% and no meaningful dry spell.
Despite the exposed position, continuous rain, 100% cloud cover, and humidity above 88% mean there has been zero effective drying and none is expected until at least August 31.
Rhyolite is non-porous so there is no structural damage risk, but the surface will be extremely slippery with standing water on holds and smears.
Late August in Snowdonia often sees Atlantic weather systems bringing prolonged wet spells; this is a classic example with nearly 70mm forecast over the next five days.
Contributing Factors
6
25.7mm of rain has fallen today with showers continuing through mid-afternoon, leaving all surfaces thoroughly wet.
Over 45mm in the past seven days with no consecutive dry days means the boulders and surrounding ground are saturated.
Humidity has been 85–93% today and is forecast to remain above 83% for days, preventing effective surface evaporation.
18.2mm, 19.0mm, 16.6mm, and 11.0mm are forecast for Aug 27–30 respectively, ensuring the rock stays wet.
The exposed position normally aids drying, but with continuous rain and cloud cover the wind simply drives more moisture onto the rock.
The favourable sun-facing aspect is completely negated by persistent 100% cloud cover and rain.
Recommendations
3
- Do not climb at Cromlech Boulders today or in the coming days — the rock is wet and will remain so.
- Monitor conditions from August 31 onward when rain is forecast to ease, but allow at least several hours of wind and sun before attempting problems.
- If visiting the Llanberis Pass area this week, consider indoor climbing at Beacon or The Indy instead.
Previous Analyses
Likely Safe to Climb
85%
57 days ago
Three consecutive dry days with falling humidity and good wind exposure have dried the rhyolite surfaces well after the wet spell ending August 21st. Today is an excellent window with clear skies, low humidity (40–50% this afternoon), and a steady easterly breeze — conditions will deteriorate significantly from Wednesday onwards.
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 are well exposed to wind from most directions, aiding rapid surface drying even after heavy rain.
- Some problems on the north-facing sides of boulders or in deep recesses (e.g. underneath roof features like Jerry's Roof) can retain seepage and condensation moisture longer than the sunlit SW/W faces.
- The Llanberis Pass funnels wind effectively, but easterly winds (as today) may not scour the SW/W-facing surfaces as efficiently as prevailing westerlies — check shaded aspects carefully.
- This is a hugely popular venue on summer weekends; expect crowds and be prepared to wait for popular problems, but the social atmosphere means good beta and condition reports from other climbers on the ground.
Warnings
1
- Significant rain is forecast from Wednesday August 26th through Friday August 28th (potentially 60mm+); do not plan on climbing later in the week.
Reasoning
The last significant rain was 15.5mm on August 20th and 3.9mm on August 21st, followed by three full dry days with moderate wind and declining humidity — non-porous rhyolite surfaces should be thoroughly dry.
The SW/W aspect receives strong afternoon sun in summer, and the exposed position with 13–23 km/h winds over the past three days has provided excellent drying conditions for surface moisture.
Rhyolite is non-porous and extremely hard; there is no structural risk from recent moisture, and freeze-thaw is not a concern in August.
Mid-to-late August offers long daylight hours and warm temperatures; today's 21°C peak and low humidity represent near-ideal summer bouldering conditions at this altitude.
Contributing Factors
6
No measurable precipitation since August 21st (3.9mm), giving 72+ hours of drying on non-porous rock.
Humidity drops to 40–45% this afternoon with clear skies, ensuring excellent friction on the rhyolite.
Steady 15–23 km/h winds through the Llanberis Pass have accelerated surface drying over the past three days.
Nearly 30mm fell August 17–21, but rhyolite is non-porous so this only affects surface moisture and local seepage, which three dry days should resolve.
Temperatures reaching 21°C today with strong solar radiation on the SW/W aspects promote rapid evaporation.
Heavy rain is forecast from August 26th onwards (nearly 65mm over four days), closing the current dry window.
Recommendations
3
- Climb today and tomorrow (Tuesday) for the best conditions; the window closes from Wednesday as rain returns.
