Does Rain Really Reduce Diving Visibility? Testing with Data from 25 Sites

"Rain makes the ocean murky" -- it is one of the most widely held beliefs among divers. But is it actually true? In this article, we measured the relationship score between observed visibility data from 25 dive sites across Japan (over 46,000+ observations in total) and daily rainfall data from Open-Meteo, putting this common assumption to a rigorous data-driven test.

The bottom line: the relationship between rainfall and visibility is remarkably weak, topping out at just -0.21. The belief that "rain equals poor visibility" is, at least for ocean diving sites, largely a myth.

How to Read Relationship Scores

Relationship scores range from -1 to +1. Values closer to -1 indicate "more rain, lower visibility," while values closer to +1 indicate "more rain, higher visibility." A value of 0 means no relationship. By convention, absolute values below 0.2 are considered "negligible," 0.2 to 0.4 "weak," and above 0.4 "moderate."

Rainfall-Visibility Relationship Across All 25 Sites

Negative Relationship (Rain Slightly Reduces Visibility)

Near-Zero Relationship

Positive Relationship (Visibility Tends to Be Better on Rainy Days)

Why Does Rain Have So Little Effect on Visibility?

1. Ocean Visibility Is Driven by Currents and Plankton

The dominant factors controlling ocean visibility are ocean currents and plankton concentrations. When a warm-water mass from the Kuroshio Current approaches, visibility can exceed 30 meters even the day after heavy rain. Conversely, a plankton bloom under clear skies can drop visibility to single digits. Compared to these fundamental drivers, rainfall is a marginal influence at best.

2. The Ocean Is Enormous

Even a heavy rainfall of 50mm per day adds just 50 liters of freshwater per square meter of sea surface. The volume of seawater at a dive site is orders of magnitude larger, so the rain is rapidly diluted to concentrations too low to affect visibility. At open-ocean sites where currents constantly flush and replace water, the influence of rainfall is essentially undetectable.

3. River-Borne Suspended Solids Arrive with a Time Lag

To the extent that rain does affect visibility, the mechanism is not direct rainfall into the sea but rather the transport of sediment and organic matter (suspended solids) via rivers. However, this runoff takes days to reach coastal dive sites. Since our analysis examines the relationship between same-day rainfall and same-day visibility, this delayed effect is largely invisible in the statistics.

4. Wave Height Is a Far Stronger Predictor

As shown in our companion article "How Waves, Swell, and Wind Affect Visibility," the strength of relationship between wave height and visibility reaches -0.27 (Hachijojima). This exceeds the strongest rainfall relationship of -0.21, and waves have a clear physical mechanism: they stir up sediment from the seabed. When checking the weather forecast before a dive, wave height and swell forecasts are far more useful than rainfall predictions.

Why Shirasaki Is the Sole Outlier Beyond -0.2

Of all 25 sites, Shirasaki (Wakayama Prefecture) is the only one where the rainfall-visibility relationship exceeds -0.2. Shirasaki sits deep inside an inlet and is close to a river mouth, making it susceptible to freshwater and sediment inflow during rain events. In such semi-enclosed bay environments, rain has a relatively larger impact compared to open-ocean sites. Even so, at -0.206, the relationship is still classified as "weak" in statistical terms.

Why Some Sites Show a Positive Relationship

It may seem surprising that sites like Mikomoto (+0.067) and Omijima (+0.060) show a tendency for better visibility on rainy days. Several hypotheses may explain this:

• Weather system coupling: The passage of low-pressure systems and fronts brings rain while simultaneously altering currents and wind direction. At open-ocean sites like Mikomoto, these weather changes can pull in warm Kuroshio water, improving visibility.
• Seasonal confounding: At some sites, the rainy season (tsuyu, typhoon season) happens to coincide with periods of high visibility, creating an apparent positive relationship.
• Freshwater stratification: Large volumes of freshwater can spread across the sea surface, forming a low-density cap that suppresses vertical mixing between surface and deeper layers. This can preserve visibility at depth in some circumstances.

What Should Divers Check Instead?

This analysis makes clear that same-day rainfall is essentially useless for predicting visibility. Here are the indicators divers should focus on instead:

1. Previous day's visibility: Ranked #2 in our AI model's impact ranking. Dive shop logs and local reports are the most reliable source.
2. Wave height and swell: Relationship scores reach up to -0.27, clearly stronger than rainfall. Checking the wave forecast is essential.
3. Water temperature changes: Ranked #1 in our AI model's impact ranking. A sudden temperature shift signals a change in ocean currents and is often accompanied by a visibility change.
4. Cumulative rainfall over several days: While same-day rain has little effect, sustained heavy rainfall over multiple days can increase suspended solids from river runoff, particularly at sites near river mouths.
5. Satellite data (chlorophyll and Kd490): Provides a broad view of ocean conditions. Our AI model incorporates these variables as well.

Summary

• Across 25 sites, the maximum relationship score between rainfall and visibility is only -0.21 (Shirasaki)
• The vast majority of sites have absolute relationship scores below 0.1, meaning rain and visibility are effectively unrelated
• Six sites show a positive relationship -- visibility actually tends to be better on rainy days
• Ocean visibility is governed by currents, plankton, and wave action; surface rainfall is a secondary factor at most
• Wave height is a far stronger predictor (relationship score up to -0.27), and divers should prioritize wave forecasts over rain forecasts
• Only at semi-enclosed bays near river mouths (like Shirasaki) might cumulative multi-day rainfall have a meaningful impact