Condition Analysis
AI-powered assessment using site data and 14-day weather history
Light but persistent rain over recent days and intermittent showers today mean the boulders are likely damp, with poor friction on the rhyolite surfaces. Tomorrow brings 12mm of rain, so conditions are not improving imminently.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The RAC Boulders sit roadside in the Mymbyr valley which can funnel moist SW airflows, keeping humidity high and prolonging surface dampness even after light rain.
- West-facing aspect means the boulders only receive afternoon and evening sun — on overcast days like today, solar drying is minimal.
- Being low-lying boulders near Llynnau Mymbyr, morning dew and lake-proximity moisture can settle on the rock overnight, requiring additional drying time even after nominally dry periods.
- The flat grassy landings can become waterlogged after sustained wet spells, affecting both approach and landing zones for bouldering.
Warnings
2
- Wet rhyolite is extremely slippery — friction loss on these boulders is sudden and can cause unexpected falls onto the landings.
- Tomorrow's 12mm of rain will ensure conditions remain poor; do not assume an improving trend.
Reasoning
The boulders have received scattered light rain over the past week (15.7mm in 7 days) with today adding 2.4mm of intermittent drizzle, and humidity averaging 81%, so rock surfaces are very likely damp.
With a west aspect, only partial wind exposure, heavy cloud cover all day, and light winds from the SW, effective drying today has been negligible — the brief dry windows mid-morning are insufficient to clear moisture from textured rhyolite surfaces.
Rhyolite is non-porous so there is no structural risk from moisture; the concern is entirely friction loss on wet or damp surfaces.
Early May in Snowdonia is characterised by changeable Atlantic weather systems; the recent dry spell (late April) has broken down and a typical unsettled pattern has returned with no sustained drying period in sight.
Contributing Factors
6
Light showers throughout the day (totalling ~2.4mm) are keeping rock surfaces damp with no sustained dry window.
Humidity has averaged around 79-81% over recent days, severely limiting evaporative drying between showers.
12.1mm is forecast for May 8th, which will re-wet any surfaces that do manage to dry today.
Partial wind exposure provides some airflow but SW winds are carrying moist Atlantic air, limiting their drying benefit.
Rhyolite does not absorb water so only surface moisture needs to evaporate, meaning recovery after a dry spell would be quick — but no dry spell is imminent.
Full cloud cover today negates any solar drying benefit from the west-facing aspect.
Recommendations
3
- Wait for a sustained dry period — current conditions and the forecast for the next several days do not support reliable friction on the rhyolite.
- If visiting the area, consider indoor climbing or sheltered alternatives until at least May 10th, and even then check surfaces carefully.
- Monitor conditions closely; if a drier window opens, the non-porous rhyolite can dry within a few hours of sun and wind, so be ready to take advantage of any break in the weather.
Previous Analyses
Do Not Climb
40%
2 days ago
The RAC Boulders have had a persistently damp period with light rain on several of the last few days (May 1–3, 5, and today), and today's forecast shows further light showers through the afternoon; surfaces are likely to be damp and greasy. Although rhyolite doesn't suffer structural damage when wet, friction on these boulders will be significantly compromised.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside at 200m in the Nant Gwryd valley, where cloud and valley mist can linger even when higher crags clear, keeping surfaces damp longer than weather station data alone suggests.
- West-facing aspect means the boulders only receive afternoon and evening sun — on overcast days like today, they get virtually no direct solar drying.
- Flat grassy landings can become saturated and slippery after prolonged wet spells, adding a secondary safety concern for spotters and fall zones.
- Being roadside and beginner-friendly, the venue attracts less experienced climbers who may not recognise subtle dampness on rhyolite — the rock can look dry while still having a thin moisture film that kills friction.
Warnings
2
- Rhyolite friction drops dramatically with even a thin moisture film — problems that feel straightforward when dry can become dangerously slippery.
- Grassy landings may be saturated and slippery after the prolonged wet spell; take extra care with pad placement and spotting.
Reasoning
Light but frequent rainfall over the last six days (totalling ~13.4 mm in the last week) combined with high humidity (77% average) and overcast skies means rock surfaces are very likely to retain a thin film of moisture, especially on shaded or sheltered faces.