- Inspect roof features and north-facing holds for residual seepage before committing to harder problems like Jerry's Roof.
- Bring a towel and brush — some sheltered surfaces may still have traces of dampness from the recent wet spell, but these should be manageable.
Marginal — Assess Conditions
62%
58 days ago
The Cromlech Boulders have had two dry days following a very wet spell (32.5mm in the last week), and with today's dry conditions and a SW/W aspect the surface rock should be largely dry, though some sheltered faces and undercuts may retain moisture. Tomorrow looks excellent for climbing, but deteriorating weather from the 25th onwards will make conditions increasingly poor through the week.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside below Dinas Cromlech and are exposed to Pass winds, which aids drying, but the enclosed valley can trap moisture and mist even on nominally dry days.
- Jerry's Roof and other overhanging problems can retain seepage and drips from above long after rain stops — always check holds on steep or roofed problems after wet spells.
- The SW/W aspect means afternoon and evening sun reaches the boulders well in summer, but the north-facing sides of individual boulders will dry significantly slower and may still be damp.
- The Llanberis Pass funnels weather from the west; the forecast 36.8mm on the 28th could arrive earlier than predicted given the site's exposure to Atlantic fronts.
Warnings
2
- Rhyolite becomes very slippery when wet — even a thin moisture film can cause unexpected foot slips on smears and slabs.
- A major rain event (36.8mm) is forecast for the 28th; do not plan outdoor climbing at this venue on that date.
Reasoning
After 15.5mm on the 20th and 3.9mm on the 21st, two full dry days (22nd–23rd) on non-porous rhyolite with moderate wind should have cleared most surface moisture, though high ambient humidity (78–83%) is slowing final evaporation.
The SW/W aspect and exposed position provide good solar and wind drying; however, winds have been from the NE today which is less effective for these aspects, and humidity has remained elevated at 78%, meaning some sheltered or north-facing surfaces may not be fully dry.
Rhyolite is non-porous and does not suffer structural weakening when wet — there is no risk of hold breakage from moisture.
Mid-to-late August in Snowdonia brings increasingly unsettled Atlantic weather; the upcoming forecast confirms this pattern with significant rain returning from the 25th and a major 36.8mm event on the 28th.
Contributing Factors
6
32.5mm fell in the last 7 days including 15.5mm on the 20th and 3.9mm on the 21st, saturating surfaces and surrounding ground.
No meaningful precipitation on the 22nd or 23rd has allowed roughly 36–40 hours of drying on this non-porous rock.
Average humidity over the last 7 days is 87%, and today is 78%, which slows surface evaporation and can leave a moisture film on rock.
The site's exposed position in the Llanberis Pass means consistent airflow assists surface drying even in humid conditions.
The afternoon and evening sun hits these aspects well in August, adding solar drying to wind effects.
Rain returns from the 25th with an escalating pattern culminating in 36.8mm on the 28th, severely limiting the climbing window.
Recommendations
3
- Tomorrow (24th) offers the best window this week — arrive early, visually and tactilely check surfaces for moisture before committing, and prioritise sun-facing problems.
- Avoid north-facing sides of the boulders and any overhanging features with potential seepage (e.g. Jerry's Roof area) until you have confirmed they are dry by touch.
- From the 25th onwards, plan alternative activities or indoor climbing — conditions will progressively deteriorate through the week with heavy rain forecast on the 28th.
Marginal — Assess Conditions
55%
59 days ago
The Cromlech Boulders have had a very wet week (32.7mm in 7 days) with the last significant rain on 20–21 August, but today is dry and tomorrow begins a promising 3-day dry window. The SW/W aspect and exposed position will aid drying, though high recent humidity (87% average) is slowing surface evaporation — climbers should inspect the rock carefully on arrival.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside in the Llanberis Pass which can funnel moisture-laden westerly winds, keeping humidity high even on nominally dry days.
- Jerry's Roof and other overhanging problems may retain seepage and drips from the heavy rain on the 20th (15.5mm) longer than vertical faces due to water tracking along crack systems.
