Rain-Resistant Dive Sites: Ito & Shirahama Stay Clear Even in Heavy Rain

Have you ever canceled a dive because of rain during the rainy season or typhoon season? Our analysis of over 46,000 actual visibility observations reveals that some dive sites maintain or even improve their visibility during heavy rain (over 30mm/day).

Sites That Stay Clear in Heavy Rain

Remarkably, Ito (Chiba Prefecture) actually improves by 0.7m during heavy rain. Shirahama (Wakayama Prefecture) similarly improves by +0.8m. Kashiwajima also maintains an excellent 14.9m in heavy rain conditions.

Rain-Vulnerable Sites

In contrast, Hachijojima drops by 2.3m and IOP (Izu Oceanic Park) drops by 1.7m in heavy rain. However, it is worth noting that Hachijojima still maintains a respectable 18.5m even during heavy rain.

Why Are Ito and Shirahama Rain-Resistant?

1. Open-Ocean Geography

Ito sits at the southern tip of the Boso Peninsula, directly facing the open ocean. It receives strong influence from the Kuroshio Current, resulting in rapid water exchange that quickly dilutes any rainwater effects. Shirahama similarly sits at the tip of the Kii Peninsula with active open-ocean water inflow.

2. Minimal River Influence

The primary pathway through which rain affects visibility is via suspended solids transported by rivers. Both Ito and Shirahama have no major rivers nearby, limiting sediment inflow.

3. Low-Pressure Systems and Kuroshio Coupling

The low-pressure systems and weather fronts that bring heavy rain simultaneously alter ocean current patterns. At open-ocean sites, these weather changes can draw in warm Kuroshio water, actually improving visibility. This coupling effect likely explains why Ito and Shirahama see improved clarity during heavy rain events.

Tips for Choosing Dive Sites in Rainy Season

1. Choose open-ocean sites: Sites like Ito, Shirahama, and Kashiwajima with active open-ocean water exchange are less affected by rain
2. Avoid river mouths: Sites near major river estuaries are more susceptible to suspended solids for several days after heavy rain
3. Check waves, not rain: As shown in our rainfall-visibility analysis, wave height has a far greater impact on visibility than rainfall

Summary

• Ito improves by +0.7m in heavy rain (>30mm): 16.2m to 16.9m
• Shirahama improves by +0.8m: 15.2m to 16.0m
• Kashiwajima maintains 14.9m in heavy rain
• In contrast, Hachijojima (-2.3m) and IOP (-1.7m) decline in heavy rain
• Open-ocean sites with minimal river influence are the most rain-resistant