Where Does Diving Visibility Surge After Japan's Rainy Season? Regional Data Analysis
2026-03-11
Post-rainy season visibility spikes most in Okinawa and Sea of Japan. Kerama jumps +1.7m in July; Echizen improves +1.7m in August. Pacific coast sees smaller gains.
Key Takeaways
- The biggest post-rainy visibility surge occurs in Okinawa (Kerama +1.7m in July) and Sea of Japan (Echizen +1.7m in August)
- Pacific coast sites (Izu, Kushimoto) see minimal improvement from the rainy season ending (+0.7m or less)
- Best post-rainy diving: July in Okinawa or August in the Sea of Japan (Echizen)
Japan's Rainy Season and Diving Visibility
Japan's rainy season (梅雨, tsuyu) moves northward: Okinawa exits first (late June), followed by Kyushu/Shikoku (early July), then Honshu (mid-to-late July). During the rainy season, heavy rainfall washes sediment and nutrients into coastal waters. When it ends, rainfall drops sharply and stable summer high-pressure sets in. This should improve visibility — but the effect varies dramatically by region.
We analyzed 46,000+ dive observations across three periods: pre-/during-rainy (May–June), rainy season exit (July), and midsummer (Aug–Sep).
Monthly Visibility: May to September
| Site | Region | May | Jun | Jul↑ | Aug | Sep |
|---|---|---|---|---|---|---|
| Kerama | Okinawa (rainy ends late June) | 19.4 | 19.7 | 21.3 | 20.9 | 19.8 |
| Yonaguni | Okinawa (rainy ends late June) | 24.2 | 24.4 | 25.6 | 26.5 | 27.3 |
| Ishigaki | Okinawa (rainy ends late June) | 20.1 | 20.1 | 20.9 | 21.0 | 21.4 |
| Tajiri | Sea of Japan (rainy ends mid-July) | 8.7 | 9.7 | 11.1 | 12.4 | 9.4 |
| Echizen | Sea of Japan (rainy ends mid-July) | 7.7 | 8.3 | 8.6 | 10.3 | 10.3 |
| Mikomoto | Pacific (Izu) | 9.7 | 12.3 | 12.4 | 12.7 | 13.2 |
| IOP (Izu) | Pacific (Izu) | 10.6 | 11.3 | 11.6 | 12.3 | 12.7 |
| Kushimoto | Pacific (Kii) | 10.8 | 12.2 | 11.5 | 12.7 | 11.5 |
The Rainy Season Exit Effect by Site
| Site | Pre-rainy exit (May–Jun) | After exit (Jul) | Δ Visibility | ||
|---|---|---|---|---|---|
| Vis (m) | Rain (mm/day) | Vis (m) | Rain (mm/day) | Δ (m) | |
| Kerama | 19.6 | 8.9 | 21.3 | 3.1 | +1.7m |
| Yonaguni | 24.3 | 7.4 | 25.6 | 2.9 | +1.3m |
| Tajiri | 9.2 | 5.4 | 11.1 | 6.2 | +1.9m |
| Echizen | 8.0 | 5.7 | 8.6 | 7.4 | +1.7m |
| IOP (Izu) | 10.9 | 9.3 | 11.6 | 10.2 | +0.7m |
| Kushimoto | 11.5 | 8.6 | 11.5 | 8.2 | 0.0m |
Okinawa (Kerama, Yonaguni, Ishigaki): Clearest Post-Rainy Surge
Okinawa's rainy season ends around June 20–25. July data captures the immediate post-rainy effect.
- Kerama: 19.7m (June) → 21.3m (July, +1.6m). Rainfall drops from 8.9 to 3.1 mm/day
- Yonaguni: 24.4m → 25.6m (+1.2m), then keeps rising to 27.3m in September
- Ishigaki: modest +0.8m in July, gradual improvement through September (21.4m)
The mechanism: during Okinawa's rainy season, heavy rains deliver sediment to coastal coral reef zones. After the rainy season, rainfall plummets, trade winds stabilize, and visibility rebounds.
Sea of Japan (Echizen, Tajiri): Biggest Improvement in August
The Sea of Japan's best visibility comes after Japan's mainland rainy season ends (mid-July). When the Pacific High locks in, the Japan Sea experiences extended calm, sunny conditions.
- Tajiri (Tottori): 8.7m (May) → 11.1m (July, +2.4m from May)
- Echizen (Fukui): 8.3m (June) → 10.3m (Aug, +2.0m). Peak in August
Pacific Coast (Izu, Kii): Muted Effect
At IOP and Mikomoto, the bigger event is spring turbidity ending (May→June: +2.6m at Mikomoto). The rainy season exit in July adds only +0.7m at IOP. Kushimoto actually dips slightly in July (12.2m → 11.5m), possibly due to seasonal Kuroshio meander or coastal current shifts.
Where to Dive After Japan's Rainy Season?
| Area | Rainy season ends | Vis gain | Best window |
|---|---|---|---|
| Okinawa (Kerama, Ishigaki) | Late June | +1.7m | July – September |
| Sea of Japan (Echizen) | Mid-July | +1.7m | August – September |
| Izu Peninsula (IOP) | Late July | +0.7m | September – winter |
| Kushimoto (Kii) | Mid-July | ±0m | August, winter (Kuroshio-dependent) |
Summary
- The biggest post-rainy-season visibility surge occurs in Okinawa (July, +1.7m) and Sea of Japan (August, +1.7m)
- Pacific-coast Izu and Kii see minimal improvement from rainy season end alone
- Best post-rainy-season diving: July in Okinawa or August in Echizen
- Izu's true clarity peak is winter (December–February), driven by thermal stratification breakdown rather than rainfall
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