- The SW/W aspect means the boulders receive afternoon and evening sun in summer, which is beneficial for drying but means morning sessions may encounter residual dampness, especially on north-facing sides of individual boulders.
- Weekend crowding can be intense at this venue — if conditions are marginal, arriving early on a weekday gives the best chance of finding dry rock and having space to assess problems.
Warnings
2
- Rhyolite becomes very slippery when wet — do not commit to problems if you encounter any surface moisture, as friction loss is sudden and falls are likely.
- The 15.5mm downpour on August 20th may have left persistent seepage in crack systems on overhanging problems like Jerry's Roof; check carefully before attempting.
Reasoning
After 15.5mm on the 20th and 3.9mm on the 21st, the rock has had only one full dry day (today, the 22nd) with high ambient humidity (85%), so surface moisture is likely still present in sheltered spots and seepage lines.
The exposed position and moderate winds (14–15 km/h) help, but the SW/W aspect receives limited solar benefit in the morning, and the persistently high humidity (85–87%) significantly slows evaporative drying — a second full dry day tomorrow should substantially improve conditions.
Rhyolite is non-porous and does not suffer structural weakening when wet; there is no risk of hold breakage from moisture.
Mid-to-late August in Snowdonia is entering the wetter autumn transition period, and the recent spell of persistent rain and high humidity is typical; temperatures around 15–17°C provide only modest drying energy.
Contributing Factors
7
32.7mm in the last 7 days including 15.5mm on the 20th and 3.9mm on the 21st means the boulders were thoroughly soaked just 1–2 days ago.
Average humidity of 87% over the past week and 85% today significantly slows surface evaporation even on non-porous rock.
With just one full day without rain since the last significant precipitation, sheltered faces and seepage lines may still hold surface moisture.
The exposed pass location and moderate winds (14+ km/h) promote air movement across the rock surface, aiding drying.
Rhyolite does not absorb water, so drying is a surface-only process that can be rapid once humidity drops.
Three consecutive dry days forecast (23rd–25th) with dropping humidity (down to 63% on the 24th) should bring excellent drying conditions.
The south-west and west-facing aspect receives strong afternoon sun in summer, which will help bake off remaining moisture.
Recommendations
3
- Wait until tomorrow (23rd) or ideally the 24th for the best conditions, as a second and third dry day with lower humidity will make a significant difference.
- If visiting today or tomorrow, inspect each problem carefully for residual dampness — pay particular attention to horizontal breaks, roofs, and north-facing boulder surfaces.
- Prioritise afternoon sessions to take advantage of the SW/W aspect's solar warming and to allow morning dampness to evaporate.
Do Not Climb
70%
60 days ago
The Cromlech Boulders have received significant rainfall over the past few days (15.5mm on Aug 20, 7.7mm today), and with high humidity (84–90%) and zero consecutive dry days, the rock surfaces are very likely still wet or damp. Although rhyolite dries relatively quickly, there has not yet been adequate drying time and conditions should improve from tomorrow.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside in the Llanberis Pass and catch prevailing SW/W winds well, which aids drying, but the NW wind today is less effective on the SW/W-facing surfaces.
- Jerry's Roof and other overhanging problems may retain seepage and drips longer than vertical faces, as water tracks along cracks in the rhyolite above.
- The pass can funnel cloud and mist even on nominally dry days, keeping humidity high and surfaces damp despite no recorded precipitation.
- Being roadside at 250m, the boulders are easily checked in person — a quick drive-by inspection is practical before committing to a session.
Warnings
2
- Rhyolite becomes extremely slippery when wet — do not attempt problems today as falls from greasy holds are likely.
- A prolonged wet spell returns Aug 25–26 with ~22mm forecast; plan sessions for the dry window of Aug 22–24.
Reasoning
With 15.5mm on Aug 20 and 7.7mm today, the rock surfaces are almost certainly wet right now, and high ambient humidity (84%) is slowing evaporation.
