The summit of Everest is normally exposed to the jet stream winds and is very windy. There is a period just before the monsoon weather system begins to arrive, pushes the jet stream off the mountain and gives a few days calmer weather before the monsoon sets in, this period is called the weather window. There are two weather windows each year, in the second half of May and a smaller less reliable on in September (it is also colder here) and this is when most people will attempt to climb Everest.
While the air will always contains around 21% oxygen, the effective oxygen levels in the air will be massively reduced by the lowering of air pressure at altitude. The level will be just over 50% of the level at sea level at base camp; 44% at ABC and falling to 32% at the summit.
Current weather forecast
