Expedition itineraries advertise a departure date and then wait. The route is climbed inside a short spell of tolerable conditions, and everything else in the schedule exists to be ready for it.
A window is a combination, not a forecast of sunshine
What matters is a coincidence of conditions: wind below a threshold on exposed ground, no significant new snow, workable temperatures and enough visibility to navigate.
Any one of those failing closes the window even if the others are excellent, which is why a clear, cold, windy day is unusable.
Thresholds are route-specific. A sheltered face tolerates wind that would make an exposed ridge impassable, so two teams on one mountain can have different windows.
Windows are short and forecast late
Reliable detail in mountain forecasting extends only a few days, and the shape of a window firms up shortly before it opens rather than a week ahead.
The usable period is often measured in a couple of days, which has to contain the ascent, the descent and any fixed camps in between.
Teams therefore position themselves high and wait, because the travel to the start of the route cannot be done inside the window itself.
Waiting has its own costs
Sitting at altitude consumes food, fuel and strength, and physical condition declines slowly at height rather than being maintained.
Support staff, permits and lodging are paid for by the day, so a long wait raises the cost of the expedition without adding progress.
These pressures accumulate into an incentive to move on marginal forecasts, which is the specific risk experienced leaders plan against.
Everyone gets the same forecast
Because forecast products are widely available, all teams on a mountain identify the same window, and the route becomes crowded within it.
Congestion at fixed sections adds hours of standing in cold and wind, which turns a comfortable window into a marginal one.
Some teams deliberately move on the day before or after the consensus, accepting worse weather in exchange for an empty route.
What it means for booking a trip
Any itinerary with a genuine weather dependency should include contingency days, and an operator that offers none is selling a schedule rather than a summit.
Return flights booked tight against the itinerary end date remove the ability to use a late window, which is when many windows appear.
Trip cancellation and delay cover matters more on these trips than on most, precisely because the dates are known to be provisional.