With only brief dry windows between showers, persistent cloud cover, and a west aspect receiving no meaningful sun today, effective drying has been minimal — the partial wind exposure at ~11 km/h from the SE provides only modest evaporative help.
Rhyolite is non-porous and not at risk of structural weakening or hold breakage from moisture, so no damage concerns apply.
Early May in Snowdonia is transitional — temperatures are still cool (averaging ~11°C) and Atlantic-driven wet spells remain common, limiting sustained drying opportunities.
Contributing Factors
6
Rain fell on May 1, 2, 3, 5, and again today, preventing any sustained drying period from developing.
Humidity has averaged 77% over the last week and is forecast at 78–91% today, severely limiting evaporative drying.
Cloud cover is near 100% for most of today, negating any benefit from the west-facing afternoon sun aspect.
Light rain is forecast between 14:00–17:00 and again at 19:00, wetting surfaces during what would otherwise be the best climbing window.
Rhyolite does not absorb water, so once a genuine dry spell arrives, surface moisture will clear relatively quickly.
Partial wind exposure at around 11 km/h provides some airflow but is insufficient to quickly dry surfaces between frequent showers.
Recommendations
3
- Wait for a sustained dry spell — the boulders need at least a full dry day with some breeze and lower humidity before friction will be reliable.
- If you visit, physically touch the rock in multiple spots (including overhangs and sloping holds) to check for the greasy film that rhyolite develops when slightly damp.
- Consider the afternoon of May 8 as the earliest possible window if the forecast trace precipitation stays minimal, but reassess closer to the day.
Marginal — Assess Conditions
70%
3 days ago
The RAC Boulders have had roughly 36 hours of drying since the last notable rain (4.5mm on May 3rd), with today showing only a trace 0.1mm. The rhyolite should be largely dry on exposed faces by this afternoon, but humidity remains moderate and some sheltered surfaces may still hold residual dampness — a visual check on arrival is warranted.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit roadside in the Dyffryn Mymbyr valley floor where cool air can pool overnight, raising humidity and promoting condensation on rock surfaces — check for dew or condensation before climbing, especially in the morning.
- West-facing aspect means these boulders only receive direct sun from early-to-mid afternoon onward; morning sessions may find surfaces slower to dry than east-facing crags in the valley.
- The flat grassy landings around the boulders can retain moisture from recent rain, and ground-level dampness can wick onto lower boulder surfaces — inspect the base of problems carefully.
- Some boulders have overhanging faces and sheltered pockets that stay damp longer than the main west-facing slabs; seepage from the grassy hillside above can feed moisture to certain problems after prolonged wet spells.
Warnings
1
- Some lower boulder surfaces near saturated grassy landings may still be damp — test friction carefully before trusting footholds close to ground level.
Reasoning
The last significant rain was 4.5mm on May 3rd, followed by a dry day on May 4th and only a trace (0.1mm) today; surface moisture on exposed rhyolite faces should be largely evaporated, though sheltered areas and lower boulder surfaces near damp grass may retain some dampness.
Around 36 hours of drying with light-to-moderate northerly winds and temperatures around 11–14°C is adequate for non-porous rhyolite on exposed faces, though the west aspect means no direct sun until afternoon and the moderate humidity (67–82%) slows evaporation somewhat.
Rhyolite is non-porous and does not suffer structural weakening when wet — there is no risk of hold breakage from residual moisture.
Early May in Snowdonia brings lengthening days and improving conditions but overnight temperatures still dip to 3–7°C with high humidity, meaning morning condensation is common at 200m altitude.
Contributing Factors
6
Only 4.5mm fell on May 3rd with a full dry day on May 4th and negligible trace today, giving ~36 hours of drying on non-porous rhyolite.
Average humidity over the past 24 hours has been 75–85%, with overnight lows near 7°C raising the risk of condensation on cool rock surfaces.
Light-to-moderate northerly winds (10–14 km/h today) provide reasonable airflow across these partially exposed boulders, aiding surface evaporation.
The west-facing aspect will receive direct sun from early afternoon, which should warm and dry any residual surface moisture on the main faces.
Over 100mm fell in the 28 days prior, and the surrounding ground and vegetation are saturated, which can maintain elevated local humidity and seepage.
Rhyolite does not absorb water and cannot be structurally damaged by climbing when damp — the concern is purely friction and safety.