The SW/W aspect receives reasonable afternoon sun in summer, and the exposed position aids wind-drying, but today's N wind is not aligned with the main faces, and only a few hours of drying have occurred since this morning's rain.
Rhyolite is non-porous and does not suffer structural weakening when wet — hold breakage is not a concern here.
Late August in Snowdonia brings increasingly unsettled Atlantic weather; the recent wet spell (36.6mm in 7 days) is typical, and shorter days reduce available drying hours compared to midsummer.
Contributing Factors
6
23.2mm fell in the last two days (Aug 20–21), leaving surfaces wet with no dry days to recover.
Humidity has averaged 86% over the past week, significantly slowing surface evaporation on the rhyolite.
There have been no fully dry days recently, meaning the rock has had no sustained drying window.
The exposed roadside position in the pass means wind can strip surface moisture relatively quickly once rain stops.
Rhyolite does not absorb water, so once surface moisture evaporates the rock will climb well — no extended drying period needed.
Three consecutive dry days are forecast (Aug 22–24), providing an excellent drying opportunity.
Recommendations
3
- Wait until at least tomorrow afternoon (Aug 22) to allow a full dry day with wind to strip surface moisture before climbing.
- If visiting Aug 22–24, visually check holds and test friction on easier problems before committing to harder lines.
- Avoid the boulders on Aug 25–26 when heavy rain (22.5mm total) is forecast to return.
Do Not Climb
35%
61 days ago
The Cromlech Boulders have received significant rain over the past few days (8.2mm on Aug 18, 2.1mm on Aug 19, and 5.3mm today with showers continuing through the morning), and today's humidity remains very high (90%). While rhyolite dries relatively quickly, the ongoing precipitation and saturated conditions mean surfaces are almost certainly wet and slippery right now — not suitable for safe bouldering.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside in the Llanberis Pass where cloud and mist can linger even when lowland areas clear, keeping humidity high and delaying drying.
- Some problems on the north-facing sides of the boulders and undercut features (e.g. Jerry's Roof) can hold seepage and condensation moisture longer than the exposed SW/W faces.
- Being at 250m in the pass, the boulders are subject to funnelled wind which aids drying but also channels damp air and low cloud through the corridor.
- Summer weekends see extremely heavy traffic — if conditions improve mid-week, visiting on a quieter day yields both better rock and less polished holds.
Warnings
2
- Wet rhyolite is extremely slippery — friction loss on steep boulder problems like Jerry's Roof creates a serious fall risk; do not attempt highball or steep problems until surfaces are confirmed dry.
- The boulders have been intermittently wet for several days; even apparently dry-looking surfaces may have a thin moisture film that drastically reduces grip.
Reasoning
With 5.3mm of rain today (including showers through to mid-morning) on top of 8.2mm on Aug 18 and 2.1mm on Aug 19, the boulder surfaces are currently wet and humidity at 90% is inhibiting evaporation.
The SW/W aspect and exposed position normally allow fast drying, but persistent cloud cover (80–100%), high humidity, and repeated light showers today have prevented any meaningful drying window so far.
Rhyolite is non-porous and does not suffer structural weakening when wet — the risk is purely friction loss on slippery surfaces, which is especially dangerous on steep boulder problems.
Mid-to-late August in Snowdonia often brings unsettled Atlantic weather; the current spell of wet westerlies is typical, though the forecast suggests a drier, cooler window developing from Aug 22 onwards.
Contributing Factors
7
Multiple showers totalling 5.3mm have fallen today, with the last significant burst at 15:00, keeping surfaces wet.
Nearly 20mm has fallen in the last 7 days across numerous rainy days, meaning the surrounding ground and sheltered boulder surfaces are thoroughly saturated.
Humidity has averaged 86% over the past week and remains around 90% today, severely slowing evaporation from rock surfaces.
The pass funnels wind across the boulders at moderate speeds (19 km/h today), which will help surface drying once rain ceases.
