1. The Definition of Flag Clouds

Flag clouds refer to flag-shaped stationary clouds that appear on the windward side of isolated snow-covered peaks under specific terrain conditions ^[1]. The term "Everest Flag Cloud" specifically denotes flag clouds that appear near the summit of Mount Everest, as shown in Figure 1. 

Everest Flag Clouds typically occur from sunrise to sunset. In addition to general cloud formation conditions, there are three contributing factors: strong solar radiation, sufficient water vapor sources, and the influence of dynamic disturbances ^[2]. You can observe from the sky by taking the most dangerous airplane- the Mt. Everest airplane. You can Above 7,500 meters on the north slope of Everest, where there is often strong wind and little snow accumulation due to constant winds, the rocky surface warms up rapidly after sunrise. The heated air rises along the slope, and simultaneously, water vapor sublimated from the ice surface condenses into clouds during the ascent. Upon reaching the vicinity of the summit, influenced by high-altitude atmospheric disturbances, flag-shaped clouds form on the windward side.

In the international classification of clouds, there are three families, ten genera, and twenty-nine species. However, this classification does not include flag clouds. The reason for the exclusion of flag clouds from the internationally recognized cloud classification is their exceptional nature; they are not representative of common clouds. Flag clouds are a highly unique type of cloud, and they are not commonly observed everywhere. Their appearance is contingent upon specific geographical conditions.

Numerous automatic meteorological observation stations have been established on Mount Everest to collect basic meteorological data, given the variable shapes of flag clouds. The shapes of these clouds are not fixed and can change due to factors such as solar radiation, moisture conditions, and the influence of various weather systems. The earliest research on Everest flag clouds dates back to the 1950s, with Mr. Xu Jinzhi being one of the first to discover and provide preliminary explanations for these clouds. Subsequent research by Gao Dengyi in "Flag Clouds of Mount Everest" further elucidated the reasons behind the formation of Everest flag clouds ^[3]. The generation of flag clouds is primarily associated with the unique altitude and geographical conditions of Mount Everest, and their strength, shape, and drifting direction can be influenced by different weather systems.

Regarding the timing of their appearance, Everest flag clouds typically form after sunrise, with a higher frequency observed between 11:00 and 15:00. After 15:00, the development of cumulus clouds becomes more pronounced, leading to increased cloud cover over the mountain. At this point, flag clouds may become intermittent, obscured, or rarely visible. Three main influencing factors contribute to the formation of Everest flag clouds: the special height and geographical location of Mount Everest, the unique surface topography of the mountain, and the impact of various weather systems.

Everest Flag Clouds provide indications of high-altitude wind direction and speed, serving as a guide for mountaineering activities. If the flag cloud drifts from west to east, with a smooth and gradually descending cloud top, it is not advisable to engage in mountaineering activities above 7,400 meters that day. If the flag cloud drifts from southwest to northeast, with significant fluctuations in the cloud top and a gradual increase in height, mountaineering activities can still take place above 7,000 meters. If the flag cloud drifts from southeast to northwest, it indicates significant snowfall in the next 1 to 3 days, and mountaineering activities above 7,400 meters are not recommended. If the flag cloud rises straight up, it indicates favorable conditions for light wind at high altitudes, making it suitable for reaching the summit ^[3].

2. How to Forcast the Weather

Observational studies indicate that the vast majority of flag clouds appearing at the summit of Mount Everest drift from west to east. However, during special weather conditions, flag clouds can also drift from east to west. Based on the direction of the flag cloud's movement, one can infer the wind direction near the summit of Everest. Additionally, by examining the undulating state of the flag cloud's top, one can estimate the level of high-altitude wind speed and the weather conditions in the Everest region for the next few days.

If the flag cloud is drifting from west to east, with a smooth and flat top, and the cloud top gradually descends after leaving the summit, it indicates that the wind speed at high altitudes is above 17 meters per second, and mountaineering activities above 7000 meters are not advisable for that day.

If the flag cloud is drifting from southwest to northeast, with significant fluctuations in the cloud top and an upward trend in the cloud top height after leaving the summit, it suggests that the high-altitude wind speed does not exceed 15 meters per second. On that day, mountaineering activities above 7000 meters can proceed. However, a day later, a high-altitude westerly trough will arrive, accompanied by strong winds and snowfall. Mountaineering activities above 7000 meters are not recommended for the next two to three days.

If the flag cloud is drifting from east to west, it indicates an easterly airflow at high altitudes. In the next 1-3 days, an Indian low-pressure system will arrive, bringing heavy snowfall with light winds. Generally, mountaineering activities above 7000 meters are not advisable.

If there is minimal cloud cover at the summit of Mount Everest, and no flag cloud is formed, two possible scenarios suggest that mountaineering activities above 7000 meters are suitable for the day and the next 1-3 days: firstly, if the clouds at the summit are slowly moving towards the southeast, indicating a weak northwest airflow at high altitudes, controlled by the Westerlies' high-pressure ridge; secondly, if the clouds at the summit and nearby rise vertically, resembling rising smoke, indicating minimal high-altitude wind speed, controlled by the subtropical high-pressure system.

Certainly, understanding the relationship between flag clouds and weather systems requires more data collection and a more in-depth study combined with the overall atmospheric circulation to truly clarify the connection between the two.