Cross-Site Analysis: How Waves, Swell, and Wind Affect Diving Visibility

"Visibility drops when waves are big" is a common rule of thumb among divers. But does the data actually support this? In this article, we measured the strength of the relationship between observed visibility data from 21 dive sites and wave height / wind speed data from Open-Meteo, putting this relationship to a quantitative test.

How to Read the Relationship Scores

The relationship scores range from -1 to +1. A negative score means "higher waves, lower visibility," while a positive score means "higher waves, higher visibility." Values near zero indicate no clear relationship. As a general guide, an absolute value of 0.2 or more represents a meaningful relationship, and 0.4 or more indicates a strong one.

Wave Height and Wind Speed Relationships Across All Sites

Key Findings

Most Wave-Affected Site: Hachijojima

Hachijojima showed the strongest negative relationship with wave height at -0.435, the highest among all sites. As a remote island directly exposed to the open ocean, it is highly susceptible to swell. On high-wave days, sand is stirred up from the seabed and visibility drops dramatically. Its wind speed relationship score of -0.227 is also notable, making it a site where both waves and wind significantly impact visibility.

Island and Open-Ocean Site Trends

Following Hachijojima, Shirasaki (-0.269), Kerama (-0.245), Yakushima (-0.242), Ishigaki (-0.223), and Shirahama (-0.206) round out the top spots -- all islands or sites facing the open ocean. These locations have little terrain to block incoming swell, so wave conditions directly affect visibility. Kerama also has a relatively strong wind relationship score of -0.195, meaning strong northerly winds in winter carry a real risk of reduced visibility.

Sites Where Waves Have Little Impact

Interestingly, IOP / Izu Oceanic Park (+0.096), Osezaki Bay (+0.090), Hirasawa (+0.075), and Kashiwajima (+0.147) show a positive relationship with wave height. While this seems counterintuitive, these sites are located in sheltered bays along the Izu Peninsula, and the positive relationship likely reflects a seasonal pattern: winter -- when waves tend to be larger -- is also when the Kuroshio Current brings clearer water to the region.

In other words, the apparent positive relationship between waves and visibility is most likely driven by seasonality as a confounding variable.

Wind Effect Patterns

Looking at wind speed relationships, Hachijojima (-0.227), Yakushima (-0.231), Kerama (-0.195), Ishigaki (-0.185), and Miyakejima (-0.171) top the list. Strong wind not only roughens the surface and reduces underwater visibility, but also triggers vertical mixing, where surface water is churned and combined with deeper cold water and suspended particles, leading to reduced visibility.

Conversely, Kashiwajima (+0.272), Osezaki Bay (+0.195), Hirasawa (+0.144), and IOP / Izu Oceanic Park (+0.142) show positive relationships with wind speed. Again, seasonality is likely the main driver -- periods of strong seasonal winds in winter coincide with higher visibility at these sites.

Takeaways for Divers

This analysis reveals that the relationship between waves, wind, and visibility varies enormously by site. At island locations like Hachijojima, Kerama, and Ishigaki, checking the wave forecast is directly useful for predicting visibility.

On the other hand, at sheltered Izu Peninsula sites, seasonal factors (such as proximity of the Kuroshio Current) and recent visibility trends matter far more than wave height alone. Understanding each site's characteristics will help you use weather data more effectively to pick the best day for your dive.