When engaging in activities in high-altitude mountainous regions, besides the aforementioned crucial climatic knowledge, it's also essential to understand some basic mountain weather patterns. Grasping this weather knowledge not only aids your mountaineering endeavors but also facilitates your nature experiences, adding enjoyment to your adventures.
Valley Wind
This is a common phenomenon in mountainous areas. The alternating occurrence of downslope and upslope winds within a 24-hour period is called valley wind. Generally speaking, upslope wind is also known as valley breeze, where air flows from the valley up the slope; downslope wind is also known as mountain breeze, where air flows from the slope down into the valley. The formation of valley winds is mainly due to the differences in the ability of the valley’s surface and the overlying atmosphere to absorb solar radiation, resulting in temperature variations between the air on the slopes and the air at the same elevation in the valley. Generally, from local noon until midnight, it is mountain breeze, while during the rest of the time, it is valley breeze. On the northern slope of Mount Everest, because the surface at elevations between 5,300 and 7,000 meters consists mostly of ice and snow, the surface temperature after sunrise remains lower than the air temperature at the same elevation in the valley, so downslope winds prevail almost all day. This type of downslope wind formed due to glacier distribution is also called glacier wind. Glacier wind is strongest between 2 PM and 6 PM local time, with average wind speeds reaching 10 meters per second below 1,000 meters above ground, and gusts reaching force 7–8. Mountaineers can plan their climbing activities based on different valley conditions.
Diurnal Variation in River Flow
Generally speaking, the diurnal variation in river flow in high-altitude mountainous regions is primarily determined by the diurnal changes in glacial meltwater, which in turn is mainly determined by daily temperature fluctuations. During spring, summer, and autumn, after sunrise, when sunlight hits the glacier surface, the glaciers gradually melt, and trickles of water slowly converge into the river valley, increasing river flow. In the Rongbuk River area of Mount Everest, about 67% of the river flow comes from glacial meltwater. Therefore, the river flow of high-altitude rivers is very low before local noon, peaking in the afternoon to evening. Mountaineers who must cross rivers on foot should do so before noon, otherwise they risk being swept away by the bone-chilling glacial meltwater, endangering their lives. A teammate from the Chinese Academy of Sciences' Qinghai-Tibet Scientific Expedition Team unfortunately lost his life after accidentally falling into a glacial meltwater stream during a post-noon river crossing.
Wind-Chill Effect
When engaging in mountaineering activities at high altitudes, in addition to adapting to the rule that ground wind speed increases rapidly with altitude—requiring you to "start early and camp early"—mountaineers must also understand the serious dangers posed by the simultaneous occurrence of strong winds and low temperatures. Here we introduce the "wind-chill effect" and "wind-chill equivalent temperature" (also called "apparent temperature"). The so-called "wind-chill effect" refers to the phenomenon where people feel even colder due to strong winds. The "wind-chill equivalent temperature," also called the "apparent temperature," means that under the same actual air temperature, the higher the wind speed, the lower the temperature people feel on their bodies. This body-perceived temperature, which differs from the meteorologically observed air temperature, is called the "apparent temperature." The reason for the significant difference between the "apparent temperature" and the observed meteorological temperature is that strong winds accelerate the loss of body heat into the atmosphere. According to measurements, when the air temperature is 0°C, if the wind speed is below 2.5 m/s, the "apparent temperature" is the same as the actual temperature; if the wind speed is 10 m/s, the "apparent temperature" is -12°C; at 20 m/s, the "apparent temperature" is -18°C. If the atmospheric temperature is -15°C and the wind speed is 10 m/s, the "apparent temperature" is -30°C, and so on.
Generally speaking, when the "apparent temperature" falls below -30°C, mountaineers are highly susceptible to frostbite. When climbing above 7,000 meters, mountaineers often encounter conditions with temperatures around -15°C and wind speeds of 10 m/s, meaning they frequently face potentially frostbite-inducing weather conditions, which is something climbers must be wary of.
