Zao Onsen and Frost-Covered Trees — Ice Monsters from the Yamagata Side

Zao Onsen in Yamagata Prefecture is one of Tohoku's foremost hot springs, boasting a history of 1,900 years since the waters first opened. The "juhyo" (frost-covered trees) that appear near the summit in winter are also called "snow monsters," because conifers clad in snow and ice take on monstrous shapes, and as a natural phenomenon rare even by world standards they draw many visitors. Unlike viewing juhyo from the Miyagi side, the Yamagata approach puts you in the hot spring village itself, making possible the incomparable combination of skiing through ice-tree forests by day and soaking in steaming open-air hot springs by night.

Highlights

The juhyo form when freezing clouds envelop the mountain's upper slopes, depositing rime ice on the fir trees (Abies mariesii) in successive layers. By January and February, individual trees may be unrecognizable beneath their ice carapaces. The gondola ride through the ice-tree forest is an experience available nowhere else in Japan at this scale.

Zao Onsen's outdoor baths (rotenburo) at the village center are legendary: strongly acidic (pH 1.25 to 1.6) sulfurous water close to milky white in open-air stone pools, steam rising against snow-covered slopes. The Dai-rotenburo accommodates many bathers at once in a communal outdoor setting that feels both ancient and extraordinary.

Ski terrain ranges from gentle nursery slopes to challenging mogul runs, with the juhyo zone traversable on designated pistes in late January and February when formations are at full size.

The History and Water of Zao Onsen

According to the official Zao Onsen website, the spring is said to have been opened 1,900 years ago, discovered by Kibi no Takayu, who accompanied Yamato Takeru no Mikoto on his eastern campaign. The water is a strongly acidic sulfur spring, with a source temperature of 45C to 66C and a pH of 1.25 to 1.6 - extremely strong acidity. The water, close to milky white, is said to make the skin smooth, and it has long been cherished as a "bath of beauty." The hot spring town spreads across a highland surrounded by mountains, and there is plenty to enjoy on a bath tour, from public bathhouses to the great open-air bath. However, because the acidity is strong, the official website lists as contraindications "people with sensitive skin or mucous membranes, especially those with photosensitivity," "dry skin in the elderly," and "acute illness." Those with delicate skin should avoid long soaks, and metal accessories may discolor, so remove them before bathing. The town flourished as a therapeutic bathing resort from long ago, and an atmospheric townscape that tells of that long history still remains.

Juhyo and the Ropeway

Juhyo are born when supercooled water droplets and snow carried by the cold seasonal wind attach to Maries' fir trees and grow. Ride the Zao Ropeway up to the summit and a field of frost-covered trees spreads as far as the eye can see. The nighttime "Juhyo Light-Up" is held for a limited period; in the 2025-2026 season it ran from December 27, 2025 to February 22, 2026. The ropeway may suspend operations in strong winds or bad weather, so check the day's operating status before you set out. It is also popular as a ski resort, and the experience of gliding between the frost-covered trees is exceptional.

The Great Open-Air Bath and Highland Nature

The Zao Onsen Dai-rotenburo, spread along a mountain stream, is a popular spot full of openness where you can savor these famous waters while surrounded by nature. However, it closes in winter (the guidance on Yamagata Prefecture's official tourism site gives roughly late November to late April, subject to change depending on snow conditions), so it cannot be used during the juhyo season. Because the water is free-flowing from the source there are no showers, and soap and shampoo cannot be used. The crater lake known as "Okama" is another landscape that represents Zao, but the Zao Echo Line that provides access is closed in winter (in 2026 it closes at 17:00 on November 4 and is due to reopen at 11:00 on April 23, 2027, on the section from the Sumikawa Snow Park entrance through the Miyagi-Yamagata prefectural border to Zao Bodaira Kogen). Just after it reopens you can see walls of snow as high as 7 m, while until mid-May it is closed at night (17:00 to 8:00). Note that Mount Zao is an active volcano, and the current eruption warning level is 1 (be aware that it is an active volcano).

Best Season and Access

According to the Zao Ropeway's explanation, the juhyo begin to ice up in November and December, grow in January, reach their most impressive size in February, and start to melt in March. The summit drops below minus 10 degrees, so prepare thoroughly against the cold. For access, it is about 40 minutes by bus from JR Yamagata Station. You can also reach it by car from the Yamagata Zao Interchange on the Yamagata Expressway. In winter, extra ski-season buses operate. Check juhyo condition reports before booking - formation quality depends on cold snaps and is not guaranteed.

Practical Information

• Peak and best season: According to the Zao Ropeway's explanation, the juhyo begin to ice up in November and December, grow in January, take on their largest and most impressive form in February, and start to melt in March.
• Event period / light-up: The nighttime "Juhyo Light-Up" is held for a limited period; in the 2025-2026 season it ran from December 27, 2025 to February 22, 2026. Check the Zao Ropeway official website for next season's dates.
• Fees: For the fees and hours of the Zao Onsen Dai-rotenburo, you are directed to check the facility's official website. For Zao Ropeway fares, refer to the fares page of the official website.
• Access: About 40 minutes by bus from JR Yamagata Station. By car, from Yamagata Zao IC on the Yamagata Expressway.
• Winter closures and road closures: The Zao Echo Line leading to Okama is closed in winter; in 2026 it closes at 17:00 on November 4 and is due to reopen at 11:00 on April 23, 2027 (section: Sumikawa Snow Park entrance through the Miyagi-Yamagata prefectural border to Zao Bodaira Kogen; Zao Town official information). Even after it reopens, it is closed at night (17:00 to 8:00) until mid-May. The Zao Onsen Dai-rotenburo also closes in winter, roughly late November to late April according to the guidance on Yamagata Prefecture's official tourism site (subject to change depending on snow conditions).
• Clothing and gear: For juhyo viewing the summit drops below minus 10 degrees, so prepare thoroughly against the cold. The ropeway's official guidance likewise urges visitors to "prepare thoroughly against the cold, as it drops below minus 10 degrees." Gloves, a knit hat and waterproof shoes are essential, and since the slopes in the hot spring town freeze over, snow boots or anti-slip attachments are reassuring.

*[Note on strongly acidic water] Zao Onsen is a strongly acidic sulfur spring; according to the official website the pH is 1.25 to 1.6 and the source temperature is 45C to 66C. The contraindications listed officially are "people with sensitive skin or mucous membranes, especially those with photosensitivity," "dry skin in the elderly" and "acute illness," and during pregnancy particular care is said to be needed in the early and late stages. Those with delicate skin should avoid long soaks and rinse off afterward as needed. Metal accessories and watches may discolor, so remove them before bathing. The water is hot, so watch out for feeling faint and drink fluids frequently. Because the Dai-rotenburo is free-flowing from the source there are no showers, and soap and shampoo cannot be used.*

*[Volcano] Mount Zao is an active volcano, and the current eruption warning level is 1 (be aware that it is an active volcano). If you enter the area around the assumed crater zone (the Umanose caldera), check the Japan Meteorological Agency's latest volcanic information.*

*[Suspension in strong wind] The Zao Ropeway may suspend operations in strong winds or bad weather. For juhyo viewing, check the day's operating status before setting out.*

(Sources: Zao Onsen official website "About Zao Onsen"; Zao Ropeway official website "Juhyo and Light-Up"; Yamagata Prefecture official tourism site "Travel to Yamagata," Zao Onsen Dai-rotenburo; Miyagi Prefecture tourism information site "Miyagi Marugoto Tanbo," Zao Echo Line winter closure; Japan Meteorological Agency "Volcanic Activity Status (Mount Zao)")