Recommendations
3
- Visit in the afternoon (after 14:00) when the west-facing boulders have had direct sun and any residual surface moisture has evaporated.
- Physically touch and test friction on each problem before committing — if the rock feels glassy or slick, move on to a drier problem or wait.
- Focus on the more exposed and prominent west-facing faces first, as sheltered overhangs and north-facing sides may still harbour dampness from the prolonged wet spell.
Marginal — Assess Conditions
65%
4 days ago
The RAC Boulders received 4.5mm of rain yesterday (May 3rd) in humid, overcast conditions, and today has been dry but with high morning humidity slowly dropping through the afternoon. Rhyolite dries quickly on the surface, and by mid-afternoon conditions should be reasonable on sun-exposed faces, but sheltered or north-facing surfaces may retain dampness — a visual check on arrival is strongly advised.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The boulders sit at road level near Llynnau Mymbyr and can collect overnight condensation and valley moisture, especially on still mornings — today's early hours showed humidity above 90%.
- West-facing surfaces won't receive direct sun until the afternoon, meaning the morning drying window relies on wind and ambient temperature alone; the NE breeze today may not reach all boulder faces effectively.
- Some boulders at RAC have sheltered overhanging faces and recessed features that trap moisture longer than the exposed slabs — these should be checked individually.
- The flat grassy landings can remain soggy after the recent wet spell (over 100mm in the past month), so pad placement and footing should be considered carefully.
Warnings
1
- Rhyolite becomes very slippery when wet — do not climb on any surface that feels damp or greasy, as friction loss on these boulders can cause unexpected falls onto the ground.
Reasoning
Yesterday's 4.5mm of rain in near-saturated conditions (91% humidity) will have left surface moisture on the boulders, though rhyolite's non-porous nature means no internal absorption — residual dampness is purely superficial.
Today has been dry throughout with humidity dropping from ~92% overnight to around 68–71% by mid-afternoon with a light NE breeze (~11–13 km/h), providing moderate but not aggressive drying; west-facing surfaces receive afternoon sun which will help from early-to-mid afternoon onward.
Rhyolite is non-porous and extremely hard — there is no risk of structural damage or hold breakage from climbing on recently wetted rock.
Early May in Snowdonia is transitional with lengthening days aiding drying, but the valley location near Llynnau Mymbyr means persistent morning dampness and mist are common; the recent prolonged wet spell (~109mm in 28 days) keeps the local environment saturated.
Contributing Factors
7
4.5mm fell on May 3rd in very humid conditions (91%), with only one dry day since to allow surface drying.
No precipitation today with humidity falling to ~68% by mid-afternoon, allowing progressive surface drying on exposed faces.
Rhyolite does not absorb water so drying is a surface-only process that can complete within hours given favourable conditions.
Humidity remained above 90% through the early hours, likely depositing condensation on rock surfaces that will take until late morning to clear.
West-facing boulders catch direct afternoon sun from roughly 1–2pm onward, which will significantly aid final drying of exposed surfaces.
Over 108mm of rain in the past 28 days means the surrounding ground and vegetation are saturated, contributing to elevated ambient moisture levels.
A light-to-moderate NE breeze (11–13 km/h) provides some airflow to assist evaporation, though partial shelter limits effectiveness on some boulder faces.
Recommendations
3
- Visit in the mid-to-late afternoon (after 2pm) when west-facing surfaces have had maximum sun exposure and humidity is at its daily low.
- Physically touch and test friction on each boulder before committing — some sheltered or recessed faces may remain greasy even when exposed slabs feel dry.
- Bring towels and check landing areas carefully, as grassy landings near the lake may be soft and waterlogged after the recent prolonged wet spell.
Do Not Climb
65%
4 days ago
Today has seen 4.5mm of rain with 90% humidity, meaning the boulders will be wet or damp right now. Although rhyolite dries relatively quickly, the combination of today's rain, light winds, and high humidity means surfaces are unlikely to dry adequately today.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The RAC Boulders are roadside and west-facing, so they will not receive direct sun until afternoon — with today's overcast, high-humidity conditions and NE wind, the west aspect is sheltered from the breeze and drying will be slow.
- These low-lying boulders sit near Llynnau Mymbyr in a lakeside setting, which contributes to ambient moisture and can delay surface drying compared to more exposed, elevated crags.
