The concept of Qiyun was first put forward by Chinese geographer Mr. Xu Jinzhi in the 1950s. He pointed out that at the summit of Mount Everest, the cloud belt fluttering with the wind looks like a flag flying over Mount Everest, so it is called flag cloud. He believes that "when the moist air flow rising from the south of Mount Everest meets the strong west wind, there is a flag cloud extending eastward on the top of the mountain". Observation and research shows that flag cloud is a kind of cloud at the top of Mount Everest. Because it hangs high on the top of the world's highest peak and floats with the wind, it can be described as "the world's highest weathervane".
On the north slope of Mount Everest, the underlying surface above 7000 meters above sea level is very different from that at 7000 ~ 5500 meters above sea level. The former is mainly gravel surface, and the latter is mainly ice and snow surface. After sunrise, at an altitude of more than 7000 meters, the surface temperature of the hillside is greater than that of the free atmosphere at the same height. According to the principle of isotherm and isobaric, the mountain wind prevails above the altitude of 7000 meters on the north slope, which transports the water vapor sublimated by ice and snow below the altitude of 7000 meters to the top of Mount Everest, providing necessary conditions for the formation of mountain clouds.
So, at what height do clouds form? Predecessors have summarized a formula to calculate the condensation height of cloud, which is related to the height of barosphere, air temperature and dew point temperature. According to the radiosonde meteorological data at rongpusi station on the North Slope in April 1959 and 1975, the monthly average height of the 400 hPa barosphere over the station in April was 7482 meters, the average temperature was - 24.2 ℃, the average dew point temperature was - 33.2 ℃, and the decline rate of the average temperature with height was 0.66 ℃ per 100 meters. With these data, we can calculate that the difference of cloud condensation height is 1364 meters. Adding the average height of the 400 hPa pressure layer, the average condensation height of the cloud is 8846 meters, which is very close to the altitude of 8844 meters of Mount Everest.
It can be seen from the above that before sunrise and sunset, the generated cloud is just near the top of Mount Everest. It moves with the wind and looks like a flag flying on the top of the world's highest peak. The direction of the flag cloud depends on the wind direction of the high-altitude wind.
On the contrary, before sunrise and after sunset, due to the lack of sunlight, at an altitude of more than 7000 meters above sea level, the temperature on the gravel surface is lower than that of the free atmosphere at the same height, forming a downhill wind. Without cloud forming conditions, it is difficult to have mountain top clouds or flag clouds on the top of Mount Everest. The change of flag cloud on Mount Everest is helpful to the short-term weather forecast when climbing Mount Everest.
In other high mountain areas, as long as there are relatively isolated mountains, flag clouds can sometimes be formed under certain surface conditions and circulation conditions.