Mount Zao's Snow Monsters Shrink as Warmer Winters and Insects Damage Their Forest

A century of photographs reveals a shrinking winter spectacle

Mount Zao's famous snow monsters once stretched five to six meters across. Since 2019, many have measured half a meter or less. An analysis presented in August 2025, using photographs dating to 1933, has put numbers to a change that visitors and residents have watched for years on the mountain's upper slopes.

The research team, led by Fumitaka Yanagisawa, professor emeritus at Yamagata University and head of a longstanding juhyo research group, compared photographs taken from identical viewpoints around the summit. Researchers graded the formations on a scale with six levels of thickness and measured the width of individual figures.

The photographs show a long decline. The broad figures of the 1930s often measured two to three meters across by the postwar decades. In many places since 2019, they have become narrow columns, carrying little of the thick ice that once filled the skyline.

Yanagisawa said the findings give a numerical basis to the changes that regular visitors have noticed:

There is now quantitative proof of just how much the ice monsters have shrunk.

Two pressures are acting together: warmer winters are changing the conditions that build the ice, while insects have damaged or killed the fir trees underneath it. Yamagata officials estimate that about 23,000 firs have died on the prefecture's side of the mountain, roughly one fifth of its stands.

The losses matter beyond the appearance of a winter landscape. Hotels, restaurants, inns and ski businesses in Yamagata plan their seasons around the weeks when the formations are at their best. Tens of thousands of visitors take ropeway rides toward the crest each winter to see them.

How wind, cloud and fir trees build juhyo

Juhyo, the Japanese name for the snow monsters, form when tiny liquid droplets in clouds freeze onto trees. The droplets are supercooled, meaning they remain liquid even below 0 degrees Celsius. When they strike a branch or needle, they freeze immediately, building a coating called rime ice.

Mount Zao provides an unusually precise combination of conditions. Persistent winds from the west can reach 26 meters per second. Surface air near the summit must be close to freezing, within a range described as roughly minus 6.3 to minus 0.1 degrees Celsius. Clouds must also contain an unusually large amount of liquid water.

Low cloud banks bring those droplets on winds arriving from the Sea of Japan. Repeated gusts add ice to the sides of branches facing the wind. The growing ridges are known as shrimp tails. When storms continue through several weeks, the coatings thicken until neighboring trees merge into white figures several meters across.

The living framework is Aomori todomatsu, also called Maries' fir, or Abies mariesii. This evergreen grows in dense stands on Zao's high slopes. Its needles and branches give droplets many surfaces to strike. Trees that shed their leaves in winter do not collect the same mass of rime.

Zao's position and shape help concentrate winds from the west and keep them arriving from a steady direction for long periods. Combined with clouds rich in moisture and consistent cold, the process works like natural three dimensional printing: each gust adds another layer to the growing form.

The result depends on both weather and a healthy tree. Strong winds and suitable clouds can supply the ice, but the fir's foliage provides the framework that allows it to grow into a broad figure.

Why the right conditions are becoming harder to find

Northern Japan has the necessary combination of vegetation, cloud and wind more often than anywhere else on Earth. Rime can form on mountain ridges elsewhere, but dense ranks of thick, merged juhyo require conditions that repeat for weeks.

Japan's Tohoku region, facing the Sea of Japan, is one of the few places where that happens. Historically, snow monsters appeared across a wider area, from Hokkaido in the north to Ishikawa Prefecture on the Sea of Japan coast.

Reliable displays are now concentrated on mountains including Zao, the Hakkoda range in Aomori, Mount Hachimantai in Iwate, Mount Moriyoshizan in Akita and Mount Azumayama, which spans Fukushima and Yamagata. The geographical range has contracted in recent decades.

Warmer winters push the altitude with suitable temperatures upward. In some northern areas, conifer forests no longer occur at the heights where the necessary cold conditions are found. Cold air and suitable trees must occupy the same place for large formations to develop.