The favourable aspect would normally aid drying, but persistent cloud cover (80–100%) today is blocking most direct solar radiation.
Rhyolite does not absorb water, so once rain stops and conditions improve, surface moisture can evaporate within hours rather than days.
A drier, cooler spell is forecast from August 22 with no precipitation and dropping humidity, which should provide good drying conditions.
Recommendations
3
- Wait until at least August 22 when the dry spell begins; by the afternoon the boulders should have had enough time to dry if the forecast holds.
- If visiting before then, check the SW-facing surfaces first — these will dry fastest and may offer usable friction before shaded or north-facing problems.
- Bring a towel and brush but do not climb if holds feel glassy or slick — rhyolite friction is excellent when dry but drops dramatically when damp.
Do Not Climb
45%
62 days ago
The Cromlech Boulders have received persistent rain over the past three days (7.9mm on Aug 18, 2.7mm today including showers through the morning), and humidity remains very high. While a dry window opens from late morning today, the rock is likely still damp in sheltered spots, and heavy rain (12.2mm) is forecast for tomorrow, making conditions poor for the near term.
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 can collect runoff and seepage from the hillside above, meaning some faces stay wet longer than the weather alone would suggest.
- The SW/W aspect means the boulders receive afternoon sun when it breaks through, but today's 100% cloud cover limits solar drying significantly.
- Jerry's Roof and other overhung problems may stay drier underneath but landing areas can be waterlogged after prolonged wet spells like this one.
- The Llanberis Pass funnels wind effectively, which aids surface drying on exposed faces, but high ambient humidity (80%+) limits evaporative drying today.
Warnings
2
- Rhyolite becomes very slippery when wet — friction-dependent slab and wall problems are dangerous in current conditions.
- Heavy rain (12.2mm) is forecast for tomorrow; do not plan a session expecting conditions to carry over from today's brief dry window.
Reasoning
Rain fell this morning (1.2mm between 01:00–07:00) on top of 7.9mm yesterday and 2.8mm the day before, so surfaces are almost certainly still damp despite a dry window opening from mid-morning.
Moderate SW wind (~19 km/h) and the exposed position help surface evaporation, but persistent cloud cover and humidity above 70% severely limit drying, meaning only the most exposed faces may approach dryness by mid-afternoon.
Rhyolite is non-porous and does not suffer structural weakening when wet, so there is no risk of hold breakage from moisture.
Mid-August should offer reasonable drying conditions, but this unsettled spell with above-average humidity and consecutive wet days is keeping surfaces damp longer than typical summer weather would.
Contributing Factors
6
Over 13mm has fallen in the past three days (Aug 17–19), with rain as recently as this morning, keeping surfaces wet.
Humidity has averaged 83% over the past week and remains 70–98% today, severely limiting evaporative drying.
12.2mm is forecast for August 20, which will re-saturate any surfaces that manage to dry today.
The pass funnels wind across the boulders at ~19 km/h today, which helps strip surface moisture from exposed faces.
Rhyolite does not absorb water so only surface moisture matters, and it will dry relatively quickly once conditions improve.
100% cloud cover for most of today prevents solar heating of the SW/W-facing rock, reducing drying rates.
Recommendations
3
- Wait for the dry spell forecast from August 23 onwards, when conditions should improve significantly.
- If visiting today, only consider well-exposed, wind-facing slabs in the dry afternoon window (11:00–18:00) and test friction carefully before committing to anything hard.
- Avoid any problems with seepage lines, mossy patches, or sheltered faces — these will remain damp for hours yet.
Do Not Climb
70%
64 days ago
Rain fell this morning (up to 2mm in one hour around 11:00) and humidity remains very high; while a dry window may open late afternoon, the rock is likely still damp and friction will be poor. The next five days bring persistent rain, making conditions unsuitable for the foreseeable future.
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 cloud/mist drifting through the pass even when surrounding areas are dry, keeping surfaces damp longer than weather data alone suggests.
- Jerry's Roof and other overhanging problems may stay drier underneath but landing areas can become slippery and muddy after sustained wet spells.
