The weather and climate of Mount Everest is one of extremes. Temperatures at the summit are never above freezing and during January temperatures can drop as low as -60° C (-76° F). Despite the low temperatures the biggest issue faced by climbers are hurricane force winds and wind chill. When the jet stream dips south during the winter hurricane force winds may develop reaching more than 285 km/h (177 mph). These winds relax in the month of May and most climbers try to attempt the peak during this short window.
The Himalayas house the 3rd largest concentration of ice on the planet after the polar regions are often referred to as the third pole of the planet.
Seasons of Everest
Mount Everest lies 28° north of the equator and is subject to a typical northern hemisphere seasonal pattern. It lies at the edge of the influence of the Indian Monsoon which brings moisture and clouds from June to September. The cold months are December/January and the best trekking is between these two seasons .during March to May and October to November, when the climate is moderate.
1. Winter Weather at Everest: Winter weather (Nov to Feb) is dominated by the southward migration of the northern hemisphere jet stream which brings high winds, cold temperatures and on occasion heavy snow falls. Winter can be a great time for trekking as the crowds trying to avoid the cold clear out. Day temperatures are comfortable and sunny skies generally prevail although it can get well below freezing at night.
2. Spring (preMonsoon)Weather at Everest: March until the unset of the Monsoon is one of the ideal times to trek. Rain is infrequent and day temperatures are warm. Lots of trekkers and climbers are out in t-shirts.
3. Monsoon Season Everest: The summer weather (June to Sept) is influenced by the Indian Monsoon which brings frequent rainfall and cloudy days. You might find solitude during this time but clouds and rain generally obscure the peaks and it’s not a good time to visit.
4. Fall Weather (postMonsoon) at Everest: The Fall season from the end of the Monsoon until temperatures get cold in December is a great time for trekking with generally clear days and comfortable temperatures.
We don’t recommend travel to the region between June and September once the monsoon sets in as generally cloudy weather and rain obscure the peaks and you might be lucky to even see Everest.
Everest Precipitation Patterns
The Everest Base Camp Region is fairly dry with an average of 18 inches of precipitation received at base camp. Most of the precipitation that does fall (80%+) occurs during the monsoon season of June to September and the rest of the year is fairly dry. Monsoon moisture comes from a south so Everest Base Camp is experiences a rain shadow effect with most precipitation falling on the south side of the mountains and raining out before the clouds reach base camp. The high elevation and generally cold temperature act as a controlling influence on the amount of moisture and limit its overall availability. This becomes evident when one compares base camp precipitation (18in) to that in Lukla (70in) at the start of the trek. The actual summit of Everest receives very little precipitation as most of it is scoured by high winds. Big winter snowstorms while infrequent are possible as strong mid-latitude storms occasionally brush the region and can deliver over a meter of snow at basecamp often taking trekkers by surprise. Its still possible to hike out but be prepared with windproof pants and gaiters if planning a winter trek.
Temperatures at Everest Summit and Base Camp
The coldest temperatures of the year occur from the 15th of December onward until the end of January with temperatures at the summit averaging -37C (-35F) while those at Everest Basecamp average -17C (1.4F). Temperatures warm rapidly in the spring months of April and May bringing t-shirt weather although nights are still cold and chilly. It’s not uncommon to see climbers wearing t-shirts all the way up to camp 3 during this time period.
During May the short window for climbing the mountain summit temperatures average -15F and -26C.
Wind and Wind Chill on Mount Everest
The summit of Everest might be the windiest place on earth with hurricane force winds buffeting the summit on over 50% of days during the windiest months. From the middle of October until the start of April the summit is subjected to winds that are almost continually in excess of 74 mph which is equivalent to a Category 1 Hurricane. During the winter Wind Chill adjusted temperatures at the summit are as low as -70C (-90F) and exposed skin would be frostbitten almost instantaneously at this temperature. Lower wind speeds in May and warmer temperatures explain the relatively short climbing window.
Wind chill and low temperatures combined with high elevation make the summit of Everest one of the least hospitable places on the planet during the winter. The adjusted wind chill temperatures of 70C (-90F) observed on the summit compare to the worlds coldest places such the record cold -67.8C (-90F) in Siberia and -89C (129F) measured at Vostok in Antarctica.
The above picture shows direction and winds at the 300mb level in January and in July (Note: the reduced strength in July and the northward migration of the Jet Stream).
How late in June can you do the Everest Base Camp Trek ?
We recommend avoiding the trek during the monsoon season but the actually timing of the monsoon varies from year to year. Some years the monsoon can start as early as late May and others not until the end of June but the average is about June 15. The monsoon lets up on average in the middle of September and in some years persists even to early October. If you are able to plan your trip without much advance notice and want to go in June keep tabs on forecasts and the progress of the monsoon across India. The monsoon typically advances as a very clear boundary and withdraws in a much less orderly fashion.
Cloud Formations of Everest
The pyramidal shape and high winds around Mount Everest produce a rare cloud type know as a “Banner Cloud.” The Banner Cloud can be seen on otherwise cloudless days on the lee side of the mountain and forms as strong winds wrap around the mountain and create a temporary low pressure cell on the lee side which draws moisture upwards. Only a few other peaks like the Matterhorn in Switzerland produce this same type of dynamic. Its commonly thought the cloud forms as Everest is the only mountain that actually projects into the jet stream but recent research shows its more likely due to the combination of high winds and the morphology of the mountain.
It’s said that the shape of the cloud on Everest is actually associated with the wind speed so that at 50mph the cloud is at a right angle to the peak and when its stronger it tilts down (conversely tilting up when its weaker).
Forecasting the Weather on Everest
The nearest weather station to the summit is at Lobuche and is known as the “Pyramid Station.” The weather station is at elevation of 5,079m and has been operating since 2006. Obtaining accurate forecasts at the summit is essential for climbers and many teams hire meteorologists to produce daily spot weather forecasts. Forecasts are made by combining on the ground observations with large scale weather models. It can generally be assumed that the temperature gradient between base camp and the summit is approximately +1ºC per 150 meter of altitude.
Climate Change and Everest
The small museum at the Everest view point in Namche Bazaar has some great historical photos of the Khumbu Glacier. Looking at these photos it’s not at all evident that the Khumbu Glacier is retreating because it is in basically the same position during the last 50 years. However, looks can be deceiving and while the glacier is not retreating it is thinning and loosing volume. Thinning occurs as a result of sublimation (direct evaporation of the ice) as opposed to melting. Sublimation occurs as temperatures are frequently below zero and the glacier never has a chance to actually melt.
Climate change in the Himalayas is underway and the rate of glacial retreat is greatly debated. This topic is best covered in a separate post and will be published shortly.
Everest and Climate Extremes
- Coldest Temperature on Summit: -41ºC (-42F)
- Warmest Temperature on Summit: -16ºC (3F)
- Highest Windspeed Everest: 175mph+ (Category 5 Hurricane Equivalent)
- Everest Summit is the Windiest Location on Earth
- Unique Clouds: Banner Clouds form only on Everest and the Matterhorn
Note : Special Thanks to Alton C. Byers for use of the photo showing retreat of the Khumbu Glacier. Alton is the director of Director of Science and Exploration at the Mountain Institute and Co-Manager of the High Mountains Adaptation Partnership (HiMAP), a partnership working with local communities to develop action plans to minimize risks related to catastrophic outbursts from glacial lakes.