There are indoor onsen pools that offer a comfortable temperature range, usually between 38-42°C (100.4°F - 107.6°F). However, having an outdoor hot spring pool with temperatures as low as -3 to -4°C (26.6°F - 24.8°F) wouldn’t be practical or safe for bathers.
Outdoor natural hot springs typically have temperatures ranging from around 30-40°C (86°F - 104°F), and some may drop significantly at night due to cooling of the surrounding environment. Additionally, it would not be feasible or enjoyable to bathe in such cold water.
Indoor onsen baths, on the other hand, can maintain a consistent warm temperature throughout the year, providing a relaxing and rejuvenating bathing experience for visitors. Some high-end resorts or spas might even offer cryotherapy or hot-cold contrast baths using ice-cold temperatures specifically designed for specific health benefits or as part of their unique packages.
So while it’s not possible to have an outdoor onsen pool with temperatures below freezing, many indoor options exist that cater to one’s bathing preferences and needs.