- The SW/W aspect means the boulders benefit from afternoon sun when available, but today's 100% cloud cover negates this drying advantage.
- Being at 250m in a narrow pass, humidity and condensation can linger on the rock surface even during nominally dry hours, particularly when humidity exceeds 90%.
Warnings
2
- Rhyolite becomes very slippery when wet — friction-dependent slab and wall problems at Cromlech are hazardous in current conditions.
- Prolonged wet weather ahead: do not plan a bouldering trip to the Pass for the next four days.
Reasoning
Rain totalling ~3.2mm fell today with the heaviest burst at 11:00, and humidity has been at or near 100% for most of the day, meaning rock surfaces are very likely still wet or at least damp.
Although a dry spell is forecast from ~14:00 onwards with humidity dropping to the low 80s and moderate westerly winds, the lack of any sunshine (100% cloud) and high ambient moisture severely limit evaporative drying on this SW/W-facing rock.
Rhyolite is non-porous and not at risk of structural damage from moisture; the concern is purely friction loss on wet or damp surfaces.
Mid-August should offer reasonable drying conditions, but this week is dominated by an unsettled Atlantic pattern bringing repeated frontal rain and persistently high humidity to Snowdonia.
Contributing Factors
6
Over 3mm of rain fell today, with the bulk arriving mid-morning, leaving rock surfaces wet.
Humidity has been 94–100% for most of today and averages 92% for the full day, severely inhibiting surface drying.
100% cloud cover throughout the day eliminates solar drying, which the SW/W aspect would otherwise benefit from.
Westerly winds of 12–25 km/h through the pass provide some evaporative drying during the afternoon dry window.
The next four days bring 25+ mm of additional rain with humidity consistently above 85%, offering no meaningful drying window.
Rhyolite does not absorb water, so once rain stops and surfaces dry, the rock recovers quickly without internal moisture concerns.
Recommendations
3
- Wait for conditions to improve — the current forecast shows no reliable dry window until at least August 22nd.
- If you are already at the Pass this afternoon (after 15:00), feel the rock carefully for residual dampness before committing to any problem; avoid slabs and friction-dependent lines entirely.
- Consider indoor alternatives or lower-commitment venues for the coming week, and monitor forecasts for the clearing trend expected around August 22–23.
Do Not Climb
55%
65 days ago
Today has seen 0.5mm of rain with moderate humidity, and the forecast shows a deteriorating weather pattern with significant rain expected over the next few days. While the rock had a good dry spell from Aug 11–13, recent light showers and today's precipitation mean surfaces may be damp, and conditions are about to get substantially worse.
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, despite their SW/W aspect, can accumulate seepage and runoff from the hillside above during sustained wet periods.
- Jerry's Roof and other overhanging problems may stay dry underneath during light showers, but the lip and topout holds collect moisture that takes longer to dry due to reduced airflow.
- The boulders' exposed position in the Llanberis Pass creates a wind-tunnel effect that aids surface drying, but high humidity (currently 77%) limits evaporation effectiveness.
- The venue gets extremely crowded on summer weekends — if conditions are marginal, heavy traffic on damp holds can polish the rhyolite surface over time.
Warnings
2
- Rhyolite becomes very slippery when wet — do not attempt slab or friction-dependent problems if any moisture is present.
- A prolonged wet spell is forecast from Aug 17–20; plan alternative activities or indoor climbing for this period.
Reasoning
Today's 0.5mm rain on top of light showers on Aug 14–15 means rock surfaces are likely damp or at best only marginally dry in sheltered spots.
The SW/W aspect and exposed wind position would normally dry light rain quickly, but moderate winds (12.6 km/h) and 77% humidity today are limiting evaporation, and the rock has not had a full dry day since Aug 13.
Rhyolite is non-porous and not at risk of structural damage from moisture; the concern is purely friction loss on wet or damp surfaces.