Zao's changing appearance reflects that narrow balance. Visitors increasingly encounter thinner outlines, sparse groups and bare trunks where packed ranks once stood. The photograph study connects this visible decline with both warming and the loss of foliage to insects.

Two waves of insects stripped and killed the host trees

Tortrix moth caterpillars began stripping needles from Maries' firs across the Zao range around 2013. Starting around 2015, bark beetles moved into trees already weakened by that damage.

Bark beetles favor stressed hosts. They tunnel beneath the bark and can kill the tree. For juhyo, the damage has a direct consequence: a trunk without needles and fine branches offers far less surface for droplets to freeze onto.

Yamagata officials put the death toll at about 23,000 firs, approximately one fifth of the stands on the prefectural side. Even when strong storms return, bare trunks cannot collect the layered masses of ice that make the largest snow monsters.

Recent winters also show how uneven the damage is across the mountain. During one particularly cold and snowy season, large formations returned at elevations of about 1,500 to 1,600 meters, where trees remained healthy.

Above 1,600 meters, extensive areas contained dead or broken trees and skeletal trunks. Healthy foliage at a slightly lower elevation could still support impressive formations, while damaged forest higher up lacked the same structure.

Forest damage and weather therefore affect the same spectacle in different ways. Cold, windy periods create opportunities for rime to grow. The condition of each stand determines how much ice its trees can collect during those periods.

Warmer winters shorten the window for ice growth

Yamagata City's average temperature from December through March has risen by about 2 degrees Celsius over the past 120 years. Across Japan's alpine zones, warming has outpaced the global average since the 1980s.

The Japan Meteorological Agency reports average warming nationally of about 1.3 degrees Celsius over the past century. Hokkaido's increase is about 1.6 degrees Celsius over 100 years. These figures describe different locations and periods, but each shows a warming trend.

Japan's projections point to warming of around 4.5 degrees Celsius by late this century, compared with the late twentieth century baseline, if effective countermeasures do not slow the trend. In winters that are warmer than usual later this century, snow monsters may not form at all.

For juhyo, even a small temperature shift matters because the process needs liquid droplets inside cold clouds. Warmer air changes the vertical structure of winter storms and can reduce the time when droplets occur within the right temperature range.

A change of a degree or two can determine whether moisture freezes onto a tree or arrives as wet snow that slides off its branches. Earlier spells of springlike weather can also melt the formations sooner, shortening the number of days when visitors can see them.

Insects add another source of pressure. Warm summers and mild winters can improve bark beetle survival and allow multiple generations within a season. Fir trees already stripped of needles capture less ice and form smaller shrimp tail ridges.

Northern Japan still has cold winters and major storms. Some seasons bring excellent stretches of wind and freezing weather. Across the longer photograph record, however, Zao's formations have become thinner, peak viewing days have declined, and the lowest altitude suitable for their formation has moved upward.

Winter businesses face a less predictable attraction

Zao's snow monsters support a winter economy built around ropeway trips, skiing, accommodation and evening excursions. Hotels and inns organize events around the peak viewing weeks. Skiers and snowboarders pass among the formations, while guided tours illuminate the figures at night when conditions allow.

Images shared on social media bring the landscape to audiences worldwide and attract new visitors to Yamagata. The appeal depends on a natural display that can vary sharply from one week to the next.

Even in an average January or February, the best viewing opportunities may amount to only a handful of days each month, when clouds clear and winds ease. Warmer weather and damaged trees make the outcome less predictable for travelers and businesses alike.

A visitor arriving after a week of thaw, or during a season affected by insect damage, may find thin trees with patchy ice rather than towering figures. That difference between the expected scene and the actual one matters to small businesses that depend on returning guests and recommendations.

The ropeway posts information about visibility and the growth of the formations. Tourism bureaus encourage flexible trips that include hot springs, local food and museums. Juhyo remain the main attraction, while other activities give travelers options when mountain conditions are poor.

A restoration effort measured in decades

Yamagata Prefecture launched the Juhyo Revival Conference in March 2023. The permanent council brings together scientists, public officials, businesses and residents to restore fir stands where possible, protect healthy areas and develop knowledge about caring for the mountain forest.

