Understanding Himalayan Weather

Nepal's mountain weather is a negotiation between two enormous systems: the summer monsoon, which floods the country from June to September, and the subtropical jet stream, which parked directly on Everest's summit for most of the year.

The climbing calendar is not a tradition. It is a direct read of the atmosphere. The jet stream moves north in May, lifting off the high summits for a few weeks; and it dips south again in the autumn, giving a second, colder window in October. Everything else — every summit, every tragedy, every fixed rope — happens inside those windows.

The monsoon, explained

From June, moist air from the Bay of Bengal marches up the valleys daily: clear mornings, thunderclouds by 11, rain or snow by afternoon. In the high country the monsoon arrives as snow — above 5,000 m, this is the season's real danger.

But the monsoon never crosses the great walls. Upper Mustang, Dolpo, and the Tibetan-border plateaus sit in the rain shadow and stay dry — which is exactly why the desert kingdoms are the monsoon-season destination.

Reading a forecast at Base Camp

Expeditions watch three or four forecast models and their disagreements: wind speed at summit altitude is the only number that matters. Under 30 km/h is a summit day. Over 50, nobody climbs. In between is where judgement, experience and luck live.

The best thing a trekker can learn is the diurnal cloud rhythm — clear dawn, building cumulus, afternoon storm — and plan to be off ridgelines by two. The mountains publish their weather every day; you just have to read it.