Why Do Zao's "Snow Monsters" Grow Toward the Wind?
― Cloud Droplets Below Freezing Turn to Ice the Moment They Hit
The "snow monsters" standing in rows on winter mountains are not just piled-up snow. They are made of cloud droplets that drifted along unfrozen below freezing, then turned to ice the instant they hit a tree, and slowly grew toward the direction the wind comes from.
In winter at Zao or Hakkoda, a herd of white monsters appears outside the ropeway window. Many people have seen them in photos or on TV.
Up close, you see jagged ice like shrimp tails lined up on one side of each branch only. The other side has almost nothing on it.
Snow falls from above, so it should pile up on top. Yet this ice reaches out sideways, and only in one fixed direction. What is going on?
Just two reasons
Droplets in the clouds and fog that wrap a mountain can drift unfrozen even below minus 10°C. The moment they hit something, they freeze onto it.
Droplets are carried in by the wind, so they strike only the side facing it. Thin, pointed spots catch them best, so the ice keeps growing upwind.
So a snow monster is not something that "piled up". It is made of water droplets the wind carries in, which the tree catches head-on and "glues" in place. Let's go through it step by step.
Why don't cloud droplets freeze below zero?
We learn that water freezes at 0°C. But that holds only when the water contains something that can "start" the ice. Cloud droplets are tiny, about 0.01 to 0.02 mm across.
The smaller a droplet is, the less likely it is to hold a speck of dust that can start ice. So many droplets stay liquid even at minus 10°C or minus 20°C. This is called supercooling.
A supercooled droplet freezes all at once at the slightest jolt. Slamming into a leaf or twig is exactly that kind of jolt. The droplet freezes on the spot, and the next one freezes on top of it.
Tiny gaps are left between the droplets. That is why the ice is not clear but white, full of trapped air. The northwest winter monsoon from the Sea of Japan carries plenty of water vapour from the sea, so these droplets keep blowing against the mountaintops.
Look at Figure 1. Ice forms only on the upwind side of the branch and grows toward the wind. The stronger the wind, the more droplets arrive each hour, so the ice grows faster.
Why does the ice keep growing upwind?
Droplets riding the wind try to slip past an object by turning with the air just before it. But droplets have weight, so they can't turn sharply. Only the ones that fail to turn hit the surface.
What matters here is how thick the object is. In front of a thick trunk, the air starts curving gently from far away. Small droplets can follow that flow, so most of them swerve around the trunk.
In front of a thin leaf or twig, the air bends sharply right at the surface. Droplets can't keep up and fly straight in. So the thinner the object, the better it catches droplets (Figure 2).
This shows how snow monsters keep growing upwind. The tip of the ice is itself a "thin, pointed bump". So the next droplets hit the tip first, and the ice juts out further into the wind.
Falling snow then catches on the shrimp tails that form on each branch. Ice and snow pile up in turn, until the tree becomes a white mass several metres tall. That is what a "snow monster" is.
Big snow monsters need three things together: strong monsoon winds carrying supercooled droplets, a moderate amount of snow, and conifers with fine needles that hold snow well (such as Maries' fir). In Japan they are said to be limited to a few mountains in the Tohoku region, such as Zao, Hakkoda and Hachimantai.
The direction the shrimp tails point is the direction the wind blew from most that winter. Winter hikers are said to use it as a clue to direction when fog hides the view.
Summary
Snow monsters form when cloud droplets that drift unfrozen below freezing turn to ice the moment they hit a tree. Droplets arrive on the wind, so they strike only the windward side. Thin, pointed spots catch them best, so the tip of the ice gathers still more droplets, and the ice keeps growing into the wind.
A snow monster is not piled-up snow.
It is the mark left where a tree met the wind's "water that wants to freeze" head-on.
For how water can stay liquid below 0°C, see "We learn that water freezes at 0°C, so why is there water that won't freeze even at minus 20°C?" For another winter case of ice growing from the ground, see "Why do frost columns sprout up out of the soil?"
- On a cold, foggy morning, look at wire fences, thin twigs and grass near power lines. See whether any white ice grains have formed on them.
- If you find ice, check which side has more. Use a compass app on your phone to note which direction that side faces.
- In the same place, compare a thin wire with a thick post. See whether the thinner one carries more ice.
Even on low ground, small amounts of rime can appear on a bitterly cold, foggy morning. Watch your step so you don't slip on frozen paths.
For those who want more ― terms, formulas and links to textbooksEach part is labelled by level, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school Earth science or physics
- High school+Advanced high school material, or textbook sidebars
- UniversityNot taught in high school; university courses (cloud physics, fluid dynamics)
- ResearchNot yet settled even at university; researchers are still investigating
Middle schoolTerms: this phenomenon has names
- Supercooling: a liquid stays liquid below 0°C because nothing is there to start freezing.
- Rime (muhyo): ice formed when fog or cloud droplets hit trees and freeze on them below freezing. Snow monsters are the classic example.
- Icing (chakuhyo): the general name for supercooled droplets freezing onto trees, power lines, buildings and other surfaces.
Middle schoolHigh schoolCheck it with a formula: how much does rime grow in an hour?