The local forest office has transplanted more than 190 naturally regenerated Maries' fir saplings since 2019. The young trees were moved from lower slopes to the summit zone near the ropeway station.

Maries' firs take 50 to 70 years to reach maturity. Rebuilding the living framework for large snow monsters therefore extends far beyond a single tourism season or a short planting campaign.

Staff monitor how many saplings survive, test different planting densities and try protection against rodents. Field mice have damaged some planting plots, adding a practical problem to the already slow task of establishing young trees at high elevation.

Methods used in other conifer forests also inform work at Zao. These include cutting dead trees for sanitation, removing material heavily affected by insects and monitoring tortrix moths and bark beetles. Lessons from those approaches are being applied to the mountain's particular conditions.

Restoration aims to rebuild forest structure so that surviving and newly established trees can collect ice when suitable weather returns. Replanting cannot reverse a warm winter. Combined with broader action to limit warming over the longer term, it can help preserve the possibility of future displays.

Students are growing trees for future winters

Students at Murayama Technical High School, about 20 kilometers northwest of Zao, have been planting Maries' fir seedlings and studying efficient ways to propagate them since 2022.

The students are enrolled in a forestry and environmental science course. With support from the Yamagata Forest Office, they collect young trees on the mountain, experiment with cuttings and test nursery methods for producing healthy planting stock.

Their work addresses a resource the snow monsters need throughout the year: living firs at suitable elevations. Winter ice lasts for weeks, but the trees beneath it must survive through many seasons before they can support large formations.

Zao's response has developed over more than a decade, from the first major insect damage to research and restoration. The main dates show how the pressures and the response have unfolded:

• Around 2013: Tortrix moth caterpillars began stripping needles from Maries' firs across the Zao range.
• Around 2015: Bark beetles began moving into the weakened trees.
• Since 2019: Many photographed formations have measured half a meter or less across, and the local forest office has transplanted more than 190 saplings.
• Since 2022: Murayama Technical High School students have planted seedlings and tested propagation methods.
• March 2023: Yamagata Prefecture established the Juhyo Revival Conference.
• August 2025: Yanagisawa's research team presented its analysis of photographs dating to 1933.

Visitors can protect the trees beneath the ice

Travelers can help protect Zao's remaining firs by staying on marked routes and avoiding contact with trunks and branches. The ice may resemble a sculpture, but its support is a living tree that remains vulnerable after the winter coating melts.

Checking ropeway and weather updates before going up the mountain helps visitors plan around poor visibility and can reduce crowding on unsuitable days. Choosing local operators supports the communities caring for the landscape.

Community forestry groups and volunteer planting activities offer other ways to support restoration. No date has been given for a full recovery of the damaged stands. The 50 to 70 years needed for firs to mature makes continued care a task spanning generations.

Zao still produces large snow monsters when healthy trees meet sustained cold, moisture and wind. Preserving that combination means caring for the forest while addressing the warming that is narrowing its winter opportunities.

Key Points

• A photograph study presented in August 2025 traced a decline from formations five to six meters across in the 1930s to half a meter or less in many places since 2019.
• Juhyo grow when supercooled cloud droplets freeze onto Maries' firs during sustained cold and strong winds from the west.
• Yamagata officials estimate that insects have killed about 23,000 firs, roughly one fifth of the stands on the prefectural side of Zao.
• Healthy trees at 1,500 to 1,600 meters have still produced large formations during cold, snowy conditions, while higher areas have suffered extensive tree losses.
• Yamagata City's average temperature from December through March has risen about 2 degrees Celsius over 120 years.
• Less predictable displays affect winter businesses and the tens of thousands of visitors who ride the ropeway each season.
• Restoration includes a council established in March 2023, more than 190 transplanted saplings since 2019 and student propagation work begun in 2022.
• Maries' firs take 50 to 70 years to mature, making forest recovery a commitment across generations.