Clouds and Weather In high-altitude mountainous regions, there is a certain relationship between clouds and weather changes. Understanding the basic relationship between clouds and weather can aid your mountaineering activities.
Here, the relationship between clouds and weather in the Mount Everest region is provided as an example for mountaineers to reference.
In spring, when mountaineering on the northern slope of Mount Everest, climbers should pay special attention to the changes in the "banner cloud" of Everest.
The so-called "banner cloud" refers to convective cumulus clouds that continuously form atop Mount Everest. Influenced by strong upper-level winds, these clouds drift and undulate like waves; from a distance, they resemble a flag fluttering at the summit, hence the name "banner cloud."
Observation studies have shown that the "banner clouds" appearing at the summit of Everest mostly drift from west to east, but when special weather systems arrive, the "banner clouds" may also drift from east to west.
From our multiple experiences creating summit weather forecasts for Mount Everest climbs, we have come to recognize that the "banner cloud" atop Everest can indeed be called the "world’s highest wind vane."
First, the direction in which the Everest "banner cloud" drifts can indicate the wind direction near the summit height (around 8–9 kilometers); second, the undulating state at the top of the "banner cloud" can help estimate the level of upper-level wind speed.
Photo 1: Banner cloud blowing strongly and descending on the eastern side of the summit, indicating strong winds, not suitable for summiting.
Photo 2: Blowing snow at the summit of Everest, indicating strong winds, not suitable for summiting.
If the "banner cloud" drifts from west to east, the top of the cloud is smooth and flattens out, and the cloud top gradually descends after leaving the peak (Photo 1), with wind speeds exceeding force 7 at 8–9 kilometers altitude, summiting is not advisable that day. If there is blowing snow at the summit of Everest (Photo 2), wind speeds at 8–9 kilometers altitude are also greater than force 7, making summiting inadvisable that day.
Photo 3: Banner cloud drifting southeast and ascending, with light winds, summiting is possible.
Photo 4: The clouds atop Everest appear braid-like, with light winds, summiting is possible.
If the "banner cloud" drifts eastward from the summit and shows an upward trend (Photo 3), or if the banner cloud drifts eastward from the summit and takes on a cloud shape resembling a little girl’s flipped braids (Photo 4), it indicates that the wind speed at altitudes of 8 to 9 kilometers is below force 6, making it suitable to attempt the summit on that day.
Photo 5: During the Spring Festival, when the banner cloud drifts northwestward, it predicts snowfall due to the influence of an Indian low-pressure system in the next 2–3 days.
If the banner cloud drifts from the northeast to the southwest (Photo 5), it indicates that a high-pressure ridge is passing near the summit of Mount Everest, with wind speeds at 8 to 9 kilometers below force 6, signifying favorable weather conditions for summiting.
Photo 6: The banner cloud drifting from the northeast to the southwest indicates the influence of a high-pressure ridge, making it suitable for summit attempts.
If the "banner cloud" drifts from the southeast to the northwest (Photo 6), it indicates the presence of southeasterly airflow at high altitudes, and an Indian low-pressure system will arrive within the next 1–3 days, bringing snowy weather with light winds. Generally, climbing activities above 7,400 meters are not advisable. However, on the north side of Everest, experienced mountaineers familiar with the terrain and routes may take advantage of this light-wind, warmer-weather period to engage in climbing activities below 8,500 meters, as the main threat to summiting from the north side is strong winds.
Photo 7: When clouds rise straight up from the summit of Everest, it indicates the best weather for summiting.
If there are very few clouds at the summit of Everest, no "banner cloud" forms, and the clouds rise almost vertically (Photo 7), it indicates that the wind speed at 8 to 9 kilometers is below force 5, representing the best possible weather for summiting.
Photo 8: The appearance of cirrus clouds over Everest indicates that the weather will deteriorate in two to three days, making it unsuitable for summiting.
If systematic high-altitude clouds appear on the western side of Everest (Photo 8), and their height gradually lowers, a westerly trough will arrive within the next 2–3 days, bringing strong winds and precipitation. Climbing activities above 7,400 meters must be suspended. (China Association for Scientific Expedition)