The weather plays a very important and often determining role in a successful summit attempt on Everest. The Jet stream is the main concern on Everest weather. This westerly wind will have a large impact on the choice for a summit day.  Other dangers caused by bad weather centre around the changes it causes in snow and rock conditions, making movement suddenly much more arduous and hazardous than under normal circumstances.  Most climbs are scheduled around May which is the safest time to climb, before the Monsoon season.

The jet stream is a fast-moving wind current that blows west to east around Earth between the altitudes of 25,000 to 45,000 feet. The speed of the average wind at the core is about 80 knots, or 120 mph, but can reach upwards of 200 mph in the winter. Usually during the scheduled climbing period on Everest the low-pressure systems will push the jet stream northward into Tibet.

On summit attempt day the winds ideally will be less than 20 mph (10m/s). Even if that doubles, there is still a good chance of making it as successful summit attempts have been made at 40mph.  Also the weather should be stable.  If the weather is inconsistent then it best to wait until there is some stability.

Storms are also a weather hazard on Everest.  Although these are typically in Monsoon season they can come earlier.  In 1996 a sudden severe storm trapped several climbers descending from the summit.  This is the biggest tragedy of Everest where eight people died.