Research reveals formation conditions of the interesting clouds that form around Mount Fuji — University of Tsukuba and collaborators

Cap clouds, Tsurushi clouds (a type of lenticular cloud), and Hata clouds (banner cloud-like clouds)-Mount Fuji is known for the diverse and fascinating cloud formations that appear around it. A research group from the University of Tsukuba and other institutions has announced findings that clarify the conditions under which these clouds form. They conducted detailed investigations through long-term observations using cameras positioned around the mountain, combined with meteorological data analysis. This represents the first scientific verification of the characteristics of these clouds, which had previously been described largely through empirical observation.

These clouds that adorn Japan's most sacred mountain are visually impressive and cherished. While general mechanisms for the formation of each cloud type had been proposed, they had not been fully clarified. The group observed clouds using what was effectively seven live cameras positioned around Mount Fuji over a three-year period from January 2019 to December 2021. Based on meteorological data from the Japan Meteorological Agency, they examined wind direction and speed at the 700 hectopascal altitude level on the windward side of Mount Fuji-approximately 3 kilometers above ground. They also verified wind speeds at different altitudes.

As a result, they identified the following characteristics for each cloud type: (1) Cap clouds, which appear as if the mountain is wearing a hat, are primarily observed on mornings during warm seasons when strong west-southwesterly winds blow and there is a layer of moist air near or above the summit.

(2) Tsurushi clouds, which appear as if suspended from the sky, tend to form with southwesterly winds when the layer of moist air is at a somewhat higher altitude and the vertical wind speed gradient is smaller than during cap cloud formation. While there were two theories about their formation mechanism, it was found that "mountain waves"-vertical oscillations in the atmosphere that occur when winds blow over mountains-are at work. Mountain waves generated by west-southwesterly winds in summer, with small vertical wind speed gradients, lift moist air to form clouds. The effect related to the alternative theory, where winds split into two streams as they pass the mountain and subsequently merge, appears to be minor.

(3) Hata clouds, which appear like fluttering flags, form frequently during daytime in cold seasons when dry northwesterly winds blow. The winds are weak at elevations below the summit, strengthen suddenly near the summit, and become even stronger aloft. In contrast to the mechanism behind Tsurushi clouds, these are conditions where mountain waves are less likely to occur. Most Hata clouds are thought to form when updrafts generated as west-northwesterly winds in winter pass over and merge around the mountain lift moist air.

The research also revealed that the main type of cap cloud is the "grounded cap type," which touches the summit, the main type of Tsurushi cloud is the "elliptical type," and the main type of Hata cloud is the "horse's mane type."

These clouds can form not only on Mount Fuji but also on isolated conical mountains such as Mount Iwaki (Aomori Prefecture) and Sakurajima (Kagoshima Prefecture).

It is fascinating that subtle differences in conditions such as wind create this diverse array of clouds. Professor Hiroyuki Kusaka, who specializes in Meteorology, from the Center for Computational Sciences at the University of Tsukuba, a member of the research group, stated: "This is research we're doing because it's interesting, rather than because of its practical utility. We want to verify this in even greater detail. We are conducting numerical simulations to investigate the specific roles of key factors such as wind direction, wind speed, vertical distribution of humidity, and the structure of the atmospheric boundary layer."

The research group consists of the University of Tsukuba, the National Institute of Information and Communications Technology, and others. The results were published in the electronic edition of Weather, the journal of the Royal Meteorological Society, on October 30, and announced by the University of Tsukuba on the 25th of November.