Last updated: August 1, 2025
Understanding the UV Index is crucial for residents of Seiyo, Japan, where the unique Cfa climate often leads to fluctuating levels of ultraviolet radiation. By staying informed about UV levels, you can effectively prevent sunburn and safeguard your skin health, ensuring that outdoor activities remain enjoyable and safe. With peak UV exposure occurring around noon—with values soaring to 12—awareness of these levels is essential for minimizing risks, especially during the warmer months when sunshine is abundant.
The UV Index throughout the day reveals a striking pattern of solar intensity. Starting at 07:00, the index is a manageable 2, but it quickly climbs as the sun rises higher. By 08:00, it's at 4, and by 09:00, it reaches 6, signaling a push towards midday sun. The peak intensity strikes at 12:00 with a blistering 12, making it essential to seek shade and wear sunscreen. From this zenith, the index gradually declines: 11 at 13:00, 10 at 14:00, and finally dipping to 7 by 15:00. As daylight wanes, the UV levels fall rapidly, resting at 2 by 17:00 and dropping to a safe 1 by 18:00 and 19:00. Enjoy the sun, but be mindful of those peak hours!
| Time | UVI | Category | Time to Burn |
|---|---|---|---|
| 07:00 | 1 | low | 45 min |
| 08:00 | 3 | moderate | 30 min |
| 09:00 | 6 | high | 25 min |
| 10:00 | 9 | very high | 15 min |
| 11:00 | 11 | extreme | 10 min |
| 12:00 | 12 | extreme | 10 min |
| 13:00 | 11 | extreme | 10 min |
| 14:00 | 9 | very high | 15 min |
| 15:00 | 7 | high | 25 min |
| 16:00 | 4 | moderate | 30 min |
| 17:00 | 2 | low | 45 min |
| 18:00 | 1 | low | 45 min |
| 19:00 | 0 | low | 45 min |
The UV Index shows significant seasonal changes, with the highest levels of ultraviolet radiation occurring from March to September, characterized by very high to extreme exposure categories. March and April see UV Index values ranging from 9 to 10, while May through August peak at 12, indicating an extreme risk of harm from unprotected sun exposure, with a notably short burn time of just 10 minutes. July and August are particularly high-risk months, necessitating careful sun protection measures. As the months progress toward October, there is a decrease to 8, moving back to very high levels, while November and December return to high and moderate levels, respectively. To ensure safety, visitors and residents are strongly advised to adopt protective measures, such as using sunscreen, wearing protective clothing, and seeking shade during peak sun hours, particularly during the high-risk months of May through September.
UV Risk Categories
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sapporo UVI | 2 | 3 | 6 | 7 | 9 | 9 | 9 | 10 | 8 | 5 | 3 | 2 |
| Yokohama UVI | 3 | 5 | 8 | 9 | 11 | 11 | 11 | 11 | 10 | 7 | 5 | 4 |
| Tokyo UVI | 3 | 5 | 8 | 9 | 11 | 11 | 11 | 11 | 10 | 7 | 5 | 4 |
| Nagoya UVI | 4 | 6 | 8 | 10 | 11 | 11 | 11 | 11 | 10 | 7 | 5 | 3 |
| Osaka UVI | 4 | 6 | 8 | 11 | 11 | 11 | 12 | 11 | 10 | 7 | 5 | 3 |
| Seiyo | 4 | 6 | 9 | 10 | 12 | 11 | 12 | 12 | 11 | 8 | 6 | 4 |