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ExWeb series - Winter climbing: The BAD chart, part 2
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Dec 17, 2004 10: 25 EST
Lately we've run a series about winter climbing in Himalaya. In high mountaineering, there are two ways to define winter; “calendar” and “permit”. The Calendar winter starts December 21, whilst a winter climbing permit begins December 1.

The Calendar winter is considered a "classic winter" climb. All first winter accents of 8000ers were made by Polish climbers - in Calendar winter. The first ascent was on Everest in 1980 and the latest on Lhotse in 1988.

But still, what does the reality look like? Let's put calendars and permits aside for a while, and examine the weather only, in order to find the worst time to climb.

Very few people have climbed 8000ers in the winter, and no one has skied to the North Pole or the South Pole in the cold season. This series is divided in 4 parts:

1. Wind Force

2. Temperature, Wind-Chill

3. Humidity

4. The “Bad” Chart – Defining Seasons

Today, part 2: Temperature, Wind-Chill

Everest summit averages -36C (-33F) during January versus -18C (0F) during the summer.
The coldest forecasted summit temperature was -41ºC (-42F). The warmest forecasted temperature was -16ºC (3F).

Basecamp Temp

Basecamp temperatures reflect summit temperature with an expected +1ºC per 150 meter of altitude drop, and range between -17C (1.4F) during Jan/Feb and -3C (27F) during summer.

Note that this is late Nepal evening temperature (6pm UTC) and it can get considerable warmer during midday. But it would also get colder in the early morning hours, when most climbers set out to climb.

Everest Winter

The coldest temperature of the year starts in the second half of December and holds until end of January. The charts show a steady drop until Dec 21, which correlates to the darkest day of the year (winter solstice) where it lands to an average of -37C (-35F) on the summit.

From that day until Feb 28 the temperature never rises above -33C (-27F) at the summit. The same pattern is true for Base Camp temperatures.

Adding Wind to Temperature

Measured temperature and experienced cold are two very different things. Applying the National Weather Service wind chill index, and the average wind speeds and temps for the periods, the average summit temperature in May drops to a wind chill of -53C (-63F). During January the average wind chill drops to -70C (-90F).

This makes Everest summit not only the tallest, but also among the coldest places on earth to humans - if not the coldest. Note also that this is an average.

The official record for the coldest Northern Hemisphere temperature is -67.8C (-90F) in Siberia, recorded on Feb. 6, 1922. The lowest temperature ever recorded on earth was -89C (129F) recorded in 1983 at the Russian Base Vostok in Antarctica. None of these data include the wind chill factor, but at sea level these kind of cold temperatures generally occur during calm conditions.

Climbing in winter

Average wind chill for Everest Base Camp is -14C (6F) during May and -30C (-21F) during winter. It’s important to consider the way an expedition is conducted. The summit is reached quickly in a couple of days when the weather briefly holds. However the tedious process of establishing camps and carrying supplies can not wait for good weather.

During the spring, climbs towards 7000 meters are often pleasant and seldom cold when the weather is fine. Climbing in t-shirt is not uncommon and down suit is necessary only from Camp 4.

Winter is completely different and temperatures call for serious gear already in Base Camp. The Wind Chill Temperature Index gives an average frostbite time of 30 minutes in BC and less than 5 minutes close to the summit.

Wrap-up winter cold

The coldest temperature of the year starts in the second half of December and holds until end of January. Everest summit averages -36C (-33F) during January. The average wind chill drops to -70C (-90F). Average wind chill for Base Camp is -30C (-21F) during winter.

Monday: 3. Humidity

Image 1 (ExplorersWeb): Average Monthly Temperature at the Summit of Everest since 2002
Image 2 (ExplorersWeb): Average Monthly Temperature at Everest Base Camp since 2002
Image 3 (ExplorersWeb): 7-day average detail of base camp and summit temperatures on Everest. Period November to Feb.


"BaseCamp" is at Pressure (hPa) 495 or app. 6100m of altitude

"Summit" is at Pressure (hPa) 346 or app. 8750m of altitude

The charts are based on forecasts delivered by SMHI and compiled by the super-computer at ECMWF. The information below is compiled from 868 forecasts from May 1 2002 to Nov 1 2004.

There is no weather station at the Summit of Everest which would have allowed exact readings, but the forecast should be fairly accurate - as it has been used by climbers for 10 Himalayan seasons and hundreds of high-altitude expeditions.

The AdventureWeather 8000 meter forecasting project is free of charge to climbers and sponsored by ExplorersWeb.com and its partners.

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