Weather and seasons
September, then July. September has July's dry ground with a fraction of the people, kinder temperatures, and the refuges staffed for the whole month. July is the driest and sunniest month on record with everything open, at the cost of heat and crowding.
The GR20 has a supported season of roughly mid-May to the start of October and the two ends fail for different reasons. Early on it is snow on the northern stages, which clear last and can hold patches into early July. At the end it is that the park's refuges stop being staffed: they stay open but sell nothing and have nobody there, so the same itinerary becomes a self-sufficient crossing. July is the driest and sunniest month and the busiest; September is the same ground with far fewer people and is staffed end to end, which makes it the best answer for most walkers. Getting off the hill is seasonal too, and on a slightly different calendar from the refuges. The bus to Col de Vergio and Castel di Vergio runs a late-spring to mid-October season, the Ponte-Leccia to Haut Asco minibus is summer only and needs a minimum party to run at all, and the taxi firm covering Calenzana, Asco, Vergio and Vizzavona gives its own season as April to the end of October, so outside the staffed window the road escapes still exist but the buses serving them do not and a retreat means a taxi or a private transfer. Vizzavona is the exception: the railway through it runs all year. Note that the published climate figures for central Corsica are valley figures from Corte at about 400 m, and the trail sits 600 to 2300 m above that, so take roughly 7 C off for a typical stage and nearer 12 C at the cols.
The figures here are Corte in the valley at about 400 m. The trail runs between roughly 1000 and 2700 m, so subtract 7 C for the average stage and nearer 12 C at the cols, which puts most of the route below freezing by day.
11°C high · 2°C low · 9.5Watch for: Short daylight against stages that already take a full summer day.The coldest month in the valley, and Corte's own average low of 1 C means hard frost the length of the route.
10°C high · 1°C low · 10.6Valley spring begins and means nothing on the route. Snow still holds the cols and the days are not yet long enough for the stage lengths.
13°C high · 3°C low · 12.1Watch for: Rapid thaw can make the unbridged stream crossings on the southern stages serious.The month walkers most often misjudge. Corte reaching 16 C reads like walking weather and the high route is still snowbound and unstaffed.
16°C high · 5°C low · 13.4Watch for: Wet spring snow over slabs, which is the worst combination the route offers.The refuges begin to be staffed from about the middle of the month, but the northern stages are the last to clear and often are not. The southern half from Vizzavona to Conca is a genuine option before the north is.
20°C high · 8°C low · 14.4Watch for: Snow above the Ortu di u Piobbu and Carrozzu stages, and the descent from Bocca Crucetta near Monte Cinto, which replaced the Cirque de la Solitude and sits higher than the line it replaced.The month the route opens up, and it improves week by week. Early June still holds snow in the north; by late June the whole traverse is usually walkable with the longest daylight of the year.
25°C high · 12°C low · 15.2Watch for: Lingering snowfields on the northern stages, and swollen streams while the melt runs.The driest and sunniest month in the record, 44 mm across about 5 rain days and 344 hours of sun, with every refuge staffed and the whole route open. The cost is heat and company.
29°C high · 15°C low · 14.8Watch for: Heat on exposed granite, and afternoon thunderstorms that build over the ridges. Start early and be off the high ground by mid-afternoon.Everything is open and the weather is reliable, but it is the hottest month, the most crowded, and the one with a real fire-risk dimension. Rated below July on those grounds rather than on conditions underfoot.
30°C high · 17°C low · 13.7Watch for: Corsica's fire risk is at its height and can close forest tracks and approach paths at short notice. Also afternoon storms and genuinely dangerous heat on the exposed southern stages.The best answer for most walkers. Ground as good as July's with far fewer people and kinder temperatures, and the refuges are staffed the whole month.
26°C high · 13°C low · 12.5Watch for: Water at its scarcest before the autumn rain. Nights are cold at altitude even when the days are warm.The first days still fall inside the staffed season, and then it ends and the autumn rain sets in. 110 mm in the valley falls as something far less pleasant at 2000 m.
21°C high · 10°C low · 11.1Watch for: First snowfalls arriving without warning on the northern traverse, with no guardian and no resupply behind you.The wettest month of the year at 124 mm over about 14 days, and that is the valley figure. The high route is taking its first serious snow.
15°C high · 6°C low · 9.9Winter conditions the length of the route and the shortest days of the year against stages that need a full summer day.
12°C high · 3°C low · 9.2