As winter approaches, frost appears on the summit of
Mount Tsurugi ( However, seeing this frost is not something you can just rely on luck.
Temperature, humidity, wind, weather...all of these factors must be perfectly aligned in order for it to appear.
This time, I used Windy.com and ChatGPT to predict the best days for frost formation, and my prediction actually came true .
In this article,
"How did we determine the days when frost would appear?"
, "What should you look at in Windy to know when there is a chance of frost?", and
" What was the climbing like on the day?"
Finally, the article touches on a topic that readers may be curious about: "Were there any bears?"
- What is frost?
- Predicting the possibility of rime ice with Windy and ChatGPT
- Departing at 2:30am. A complete night hike.
- And then──the frost really "existed"
- Video from the day (DJI Osmo Action5 Pro, audio intact)
- By the way, were there any bears?
- On the way home, I realized "Why there are so few bears in Shikoku"
- Summary: Rime can be "predicted"
What is frost?
Hoarfrost is a phenomenon in which supercooled water droplets in the air (liquid mist even at temperatures below 0°C) freeze the moment they come into contact with tree branches and other surfaces .
The shining white ice, like a coating of powdered sugar, is a beautiful natural phenomenon unique to the mountains in winter.
The conditions for its appearance are surprisingly strict.
- Temperature: below 0°C (ideally -2°C to -5°C)
- High humidity: Fog is occurring
- Weak wind: Ice does not grow if the wind is strong.
If these three conditions are not met, frost will not appear.
Therefore, you won't be able to see it just by climbing on a "cold day."
Predicting the possibility of rime ice with Windy and ChatGPT
To ensure a successful frost photography this time, I checked the weather conditions at the summit of Mt. Tsurugi (altitude set to 2,000m) in detail on Windy.com the day before my climb.
Conditions as of November 20, 2025 (from Windy)
- Temperature: Around -2°C overnight
- Wind speed: 1-5 m/s (a gentle wind ideal for frost growth)
- After 7:00 AM → Clear sky mark (the frost glitters, making it ideal for photography)
After seeing the screenshot, ChatGPT responded with this:
"These conditions are perfectly aligned for rime to form," he said.
"If you climb the mountain early on the morning of the 20th, there's a very good chance you'll see rime."
Encouraged by this "prediction," we decided to set the date for the climb as November 20th.
Departing at 2:30am. A complete night hike.
We set off at 2:30 a.m. on the day,
took a break at the Lawson Sadamitsu store, and arrived at the trailhead just after 5 a.m.
There were a few cars parked there, but I was the only one out for a night hike.
The trail was pitch black, and I had to rely on my headlamp.
I dug out an action camera light I'd bought on Amazon a few years ago and had left lying around in storage, and used it to light the way as I climbed.
Along the way, we heard the cries of what seemed to be deer several times.
We were so wary that even the slightest swaying of grass would make us turn around, but we continued to gain altitude in silence.
And then──the frost really "existed"
As the Nishijima Lift Station came into view, the surrounding area gradually became brighter.
Suddenly, I looked up towards the summit...
Only the trees near the summit of Mount Tsurugi were frozen white.
At that moment, I was convinced, "Yes, I did. This is frost."
Take a photo immediately and send it to ChatGPT,
"This is definitely frost!!"
When I heard this comment, I realized that Windy and GPT's weather analysis had been spot on.
Video from the day (DJI Osmo Action5 Pro, audio intact)
The beauty of the frost is hard to convey in still images, so please take a look at the video.
The sound of the wind, footsteps, and low temperature - the atmosphere of the day are all captured in the video.
By the way, were there any bears?
Conclusion: I didn't encounter it this time either.
The route I walked was
- Tsurugiyama trailhead → Nishijima Station
- Nishijima Station → Beginner Course
- To the observation deck overlooking Jirogui
- Continue walking along the ridgeline for a bit
We didn't encounter any other hikers on this journey, and we certainly didn't encounter any bears.
On the way home, I realized "Why there are so few bears in Shikoku"
On the way back, we took National Route 438 → National Route 492 → National Route 193 back to Kagawa.
In the village visible from the train window, all the persimmon trees remained intact.
In Honshu, there are constant reports of Asiatic black bears descending into human settlements due to food shortages and destroying fruit, but there is almost no evidence of this in Shikoku.
The reason is probably—
- The bear population in Shikoku is extremely low
- There may still be plenty of food left in the mountains
This is purely my personal observation, but I get the impression that the relationship between the mountains and bears in Shikoku is completely different from that in Honshu.
Summary: Rime can be "predicted"
During this climb, I realized that frost can be predicted to a certain extent by interpreting meteorological data, rather than relying on experience or intuition
Windy's temperature, humidity, and wind
analysis by ChatGPT
and the day's actions
By combining these factors, we were able to successfully find a day when frost was most likely to form.
I hope this will be helpful for those who are aiming to photograph frost next year and beyond.
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