1. What kind of night creates a "Sea of Fog" over a town in the Miyoshi Basin?
The "Sea of Fog" in the Miyoshi Basin is featured in a public relations magazine by JR West. It describes how, on clear nights with large temperature differences from autumn to early spring, water vapor rises from the river surface, turns into fog, and spreads along the mountain slopes to cover the entire town. The fog, dyed by the morning sun, is said to look like the sea [9]. However, since this is a tourism-focused publication, we will verify the mechanism using other materials.
According to the Japan Meteorological Agency (JMA), fog is a phenomenon where countless tiny water droplets float in the air, reducing horizontal visibility (the distance you can see) to less than 1 km (about 0.6 miles) near the ground. If visibility is between 1 km and 10 km (about 6 miles), it is called "haze," not fog [1].
Why is fog particularly common in basins? In short, on clear nights with weak winds, cold, heavy air from the mountain slopes flows down and pools at the bottom of the basin. Water vapor from rivers and other sources adds to this air [4][5][6]. We will look at the observation of fog starting along the river in section 4. First, let’s look at how cold air flows downward.
2. Why does heavy cold air flow down slopes and pool on the basin floor?
Usually, the ground is warmer, and the air higher up is colder. However, because cold air is heavy, it can pool near the ground. The JMA explains this state as a "ground inversion" [4].
On clear nights, radiative cooling occurs, and heat from the ground escapes into the air [2]. The same thing happens on mountain slopes. Air that cools and becomes heavy flows down the slope and pools at the bottom of the basin. This stable layer of cold air is called a "cold air lake." The bottom of the basin becomes an inversion layer where the temperature is higher above, and it cools more strongly than the surrounding flatlands [5]. A meteorological explanation from Gifu Prefecture also states that because cold air is heavy and tends to pool at the bottom, cold air easily collects in basins [6].
This is somewhat similar to how water poured on a mountain slope collects in low places. However, air is invisible and mixes more easily than water, so it does not move as clearly in reality.
Even if cold air collects, wouldn’t it scatter if the wind blows?
3. Why does wind scatter the cold air and stop fog from forming?
If the wind is strong, the air near the ground and the air above are mixed, making it difficult for the temperature to drop significantly [3]. The JMA explanation also states that the mass of cold air near the ground is blown away if there is wind [4].
In an example from Tohoku University’s explanation, when the average nighttime wind speed near the mountain top was less than 2 m/s (about 4.5 mph), the depth of the cold air lake was about 70 m (about 230 feet), close to the depth of the basin. If the wind speed exceeded 5 m/s (about 11 mph), no cold air lake formed [5]. 5 m/s converts to 18 km/h (about 11 mph) in hourly terms, which is about the speed of riding a bicycle slowly.
The figures of 70 m and wind speeds of 2 m/s and 5 m/s are specific to the conditions of the example given in that explanation. They are not fixed values that apply to every basin.
Therefore, radiation fog is often seen in basins on clear, calm nights from night to dawn, and it is said to be particularly common in autumn [3]. If cold air collects, does that alone create fog? Observations report something slightly unexpected.
4. Why does fog not first appear in the coldest place in a basin?
In observations conducted in the Ena region of Gifu Prefecture (reported by the Small Climate Group Research Association in 1994), typical basin fog appeared mostly on days when the previous day was clear and the morning of the day was also clear. It appeared at the bottom of the basin, centered along the Kiso River [7].
The surprising part is how it formed. The report states that fog did not occur in the middle of the basin floor, where it was coldest. Instead, it formed along the river, where temperatures were relatively higher, and then spread to the surroundings [7].
In Miyoshi’s "Sea of Fog," it was also introduced that the source of the fog was water vapor rising from the river surface [9]. If you place these two materials side by side, you might think that because the river supplies water vapor to the air, fog begins to form along the river. However, this is a view that connects two sources; it is not the reason stated in the Ena report itself.
The report also divides the life cycle of fog into four stages: formation, change, development, and dissipation [7]. If cooling alone is not the reason, what else is added?
5. Why does air need to mix, and not just cool down, for fog to form?
A 2022 report by the same research group (in *Tenki*, Vol. 69, No. 8) observed a wide basin along the Kiso River and a narrow basin downstream of the Agui River Dam. The abstract listed on J-GLOBAL summarizes that the process of mixing air of different temperatures plays a major role in the formation of basin fog, in addition to radiative cooling [8].
The previous explanation of "cooling to form fog" has been joined by "mixing." For example, you might imagine mixing the air near a stove with the air near a window. However, the movement of air in the mountains is much more complex than in a room.
If you want to know the fine details of what kind of air mixes where and how much, it is worth looking at the original text.
How long does the fog created this way stay in the basin?
6. Why does morning fog disappear when the sun rises and temperatures go up?
According to a disaster prevention explanation from Kobe City, radiation fog is thickest around sunrise and disappears when the sun comes out and temperatures rise [2].
The Ena observations divide the life of basin fog into four stages: formation, change, development, and dissipation [7]. The specific times and temperatures for each stage are not covered here.
Connecting the flow so far, it becomes a story of one night in a basin: on clear nights with weak winds, cold air from the slopes collects at the bottom of the basin, fog is born and spreads along the river, and it disappears in the morning sunlight.
Finally, let’s think about how to verify this flow with your own eyes.
7. How can you find the whitest places by looking at rivers and low land from your window on a foggy morning?
On a morning with fog, look outside from your home window and try to find the whitest places. Are places near the river or low-lying areas turning white? It is also an interesting method to check the positions of rivers and mountains on a map and compare them with the white places.
On foggy mornings, visibility is poor, so do not approach rivers or waterside areas. Please observe from a safe place.
Even on mornings without fog, the air temperature before dawn on clear, calm days may be lower than during the day. Comparing the lowest temperature in the weather forecast with the actual coldness of the morning air is also a clue.
The Ena observations are published in the Japanese academic journal *Tenki* [7][8]. Some issues contained plates and figures. If you want to read them, you can try searching for them in a library.
Safety Note: When observing, do not approach rivers or waterside areas. Always observe from a safe place.