- The predominantly easy problems (V0–V4) with flat grassy landings reduce the consequence of a slip, but rhyolite friction loss when damp is still significant and should not be underestimated on slab or smear-dependent problems.
- The preceding week (Apr 20–30) was exceptionally dry with 10 consecutive rain-free days, but the last three days (May 1–3) have brought ~10.5mm of cumulative rain, re-wetting surfaces that had previously dried out fully.
Warnings
2
- Rhyolite becomes very slippery when wet — do not attempt slab or friction-dependent problems until surfaces are confirmed bone-dry.
- The lakeside setting can maintain higher ambient humidity than surrounding crags, potentially prolonging dampness on sheltered faces.
Reasoning
With 4.5mm of rain today and 90% humidity, rock surfaces at the RAC Boulders are currently wet or at best damp, and any sheltered or north-facing facets will retain moisture.
The west aspect is sheltered from today's NE wind, limiting air circulation across the rock faces, and overcast skies with high humidity will severely slow evaporation — meaningful drying is unlikely before end of day.
Rhyolite is non-porous and does not suffer structural damage when wet, so there is no hold-breakage or route-damage concern.
Early May in Snowdonia brings lengthening days and reasonable temperatures (~12°C), but Atlantic-influenced weather means rapid transitions between dry spells and wet fronts are common.
Contributing Factors
6
Active precipitation today means rock surfaces are currently wet, eliminating the friction advantage rhyolite normally provides.
Ninety percent humidity dramatically slows evaporation, keeping surfaces damp even after rain stops.
The NE wind at only 13 km/h offers little drying benefit to west-facing rock, which is sheltered from this wind direction.
Rhyolite does not absorb water, so once surface moisture evaporates the rock will be ready to climb — no extended internal drying needed.
After an excellent dry period in late April, the last three days have brought cumulative rain that has thoroughly re-wetted surfaces and surrounding ground.
Temperatures are mild enough to support drying but not warm enough to accelerate it significantly, especially under overcast skies.
Recommendations
3
- Wait until tomorrow (May 4) at minimum, and check for dry surfaces in the afternoon when any residual moisture has had time to evaporate.
- Prioritise overhanging or steeper problems that shed water more readily, and avoid slabs or friction-dependent routes until surfaces are confirmed dry.
- If visiting tomorrow, bring a towel to dry holds and test friction on easier problems before committing to anything committing or highball.
Do Not Climb
55%
4 days ago
Rain today (4.6mm) with high humidity (87%) means the boulders are likely wet right now. Although rhyolite dries quickly, the combination of today's rain, light winds, and a NE wind direction (sheltering the west-facing rock) means surfaces are unlikely to have dried sufficiently this afternoon.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The west-facing aspect means these boulders won't catch any morning sun and rely on afternoon/evening sun to dry — with overcast, humid conditions today, solar drying will be minimal.
- Roadside boulders at valley floor level near Llynnau Mymbyr can accumulate runoff and splash moisture from passing traffic and surrounding grass, prolonging surface dampness after rain.
- The NE wind today is blowing onto the sheltered (back) side of the west-facing boulders, providing minimal direct airflow across the main climbing faces.
- Some of the lower problems and slab faces at RAC Boulders are notorious for retaining a thin film of moisture longer than the steeper faces — check these carefully even when upper problems feel dry.
Warnings
2
- Rhyolite becomes extremely slippery when wet — friction-dependent slab problems (common at RAC Boulders) are particularly dangerous in damp conditions.
- Three consecutive days of rain mean even apparently dry-looking holds may have a thin moisture film — test friction carefully before committing to any moves.
Reasoning
With 4.6mm of rain today on top of ~6mm over the previous two days, and humidity at 87%, the rock surfaces are almost certainly still damp or wet.
Light NE winds (12.2 km/h) are blowing away from the main west-facing surfaces, limiting air-drying, and overcast humid conditions mean negligible evaporative drying today.
Rhyolite is non-porous and does not suffer structural damage when wet — there is no hold-breakage risk from climbing on damp rock here.
Early May in Snowdonia brings variable weather; the recent wet spell (111mm in 28 days) is typical for spring, but the preceding dry week (Apr 21–30) was unusually good — conditions are now reverting to a more unsettled pattern.
