Conditions

Seasons

When each range is in condition. Pick a month to see which ranges are at their best; the rest dim out.

Month

Generalised windows. Local conditions in any given year override this.
Range	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Himalaya (Nepal)	Closed	Closed	Closed	Workable	Prime	Closed	Poor	Poor	Poor	Prime	Workable	Closed
Karakoram	Closed	Closed	Closed	Closed	Closed	Workable	Prime	Prime	Workable	Closed	Closed	Closed
Alps (summer rock)	Poor	Poor	Poor	Workable	Workable	Prime	Prime	Prime	Prime	Workable	Closed	Poor
Alps (winter mixed)	Prime	Prime	Prime	Workable	Poor	Closed	Closed	Closed	Closed	Workable	Poor	Prime
Andes (Cordillera Blanca)	Poor	Poor	Poor	Workable	Prime	Prime	Prime	Prime	Workable	Poor	Closed	Poor
Patagonia	Prime	Prime	Workable	Poor	Poor	Closed	Closed	Closed	Poor	Workable	Prime	Prime
Alaska Range	Closed	Closed	Closed	Workable	Prime	Prime	Prime	Workable	Poor	Closed	Closed	Closed
Scandinavian rock	Closed	Closed	Closed	Poor	Workable	Prime	Prime	Prime	Workable	Closed	Closed	Closed
Romsdalen (winter walls)	Workable	Prime	Prime	Prime	Workable	Closed	Closed	Closed	Closed	Closed	Poor	Workable
Caucasus	Closed	Closed	Closed	Poor	Workable	Prime	Prime	Prime	Workable	Poor	Closed	Closed
East Africa	Workable	Poor	Closed	Prime	Prime	Workable	Poor	Poor	Prime	Prime	Workable	Closed
Antarctica (Vinson)	Prime	Prime	Workable	Closed	Closed	Closed	Closed	Closed	Closed	Closed	Workable	Prime

Prime Workable Poor Effectively closed

Notes by range

Range	Window
Himalaya (Nepal)	Pre-monsoon April to May and post-monsoon October. The monsoon shuts June to September.
Karakoram	A single window, late June to mid-August. Nothing else is realistic.
Alps (summer rock)	Huts open roughly mid-June. July and August are the settled months; September is quieter and often clearer.
Alps (winter mixed)	February to April for frozen faces and consolidated snow.
Andes (Cordillera Blanca)	The dry season is May to September, with June to August the most settled.
Patagonia	Southern summer, November to March. Windows last two to four days.
Alaska Range	May to early July. Later than that the snow bridges rot and the glaciers open up.
Scandinavian rock	June to early September, with July most reliable. Midnight sun through mid-July.
Romsdalen (winter walls)	February to April. The face needs to be frozen or it sheds rock continuously.
Caucasus	Late June to early September.
East Africa	Two dry seasons: January to February and July to October, between the rains.
Antarctica (Vinson)	November to January only, when there is daylight and the runway is usable.

What sets the window

In the Himalaya it is the monsoon. Moist air off the Bay of Bengal arrives in early June and does not clear until late September, burying the range in unstable snow. The two climbing seasons sit either side of it, and the pre-monsoon window is preferred on Everest because the jet stream lifts off the summit for a week or two in mid-May.

In the Karakoram, which sits north of the monsoon's reach, the constraint is instead the short period when the glaciers are passable and the high camps are not being scoured by wind.

In Patagonia it is pure wind. The Southern Ocean has no landmass to slow it at these latitudes, so the westerlies arrive on the icefield unobstructed. Forecasting improved enormously once high-resolution models became available, and the practical effect was that parties stopped making long sieges and started waiting in town for a two-day gap.

In Alaska the limit is the freeze. The West Buttress depends on snow bridges over crevasses on the Kahiltna, and by late July those are gone.