When that air reaches altitude, it cools off, and any water held in it condenses and falls as rain or snow. The taller a mountain is, the more air is forced over it, creating even more potential for precipitation.
Winds can also exacerbate the severity of high-elevation storms if they pass over major bodies of water on their way to a mountain. Peaks in the Cascades, for example, get precipitation from winds that blow in from the Pacific. On Everest, winds can carry water in from the Bay of Bengal to the south.
Everest is always formidable but for most of the year, it is also un-climbable. The monsoon season from June through September brings drenching rains down low and heavy snow up high. Conditions dry out in October, and some climbers choose to make a summit push then, but by late fall, short days and frigid temperatures increase the difficulty of climbing.
The majority of expeditions arrive at Base Camp in April, when temperatures warm up enough to make climbing possible. Then, all eyes turn to the jet stream.