Contributing Factors
6
Active precipitation today means surfaces are freshly wet and unlikely to have dried.
Very high humidity severely slows evaporation from rock surfaces.
The NE wind direction is sheltered from the main west-facing climbing surfaces, reducing airflow-driven drying.
Rain on May 1st (2.9mm), May 2nd (3.1mm), and today (4.6mm) means three consecutive days of precipitation with no drying window.
Rhyolite does not absorb water, so once surface moisture evaporates the rock will be fine — no extended internal drying needed.
Spring temperatures are adequate for drying but not warm enough to rapidly evaporate surface moisture in humid conditions.
Recommendations
3
- Wait for tomorrow (May 4th) which is forecast dry — assess conditions on arrival as surfaces may have dried by afternoon with any sun or breeze.
- If you visit today, bring alternative plans (walking, café in Capel Curig) as the boulders are very likely too wet for safe friction-dependent climbing.
- Prioritise steeper, overhanging problems first when returning after rain, as these shed water fastest — save slabs and lower-angle faces for last.
Do Not Climb
40%
4 days ago
Rain today (4.5mm) on top of recent showers on May 1st and 2nd means the boulders are likely wet right now. With high humidity (86%), light NE winds, and a west-facing aspect receiving no direct drying wind, surface moisture will linger into the evening.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The west-facing aspect means NE winds today provide minimal direct airflow across the boulder faces, significantly slowing surface drying.
- Roadside boulders at RAC sit in a relatively sheltered valley position between Llanberis Pass and Capel Curig, which can trap damp air and reduce drying potential compared to more exposed crags.
- The flat grassy surroundings and lakeside setting near Llynnau Mymbyr contribute to local humidity, and ground moisture can keep the lower portions of boulders damp longer than the tops.
- After the prolonged wet spell from mid-April (over 100mm in 28 days), moss and lichen growth on lesser-trafficked problems may be more pronounced than usual, further reducing friction.
Warnings
2
- Rhyolite becomes very slippery when wet — friction loss on damp boulder surfaces is a genuine safety risk, especially on slab and lower-angle problems.
- The lakeside setting and saturated ground after a wet April mean localised dampness may persist longer than weather data alone suggests.
Reasoning
With 4.5mm of rain today following 6mm over the previous two days, surface moisture is almost certainly present on the boulders right now.
NE wind at only 11.5 km/h offers minimal drying benefit to west-facing rock, and with 86% humidity, evaporation will be very slow through the remainder of the day.
Rhyolite is non-porous and suffers no structural damage from moisture — the concern here is purely friction loss and climber safety on slippery wet surfaces.
Early May in Snowdonia is transitional; temperatures are moderate (~13°C) but not warm enough to drive rapid drying, and the valley location near lakes keeps ambient moisture high.
Contributing Factors
7
Active or very recent rainfall today means boulder surfaces are wet or damp right now.
No dry days have accumulated — rain fell today and on the two preceding days (2.9mm and 3.1mm), so there has been no meaningful drying window.
Humidity at 86% severely limits evaporation, meaning even light surface moisture will persist for many hours.
The NE wind at 11.5 km/h blows largely from behind the west-facing boulders, providing very little direct airflow across climbing surfaces.
Rhyolite does not absorb water, so once surface moisture evaporates the rock will offer excellent friction without needing deep drying time.
Temperatures are adequate but not warm enough to drive rapid drying, especially combined with high humidity and sheltered aspect.
Over 111mm of rain in the past 28 days means the surrounding ground is saturated, contributing to local humidity and potential seepage around boulder bases.
Recommendations
3
- Wait for tomorrow (May 4th) when dry conditions are forecast — assess the rock visually before climbing, particularly checking lower boulder faces and north-facing sides for residual dampness.
- If visiting today, consider it a scouting trip to check conditions and identify which problems have dried, but do not commit to climbing on damp surfaces.
- Bring a towel and soft boar's hair brush to check holds, and focus on the upper portions of sun-catching faces if any problems appear dry by late afternoon.
Marginal — Assess Conditions
60%
5 days ago
Today saw 2.1mm of rain with high humidity (84%), so surfaces are likely still damp this evening. However, the preceding 10-day dry spell (Apr 21–30) thoroughly dried the rock, and rhyolite sheds surface water quickly — if rain has stopped and afternoon sun reaches the west-facing boulders, conditions may be climbable later today but need visual confirmation.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The west-facing aspect means the boulders only receive direct sun from mid-afternoon onward, so morning dampness from today's rain will linger longer than at a south-facing venue.