We work out how much water the wind carries onto a branch each second, and how thick it would be as ice. All values are rough; real growth varies a lot with the weather.
| In symbols | (growth rate) = E × w × v ÷ ρ |
| In words | Thickness grown per second = catch rate × water per cubic metre of air × wind speed ÷ density of rime |
| Where it comes from | Conservation of mass: of the water the wind carries to a surface (water amount × wind speed), the caught part all turns to ice. Dividing mass by density gives volume, that is, thickness |
| Symbol | Meaning and unit |
| w (cloud water) | 0.3 g per cubic metre of air (roughly 0.1 to 0.5 g in mountain fog) |
| v (wind speed) | 10 m per second |
| E (catch rate) | 0.5 (a guide for thin leaves and twigs; much smaller for thick trunks) |
| ρ (rime density) | 400000 g per cubic metre (less than half that of ordinary ice because it holds air) |
| Water carried to 1 square metre per second (g) | 0.3 × 10 ＝ 3 |
| Part that is caught (g) | 3 × 0.5 ＝ 1.5 |
| Built up in 1 hour (3600 s) (g) | 1.5 × 3600 ＝ 5400 |
| As ice thickness (m) | 5400 ÷ 400000 ＝ 0.0135 |
| In centimetres (cm) | 0.0135 × 100 ＝ 1.35 |
| Growth in 1 day (24 hours) (cm) | 1.35 × 24 ＝ 32.4 |
That is about 1.4 cm in an hour and over 30 cm in a day. In a single night, ice longer than the width of your palm grows upwind. As the formula shows, the growth rate is proportional to wind speed. The slider in Figure 1 uses this formula as it is.
High schoolHigh school+Why don't droplets freeze, and why is the ice white?
High schoolFor water to freeze, a tiny seed of ice must form. The smaller the droplet, the less likely it holds the dust (ice nuclei) that seeds it, so it stays supercooled. The heat released on freezing (latent heat of fusion) escapes quickly into the cold air, so a droplet that hits freezes in an instant.
High school+When droplets freeze one by one at once, they stack up keeping their shapes, and air is trapped in the gaps. This is white, light rime. When the air is near 0°C and the droplets are large, they spread out before they freeze, giving clear, heavy ice. This kind can cause damage by its weight when it builds up on power lines and the like.
UniversityCatch efficiency and the Stokes number
Whether a droplet can bend with the flow is expressed by the Stokes number in fluid dynamics. It is the ratio of "the time a droplet needs to change direction" to "the time the flow takes to bend in front of an obstacle". If the Stokes number is well above 1, the droplet goes straight and hits; if it is small, the droplet rides the flow and swerves.
The obstacle's width is in the denominator, so the thinner it is, the larger the Stokes number. Estimating with a 0.02 mm droplet and a 10 m/s wind, it is far above 1 for a 1 mm needle and about 0.05 for a 30 cm trunk. In cloud physics this catch rate is called the collection efficiency (collision efficiency), and the same idea is used to predict icing on aircraft and power lines.
📖 Derivations and further reading: Wikipedia, "Stokes number" (Japanese) / Wikipedia, "Supercooling" (Japanese)
ResearchWhat is still not clear
- How far will snow monsters shrink? At Zao, reports say the area where snow monsters can be seen has narrowed since several decades ago, and the height at which they appear has risen. Observations continue to study the link to warming.
- The trees themselves are disappearing. In recent years, many of the Maries' firs that form the base at Zao are said to have died from insect damage. How to restore the forest that supports the snow monsters is a challenge.
- How snow and rime combine. In what proportions, and in what way, do falling snow and blowing supercooled droplets pile up to make a "monster" shape? The details of how the shape is decided are not yet fully explained.
So this article, too, is "an explanation of what is known so far". How droplets freeze is well established, but how the snow-monster scenery will change is still being watched.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "Changes of state", "Weather (monsoons)" | Water turning to ice; the northwest winter monsoon |
| High school | Earth science: "Structure of the atmosphere, how clouds form" | Supercooled cloud droplets, ice nuclei |
| High school+ | Physics: "Heat (latent heat of fusion)" | White rime versus clear ice |
| University | Fluid dynamics, cloud physics | Stokes number, collection efficiency, icing |
| Research | Climate change and mountain ecosystems | Shifts in snow monster range, forest restoration |
| ― | Links to daily life | Winter mountain scenery, wind direction clues, icing on power lines |
- Wikipedia (Japanese), "Muhyō (霧氷)" (rime, including snow monsters)
- Yoshimitsu Ogura (小倉義光), Ippan Kishōgaku (一般気象学, "General Meteorology"), University of Tokyo Press (東京大学出版会)
- H. R. Pruppacher, J. D. Klett, Microphysics of Clouds and Precipitation, Springer
- Japan Meteorological Agency (気象庁), "Yohō Yōgo" (予報用語, "Forecast Terminology"), entries on rime and icing
- Wikipedia (Japanese), "Stokes number (ストークス数)"
*This article is a general-audience science explainer. The figures given are rough estimates to help you understand the mechanism. When heading into the winter mountains, follow local weather information and facility guidance.