Mid-August in Snowdonia is prime climbing season but Atlantic weather systems can bring extended wet spells, and the incoming front suggests several days of poor conditions ahead.
Contributing Factors
6
0.5mm recorded today means rock surfaces are likely still damp or only partially dried.
6.1mm forecast tomorrow (Aug 18) and 12.1mm on Aug 19 will keep conditions poor for several days.
Humidity at 77% today and rising to 89–94% over the next few days severely limits surface evaporation.
The pass funnels wind across the boulders, which helps clear surface moisture when rain stops.
The SW/W-facing boulders receive afternoon sun when available, boosting drying potential in clear spells.
Rhyolite does not absorb water so only surface moisture needs to evaporate, enabling faster recovery after rain.
Recommendations
3
- Wait for conditions to improve — the next realistic climbing window is Aug 21 at the earliest, and even then a visual check for residual dampness is essential.
- If you visit today hoping for dry rock, focus on steep overhanging problems where surfaces may have stayed sheltered, but expect reduced friction on slabs and vertical faces.
- Check UKClimbing and local social media for real-time condition updates from the Pass before travelling.
Marginal — Assess Conditions
70%
66 days ago
The Cromlech Boulders have had only one full dry day after a light 0.6mm shower on the 14th, but today is dry with moderate wind and the rock's non-porous rhyolite surface should have dried. However, humidity remains elevated at 81% and a deteriorating forecast from tomorrow onwards means today may be the last viable window for several days.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside in the Llanberis Pass and catch SW/W winds well, which aids drying, but the pass can funnel moist air and low cloud that keeps surfaces damp even without measurable rain.
- Jerry's Roof and other overhung problems may retain seepage or condensation on their undersides longer than vertical faces, so inspect holds carefully on steeper lines.
- The venue is extremely popular in summer — if conditions are marginal, arriving early gives the best chance of dry rock before any afternoon moisture builds.
- Several problems on the north-facing sides of the boulders will dry significantly slower than the SW/W-facing aspects and may still be damp from the 14th's shower.
Warnings
2
- A significant multi-day rain event is forecast from Aug 17–20 with over 50mm total precipitation; do not rely on quick drying between showers.
- North-facing and shaded boulder surfaces may still be damp despite the main faces appearing dry — test friction before committing to moves.
Reasoning
Only 0.6mm fell yesterday (Aug 14) and today has been dry, so surface moisture on exposed SW/W faces should have evaporated, though sheltered or north-facing surfaces may retain dampness given 81% humidity.
One full dry day with moderate 15 km/h northerly wind on non-porous rhyolite with good SW/W aspect is generally sufficient to clear light surface moisture from a 0.6mm shower, but high humidity limits evaporation rates.
Rhyolite is non-porous and not susceptible to structural weakening when wet; there is no hold breakage risk from recent moisture.
Mid-August offers reasonable daylight hours and temperatures around 17°C, but the incoming Atlantic frontal system from tomorrow will bring sustained heavy rain through to the 20th, closing the window rapidly.
Contributing Factors
6
Only 0.6mm fell on Aug 14, a trivial amount that non-porous rhyolite sheds quickly.
Today (Aug 15) has been dry with no precipitation, allowing surface evaporation.
Humidity at 81% slows evaporation and can cause condensation on cooler rock surfaces, particularly in shaded areas.
15 km/h northerly wind across the exposed pass aids surface drying effectively.
54.8mm of rain is forecast over the next 5 days starting tomorrow, which will make conditions unsafe from Aug 17 onwards.
The south-west and west-facing aspects receive good afternoon sun, promoting drying on the main faces.
Recommendations
3
- If visiting today, visually inspect and touch-test holds before committing — SW/W-facing problems are most likely dry, but check north-facing sides carefully.
- Plan to climb this afternoon when solar radiation on the SW/W aspects is strongest and any residual dampness has had maximum drying time.
- Avoid the Cromlech Boulders from Aug 17–20 as heavy sustained rainfall (up to 17.8mm/day) will make the rock dangerously slippery.
Climbing Outlook