Name            Pressure (hPa)            Appr. altitude
BC                495                            6100 m
Summit          346                            8750 m

Everest Temperature (all values Celsius)

Everest summit temperature roughly fluctuates between -20ºC during summer and -35ºC. The coldest forecasted summit temperature was -41ºC (on several occasions in Dec 2002, Jan and Feb 2003. The warmest forecasted temperature is -16ºC.

Basecamp temperatures reflects summit temperature with an expected +1ºC per 150 meter of altitude drop.

Note the temperature drop for February 2003. 8 of 12 summit temperatures below -40ºC happened during a two week period mid-February 2003. Two season average temperature show January as the coldest month followed closely by February and December.
 

Everest Wind speed (all values meter/second)

It's always cold on the summit but not always windy. From end of May until the third week of October there typical summit winds will range between calm and 15 m/s.

The highest forecasted wind speed, 78 m/s (175miles/hour) was for Feb 6, 2004. This is well above the 156 miles/hour threshold for a Category 5 Hurricane.

There is a Category 1 Hurricane (32m/s - 74miles/hour) blowing 1 out of every 4 days at Everest Summit. From late autumn (app. Oct 20) until end of January there is almost constant hurricane force winds (more than 3 out of 4 days) at the summit. Climbing to Everest summit in a wind force of 32 m/s should be considered close to impossible, which means an Everest Winter Expedition has to be lucky to be able to stand even a slim chance of making the summit.

From this data February comes out as an unstable month. In the winter of 2003-2004 it was the most severe winter month, but in 2003 it was better than both January and December.

Everest Humidity

During summer month (Monsoon) there will be almost daily snow- or rainfall at BC. At Everest Summit chances of precipitation are generally very low, except for the summer month.

Wind Chill

During the windy winter months of Everest the Wind Chill factor is constantly around -100C, and it doesn't even make sense to measure wind chill since winds above 40 miles/hour doesn't really lower the wind chill max. Climbing Everest in the winter is simply put "beyond wind chill".

The "BAD" chart

When is Everest at its worst?

Comparing the factors above and adding the sunlight factor gives an interesting picture - besides from two very short periods in May and October it's almost never good to climb Everest.

Below is an attempt to define the climbing seasons of Everest: