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Summiting Everest: Weather's narrow window of opportunity

Craig Mitchell, Saturday September 21, 2024 - 10:15 AEST

For professional climbers and commercial mountaineering companies, April and May are the optimal months to reach the "top of the world", and within these two months, there may only be a few days when weather conditions near the Himalayan mountain peaks are suitable for a summit attempt.

Image source: Simon, Pixabay Founder

The Himalayan Mountain Range, with the highest concentration of peaks above 7,000 meters, serves as a natural boundary between the cold, dry winters of the Tibetan Plateau to the north and the humid, rainy summer monsoon season over Southeast Asia to the south.

March is considered the month marking the end of winter in Central Asia with April bringing a gradual increase in temperatures and a general decrease in wind speeds at higher elevations across the Himalayan Range. While winter temperatures above 7,000 meters can range from -20°C to -30°C and drop as low as -50°C to -60°C above 8000m, temperatures in May are more stable and typically range between -10°C and -20°C, making conditions more manageable for climbers.

Wind speed is another crucial factor for climber safety, as high winds can exacerbate the effects of cold temperatures, leading to severe frostbite and hypothermia. Climbers often face fatigue while trying to maintain core body temperature and maintaining balance against the physical forces. Strong winds can also cause blowing snow and reduced visibility, complicating navigation on exposed ridges.

At 8,849 meters, Mount Everest lies just below the tropopause and is affected by fast-flowing jet streams, where wind speeds typically exceed 140 km/h. The presence of a jet stream and extreme wind chill can increase the rapid onset of hypothermia.

An example of extreme wind conditions occurred on February 7, 2019, when the German Weather Agency's ICON weather prediction model forecast winds of over 140 km/h extending from Saudi Arabia (across the Himalayas) to southern China and wind speeds near Everest reaching 200 km/h (image below).

Wind forecast at 9000m on February 7, 2019 with annotated Jetstream (arrows) [Source: Ventusky - ICON model]

Even during April and May, strong winds associated with the jet stream pose significant risks, creating dangerous and potentially life-threatening situations at high elevations.

To ensure the safety of climbing teams and increase the likelihood of a successful summit, expedition organisers often employ their own meteorologists to advise on the best dates for the summit ascent. Meteorologists look for high-pressure systems moving over the region, which can bring clearer skies and lighter winds. On April 24, 2019, a high-pressure ridge west of Mount Everest resulted in mostly clear skies and forecast wind speeds of less than 30 km/h at 9,000 meters. On this day, a jet stream was located over Southeast Asia, with another positioned over the Middle East (image below).

Wind forecast at 9000m on April 24, 2019 with annotated Jetstream (arrows) [Source: Ventusky - ICON model]

The race to the Top

Setting off on the upward climb to the next camp [Source: unknown photographer - Pixabay]

The race is on during May to complete summit attempts and retreat from the mountain peaks before the arrival of the monsoon and the heavy snowfalls, frequent thunderstorms, severe cold and increasing risk of avalanches. The monsoon typically reaches the eastern Himalayas in early June but can arrive as early as late May. Once the monsoon season has begun, it will typically continue across the Himalayan Range until around September.

Monthly rainfall accumulations using CAMS (Climate Anomaly Monitoring Systems) modelling data from the US Climate Prediction Centre (image below) show the development of the monsoon season across Southeast Asia and Bhutan in May and progression of the monsoon across India and Nepal during June and July.

CAMS (satellite monitoring system) Monthly climatic rainfall accumulations in mm/month (based on 1991-2020 period) with annotated monsoon trough position [Source: IRI (Columbia Climate School - International Research Istitute): images from the US Climate Prediction Centre]

The pre-monsoon months of April and May offer the best conditions for summiting Mount Everest and other Himalayan peaks with a more predictable weather pattern, manageable temperatures and a higher chance of lower wind speeds at higher altitudes.

If your dream is to reach the top of the world, place a circle around April and May on next year's calendar.

- Weatherzone

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Summiting Everest: Weather's narrow window of opportunity

10:15 AEST For professional climbers and commercial mountaineering companies, April and May are the optimal months to reach the "top of the world", and within these two months, there may only be a few days when weather conditions near the Himalayan mountain peaks are suitable for a summit attempt.