Is Winter Really the Clearest? Testing a Common Diving Myth with Data from 25 Sites

Among divers in Japan, there is a widely held belief that "winter seas have the best visibility." And indeed, anyone who has dived in Izu during January can attest to the stunning clarity. But does this hold true across the entire country? We analyzed over 46,000 actual observations from 25 dive sites nationwide to put this common wisdom to the test.

The Verdict: "Winter = Clearest" Only Applies to the Pacific Coast

Let us start with the conclusion: "Winter has the best visibility" is true for the Pacific coast of mainland Japan, but false for the Sea of Japan coast and Okinawa/subtropical areas. On the Sea of Japan side, winter is actually the worst time for visibility, while Okinawa-area sites peak in summer.

Pacific Coast: Winter Peaks (The Myth Holds True)

Dive sites along the Pacific coast, centered on the Izu Peninsula, show a pronounced pattern of peak visibility during winter (December through February).

All seven sites record their highest annual visibility in January. The contrast is especially dramatic at Izu Oceanic Park, where the winter average of 17.8m is a full 6.1m higher than the summer average of 11.7m. Ito (Chiba Prefecture) also shows a clear gap: 18.3m in winter versus 14.6m in summer.

Why Is the Pacific Coast Clearest in Winter?

Two main factors are at work. First, plankton activity decreases in winter, reducing suspended particles in the water. Second, the Kuroshio Current plays a crucial role. The Kuroshio carries oligotrophic (nutrient-poor) water that is exceptionally clear, and with reduced plankton in winter, the Kuroshio's inherent clarity is more fully reflected at dive sites.

Conversely, spring brings the spring phytoplankton bloom, causing a sharp drop in visibility. The April figure of 10.1m at Izu Oceanic Park and May's 9.2m at Futo vividly illustrate this "spring turbidity" phenomenon.

Sea of Japan Coast: Winter Is the Worst, Summer Is Best

In stark contrast to the Pacific coast, sites on the Sea of Japan coast experience their lowest visibility in winter.

The Echizen data is particularly striking. Winter average visibility is just 6.0m, roughly two-thirds of the summer average of 9.1m. December's 5.0m is the annual low — the exact opposite of Pacific coast sites, where December is typically the annual high.

Why Is the Sea of Japan Coast So Poor in Winter?

The Sea of Japan coast is battered by strong northwesterly seasonal winds during winter, creating rough seas. Heavy wave action churns up bottom sediment, drastically reducing visibility. Additionally, the Tsushima Current that flows along this coast is not as oligotrophic as the Kuroshio, offering less benefit to water clarity.

In summer, the Sea of Japan calms down considerably, and the relatively warm and clear water from the Tsushima Current flows in, resulting in peak visibility in August. At Echizen, the difference between summer peak (10.3m) and winter low (5.0m) reaches a remarkable 5.3m.

Okinawa and Subtropical Areas: High Year-Round, But Summer Peaks

Okinawa and other subtropical island sites maintain excellent visibility year-round but do not follow the "winter is best" pattern either.

Yonaguni records an astounding 27.3m in September but drops to its annual low of 22.4m in January. Kerama follows a similar pattern, peaking at 21.4m in July and bottoming out at 18.1m in January. Winter is the annual minimum for these sites — the opposite of the Pacific coast pattern.

Amami Oshima is an interesting exception, peaking in December at 24.6m. Its geographic position between mainland Japan and Okinawa likely means it receives stronger direct influence from the Kuroshio Current.

Sites with Unique Patterns

Mikomoto: Weak Winter Pattern

Mikomoto peaks in December at 13.7m, showing a winter-leaning pattern. However, the seasonal difference is smaller than at other Pacific sites — summer (the hammerhead shark season, July through September) still maintains 12 to 13m. Being an exposed offshore island, visibility is heavily influenced by the Kuroshio Current's proximity.

Kannoura: Autumn Is Best

Kannoura in Kochi Prefecture peaks at 11.5m in November, following neither the winter nor summer pattern. This unique autumn peak may be related to a branch of the Kuroshio reaching the coast most consistently during the fall months.

Winter vs. Summer Averages: The Big Picture

To make the regional differences even clearer, here is a summary comparing average winter and summer visibility by area.

The Pacific coast shows a +4.0m advantage for winter over summer, making winter the clear winner. The Sea of Japan coast, however, shows a -1.7m gap in the opposite direction, favoring summer. Okinawa sites are essentially a wash, with negligible seasonal variation.

Conclusion: The Best Season Depends on Where You Dive

The belief that "winter has the best visibility" is a fact specific to the Pacific coast of mainland Japan. Here is what 25 sites worth of data tells us:

1. Pacific coast (Izu, Kushimoto, Chiba, etc.): Winter is best. The Kuroshio Current brings oligotrophic, crystal-clear water, while plankton levels drop to their annual minimum.
2. Sea of Japan coast (Echizen, Tajiri, Omijima): Summer is best. Winter storms and rough seas drive visibility as low as 5m.
3. Okinawa/Subtropical (Yonaguni, Kerama, Amami): High year-round, with a slight summer peak. Any season works.

When planning your dive trip, do not assume that "winter means clear water" regardless of destination. Choose the optimal season based on the specific region you plan to visit. Our forecast tool provides real-time visibility predictions for each site to help you plan.