September Has the Biggest Visibility Swings — Monthly Variance Analysis
2026-03-16
September is Japan's highest-stakes diving month. Typhoons can drop visibility to 1m, but post-typhoon clarity can hit 30m+. Hachijojima September ranges 4–30m (26m spread); Ito spans 3–35m (32m spread). In September, site selection is everything.
September Visibility by Site (Average & Range)
Why September Is Most Volatile
Peak Typhoon Season
September sees the most typhoon approaches statistically. Passage drops visibility to 1–3m, but recovery to 30m can happen within a week. Kerama's 29m range (1–30m) reflects this typhoon cycle. (JMA Typhoon Statistics)
Kuroshio Seasonal Shift
September sees variable Kuroshio approach distance. Hachijojima (21.9m) and Yonaguni (27.3m) ride the Kuroshio, but drops sharply when it retreats. Sea of Japan sites face a double hit from typhoons and autumn cooling.
Reliably Good Sites in September
- Yonaguni (27.3m): Floor of 15m. Recovers fast even after typhoons
- Ishigaki (21.4m): Floor of 5m but 21m average. Fairly stable
- Akinohama (14.9m): 13.5m range — most stable on Honshu
About the Data
September observations from sites with 20+ data points (28 sites). Range = max − min.
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