Nobody tells you Everest can only be climbed a few days a year, and the jet stream is why

Mount Everest can only be climbed for a handful of days each year, and the reason is a river of wind in the sky called the jet stream. For most of the year it sits directly on the summit, blasting the top of the mountain with sustained winds of 160 km/h and gusts that can top 250 km/h — conditions no human can climb or survive in. For a few weeks each spring, as the monsoon shoulders the jet stream north, that wind lifts off the summit and the mountain briefly falls calm. That short calm is the only window climbers get. Miss it, and the mountain closes again for a year.

The jet stream that owns the summit

The jet stream is a band of very fast, high-altitude wind that circles the planet at roughly the height of Everest’s summit, around 9,000 m. For most of the year it flows straight across the Himalaya, and the top of Everest sits squarely in it. That is why, from autumn through early spring, the summit is a vertical desert of hurricane-force wind: sustained speeds around 100–180 km/h, with gusts that have been measured well above 250 km/h. At those speeds a climber cannot stand, tents are shredded, exposed skin freezes in minutes, and the wind chill drives an already lethal cold far lower. It is not a matter of toughness. The upper mountain is simply uninhabitable while the jet stream is parked on it.

Why May opens the door

Everything changes when the South Asian monsoon begins its annual march north in late April and May. As that vast weather system advances, it nudges the jet stream off the Himalaya and up over the Tibetan Plateau to the north. With the jet gone, the winds over the summit collapse, the pressure edges up, and for a short spell the top of the world becomes climbable. This is the “weather window,” and it is why almost every Everest summit in history has happened in a narrow band of days, usually in the second half of May. Expedition teams spend six weeks acclimatising on the lower mountain doing nothing but waiting for the forecast to show that the jet has lifted.

The pattern shifts year to year, and forecasters now model the jet-stream position days ahead so teams can time their push. But the fundamentals do not change: a usable window is measured in days, and the mountain grants only one or two of them per season.

Why the window itself is dangerous

The cruel twist is that the same narrow window that makes Everest climbable also makes it crowded and deadly. When hundreds of people have spent weeks and fortunes waiting, and the forecast finally shows three calm days, everyone goes at once. That is exactly what produced the notorious 2019 traffic jam, when a queue of climbers stacked up in the death zone during a two-day window and eleven people died — a story we tell in full in why the Everest traffic jam of 2019 turned deadly. A short window forces a gamble: push when the crowd pushes, or hold out for a cleaner day that may never arrive before the monsoon slams the door. Thin air makes that decision even harder — see our guide to altitude sickness, AMS, HACE and HAPE.

Cover photo: Nir B. Gurung via Wikimedia Commons (CC BY-SA 4.0).