- These roadside boulders sit in the Mymbyr valley corridor which can channel moist SW airflow, keeping humidity high even when rain has stopped — check for condensation on the rock surface.
- The flat grassy landings can become saturated and slippery after recent rain; with 5mm in the last two days, spotters should be aware of soft, potentially muddy ground.
- Some of the lower boulders near Llynnau Mymbyr may retain seepage or splash moisture from the lake edge, particularly after wet spells — the higher problems further from the water tend to dry first.
Warnings
2
- Do not assume the rock is dry just because rain has stopped — high humidity and sheltered faces can keep rhyolite greasy for hours after light rain.
- Grassy landings may be slippery after recent rain; take extra care with spotting and pad placement.
Reasoning
Today's 2.1mm of rain on top of yesterday's 2.9mm means surfaces are likely damp right now, though the non-porous rhyolite will not have absorbed any water internally.
With only moderate wind (17.6 km/h from SW) and high humidity (84%), surface drying will be slow today; the west aspect won't see direct sun until mid-afternoon, further delaying evaporation.
Rhyolite is non-porous and extremely hard — there is no risk of structural damage or hold breakage from this moisture, so the concern is purely friction and safety.
Early May in Snowdonia brings lengthening days and reasonable temperatures (~14°C), but Atlantic-influenced weather keeps humidity high and showers frequent; conditions are improving but not yet reliably dry.
Contributing Factors
6
2.1mm of rain today with 84% humidity means rock surfaces are likely still damp, especially on sheltered faces.
Ten consecutive dry days (Apr 21–30) with warm easterly winds thoroughly dried the rock before this recent return of moisture.
Rhyolite sheds water quickly from its surface and does not absorb moisture internally, so drying time after light rain is measured in hours rather than days.
Humidity at 84% today and forecast 88% tomorrow slows surface evaporation and may cause condensation on cold rock faces.
West-facing boulders will receive direct afternoon and evening sun which aids drying, though morning and early afternoon will remain shaded.
Partial wind exposure at 17.6 km/h provides some airflow for drying but is not strong enough to rapidly clear surface moisture in high humidity.
Recommendations
3
- Wait until late afternoon when the west-facing boulders have had direct sun exposure, and physically touch the rock to check for residual moisture or greasiness before climbing.
- Start on the more exposed, higher boulders away from the lakeside which will dry fastest, and avoid any problems with visible dampness or lichen-covered holds.
- Bring a towel to dry holds if needed, use minimal chalk, and be prepared to walk away if friction feels compromised — the easy-graded problems here can become deceptively slippery when damp.
Marginal — Assess Conditions
60%
6 days ago
After an excellent 10-day dry spell (April 21–30), today's 3mm of rain has broken the run, and the rock will likely be damp right now. However, rhyolite dries quickly and if the rain has passed, the boulders should dry within hours in moderate wind — a visual check on arrival is essential.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The RAC Boulders are roadside and west-facing, so afternoon and evening sun will help dry any surface moisture from today's rain, but the boulders will be shaded through the morning.
- Some of the lower boulders near Llynnau Mymbyr can retain splash moisture and ground dampness longer than the more exposed blocks further from the lake edge.
- The flat grassy landings can become saturated after rain, and moisture wicking up from wet grass onto the base of problems can keep lower holds damp even when upper surfaces have dried.
- The extended dry spell from April 21–30 (10 consecutive dry days) means the ground and rock were thoroughly dry before today's rain, so recovery should be quick once precipitation stops.
Warnings
2
- Rhyolite becomes very slippery when wet — the friction loss on slab and vertical problems can be severe and lead to unexpected slips; do not climb if rock feels greasy.
- The forecast shows intermittent light rain over the next few days; conditions may fluctuate and a dry morning does not guarantee a dry afternoon.
Reasoning
Today saw 3mm of rain breaking a 10-day dry spell, so surface moisture is present but the rock has no residual deep saturation from prior weeks.
With partial wind exposure (18 km/h SW) and west-facing aspect catching afternoon sun, surface moisture on rhyolite should evaporate within a few hours once the rain stops, though shaded faces and lower sections may linger.
