Days When Visibility Exceeded 30m: Japan's All-Time Clearest Dives — 1,840 Records
2026-03-11
Analyzing 1,840 observations of 30m+ visibility. Yonaguni accounts for 81% of all records.
Analyzing 30m+ Visibility: 1,840 Records from 46,000 Dives
Every diver dreams of 30-meter visibility. Our database of 46,000+ real dive log observations across 24 sites in Japan contains 1,840 records of visibility at or above 30m (capped at realistic ≤55m to exclude anomalies). Which sites, which months, and how often?
Site Rankings: 30m+ Record Counts
The data shows extreme concentration at specific sites.
| Rank | Site | 30m+ Records | Total Obs. | Hit Rate |
|---|---|---|---|---|
| 1 | YonaguniOkinawa | 1500 | 4,826 | 31.1% |
| 2 | ItoChiba | 91 | 1,981 | 4.6% |
| 3 | YakushimaKagoshima | 65 | 157 | 41.4% |
| 4 | Amami OshimaKagoshima | 46 | 1,450 | 3.2% |
| 5 | KeramaOkinawa | 34 | 1,533 | 2.2% |
| 6 | IshigakiOkinawa | 32 | 1,473 | 2.2% |
| 7 | IOPShizuoka | 16 | 3,151 | 0.5% |
| 8 | FutoShizuoka | 15 | 3,498 | 0.4% |
| 9 | HachijojimaTokyo | 7 | 326 | 2.1% |
| 10 | KashiwajimaKochi | 5 | 1,139 | 0.4% |
Yonaguni: 30m+ Is Almost Normal
Yonaguni (Okinawa) accounts for 81.5% (1,500 records) of all 30m+ sightings. Its hit rate of 31.1% means one in three dives there exceeds 30m visibility.
Yonaguni: Monthly 30m+ Hit Rates
| Month | 30m+ Count | Total | Hit Rate | Monthly Avg. |
|---|---|---|---|---|
| Jan | 71 | 476 | 14.9% | 22.4m |
| Feb | 74 | 424 | 17.5% | 22.6m |
| Mar | 102 | 435 | 23.4% | 23.5m |
| Apr | 108 | 401 | 26.9% | 23.9m |
| May | 118 | 416 | 28.4% | 24.2m |
| Jun | 105 | 347 | 30.3% | 24.4m |
| Jul | 182 | 444 | 41% | 25.6m |
| Aug | 197 | 402 | 49% | 26.5m |
| Sep | 193 | 373 | 51.7% | 27.3m |
| Oct | 144 | 289 | 49.8% | 26.8m |
| Nov | 101 | 339 | 29.8% | 24.4m |
| Dec | 105 | 480 | 21.9% | 23.4m |
September peaks at 51.7% — more than one in two dives exceeds 30m. July through October all exceed 40%. Even in January, the rate holds at 14.9%.
Best Strategies for 30m+ Visibility
Surest bet: Yonaguni in September–October
Near-50% hit rates make September/October Yonaguni the statistically strongest combination. January–March (hammerhead season) still holds 15–20% — combining big animals with clear water.
Best on Honshu: Ito (Chiba) in December–January
91 records and a 4.6% hit rate make Ito the Honshu champion. Just 1.5 hours from Tokyo, winter months average 17–18m with occasional 40m days after calm spells.
Summer surprise: Yakushima in July–August
A 41.4% hit rate from limited data (157 obs). July average of 32.1m exceeds Yonaguni's 25.6m. Worth pairing with the UNESCO World Heritage forest.
Key Takeaways
- 81.5% of all 30m+ records are from Yonaguni (1,840 total)
- Yonaguni September: 1 in 2 dives exceeds 30m (51.7%)
- Honshu leader: Ito (Chiba) with 91 records / 4.6% hit rate
- Peak months globally: August (234), September (230), July (215)
- Yakushima shows 41.4% from small sample — a compelling outlier
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