The forecast says 20% chance of rain. Should you drive three hours to the crag? The weather is beautiful now but a thunderstorm is forecast for 2 PM. Should you start that multi-pitch? These are the questions that separate safe climbers from statistics. Weather is the leading cause of climbing accidents that don't involve human error, and it's almost entirely predictable if you know how to read the signs.
Reading a Climbing Forecast
Don't rely on a single forecast app. For climbing decisions, use dedicated weather services that include: cloud base altitude, precipitation probability, wind speed and direction, and especially the gust forecast. General apps like generic weather services are optimized for daily life — climbing-specific forecasts (like Mountain Forecast for specific peaks, or WUnderground for specific weather stations) provide the data that matters.
The critical numbers for climbers: cloud base below 3000m means potential for rain and reduced visibility. Wind above 25mph makes leader climbing dangerous and increases fall consequence. Gusts above 40mph are a no-go for most climbing. Temperature below freezing at altitude means ice risk on north-facing routes; above 30°C dramatically increases heat exhaustion risk.
The Afternoon Thunderstorm Pattern
In summer across most of North America and Europe, the pattern is predictable: morning clear skies, afternoon thermal buildup leading to thunderstorms between 2-5 PM. This isn't a forecast issue — it's a certainty of physics. If you're doing a multi-pitch, start at first light (5-6 AM summer) and plan to be off the route by 1 PM. This gives you margin for delays without being caught on a wall in lightning.
The lightning decision: if you can hear thunder, you're within 10 miles of the storm and at risk. The 30-30 rule — if the time between flash and thunder is less than 30 seconds, lightning is close enough to be dangerous. Seek shelter immediately, preferably in a building, or if in the open, in a forest with trees of similar height. Never shelter under an isolated tree or near a cliff top.
Wind: The Overlooked Killer
Wind is more consistently dangerous than rain for climbers. Steady wind above 25mph makes precise footwork nearly impossible and increases the consequence of a fall. Gusts can exceed 50mph even when steady winds are moderate. These gusts can: blow a leader off balance on a bolt, create unexpected rope drag, and make communication with your belayer difficult.
Wind direction matters as much as speed. A headwind on the approach is favorable; a tailwind on a high alpine route means you're climbing into weather. Watch for lenticular clouds — smooth, lens-shaped clouds forming near peaks in strong wind — they're indicators of rotor turbulence and extreme winds at your elevation.
Temperature Management
Cold climbing (below 5°C) requires specific preparation: chemical hand warmers in pockets, vapor barrier liners to manage sweat, belay jackets that actually work (down or synthetic puffy, not softshell). Fingers lose dexterity below 10°C, making small holds and difficult clipping much more dangerous. The warm-up problem: fingers need warmth for blood flow, but you're standing in a cold belay.
Hot weather (above 28°C) increases dehydration risk dramatically. Sweat rates can exceed 1 liter per hour in direct sun at altitude. Combined with the physical exertion of climbing, this can lead to heat exhaustion in 30 minutes. Hydration before the climb, during the climb, and after is non-negotiable in heat. Start early, finish by midday.