Rhyolite is non-porous and does not suffer structural weakening or hold damage when wet — there is no conservation concern here, only friction loss.
Early May in Snowdonia offers lengthening days and reasonable temperatures (~15°C), which aid drying, though spring showers remain frequent and humidity is elevated today at 75%.
Contributing Factors
6
3mm of rain today has wetted the rock surface, breaking a long dry spell and meaning the boulders are likely damp right now.
Ten consecutive dry days (April 21–30) with low humidity ensured the rock and ground were thoroughly dried out before today's rain, meaning no deep residual moisture.
Non-porous rhyolite sheds surface water rapidly and does not absorb moisture, so once rain stops the rock can dry within hours in favourable conditions.
The west-facing aspect will receive direct afternoon and evening sun, which will accelerate drying of today's surface moisture if skies clear.
Humidity at 75% with SW winds will slow evaporation somewhat compared to the very dry easterly spell that preceded it.
Light but persistent precipitation is forecast over the next several days (0.5–2.2mm daily), meaning fully dry conditions will be hard to sustain.
Recommendations
3
- If visiting this afternoon, check that the rain has fully stopped and allow 2–3 hours of sun and wind before committing to problems — test friction on easy slabs first.
- Focus on steeper overhanging problems which shed water fastest, and avoid slabby or low-angle faces which retain surface moisture and offer dangerously reduced friction.
- Consider returning later in the week around May 6–7 when a drier window may open with cooler, less humid conditions.
Likely Safe to Climb
92%
9 days ago
The RAC Boulders have had 9 consecutive dry days following a very wet April, with humidity dropping steadily and today bringing warm, breezy easterly conditions at 57% humidity. Rhyolite dries quickly as a non-porous rock, and surface moisture will have long since evaporated — conditions should offer excellent friction.
Based on weather conditions only — does not cover bird nesting restrictions or other access issues.
- The west-facing aspect means the boulders receive afternoon and evening sun, which combined with 9 dry days and recent easterly winds will have thoroughly dried all surfaces including any sheltered faces.
- Some boulders closer to Llynnau Mymbyr may retain ground-level dampness or have mossy/lichen-covered bases that stay greasy — check lower landings and slab feet before committing.
- The roadside location in the Nant Gwryd valley can funnel mist and low cloud even on otherwise clear days, so be aware that sudden humidity spikes could make friction feel inconsistent.
- After 142.7mm of rain in the past 28 days, any natural seepage lines or drainage channels on the boulders (particularly on overhanging faces with cracks above) should be checked visually, though 9 dry days makes residual seepage very unlikely.
Reasoning
Nine consecutive dry days on non-porous rhyolite, with humidity now at 57% and warm temperatures (16°C today), means surface moisture will have fully evaporated days ago.
The recent run of easterly winds and low humidity (averaging 62% over the last week) has provided excellent drying conditions, and the west-facing aspect has caught afternoon sun during a warm spell peaking at nearly 19°C.
Rhyolite is non-porous and extremely hard — there is no risk of structural weakening or hold breakage from the preceding wet period.
Late April is transitional but conditions are unusually favourable; daylight hours are long, temperatures are mild, and the extended dry spell is well-timed for spring climbing.
Contributing Factors
5
Nine consecutive dry days have allowed all surface moisture to evaporate completely from the non-porous rhyolite.
Today's humidity of 57% with a brisk easterly wind at 26.6 km/h ensures dry, high-friction conditions on the rock surface.
The last week has seen temperatures reaching 18.8°C with today at 16.1°C, accelerating evaporation from any residual dampness.
142.7mm fell in the preceding 28 days, but rhyolite does not absorb water, so this only matters for seepage and ground conditions which 9 dry days will have resolved.
Humidity reached 94% in the early hours of April 28th, but daytime conditions today with wind and sun will quickly dispel any overnight condensation.
Recommendations
3
- Conditions are excellent — head out and enjoy the friction, particularly in the afternoon when the west-facing aspect catches direct sun.
- Inspect any north-facing or deeply recessed surfaces for residual dampness or lichen grease before pulling on, especially on lower-tier problems near the lake.
- Check BMC RAD for any active bird nesting restrictions at the venue, as April falls within the peregrine and raven nesting season.
Climbing Outlook