Weather Windows and Expedition Planning at Altitude

Expedition mountaineering at high altitude is in large part a discipline of patience. A climber who has acclimated, established high camps, and assembled gear still cannot summit without an acceptable weather window for the attempt, and recognizing the window when it appears is the critical judgment call in most expeditions. The climbers who succeed on demanding routes typically do so not by having exceptional weather during the expedition but by correctly identifying the three or four day period in a thirty day expedition when the conditions are acceptable.

The forecast products used for expedition planning have become substantially more accurate over the last fifteen years. Dedicated mountain forecasters serving the climbing community now produce forecasts at specific summits and ridgelines, with resolution that general aviation or commercial forecasts cannot match. The forecasts integrate multiple global models, regional models tuned for terrain, and upper air soundings from the nearest stations. Climbers on Denali, Aconcagua, Everest, and the Karakoram giants now plan around forecasts that accurately resolve summit weather three to five days in advance.

The window itself is defined by a combination of wind, precipitation, and temperature. Summit winds above forty to fifty miles per hour make technical climbing dangerous independent of any other factor. Precipitation on the route produces dangerous snow loading and avalanche conditions. Temperatures that push wind chill into severe frostbite range make fingers and toes unreliable within an hour of exposure. A window exists when all three variables are manageable simultaneously, and the number of consecutive days with all three in range is typically small on any demanding route.

The decision to commit to a window is the hardest one. Once a party leaves high camp for a summit attempt, the attempt typically takes twelve to twenty hours round trip on the largest peaks, and the weather in hour eighteen may differ from the weather in hour one. Committing requires confidence that the forecast window will hold at least that long, and the experience of reading the early hours of the attempt against what the forecast predicted is how climbers learn to trust or question the product. Patterns that deviate early from the forecast are a signal that the rest of the forecast is also in doubt.

Retreat is the decision that gets fewer headlines than summits but that keeps climbers alive for the next expedition. A window that closes prematurely, a forecast that deteriorated after the party committed, or conditions that were worse in execution than the forecast suggested are all legitimate reasons to turn around, and the discipline of turning around is what separates careers from single seasons. The mountains remain in place. The window that closed on one attempt will open on another. Expedition mountaineering is not a sport where persistence in adversity is rewarded. It is a sport where recognizing the limits of the conditions and the body is rewarded, and the weather window is usually the constraint that determines both.