Kumomi Visibility Analysis: Inside West Izu's Premier Cave Diving Site

Kumomi's caves look best at 14m in winter. But most divers visit in summer at 8.4m. 1,980 days of data reveal the right time to see Kumomi at its best.

In this article, we analyze roughly 1,980 days of actual visibility measurements collected from 2013 to 2026 to uncover Kumomi's true visibility patterns. The bottom line: Kumomi's seasonal visibility pattern is quite different from what many divers might expect.

Monthly Visibility Data: Winter Is the Best Season

Let's start with the monthly averages. The highest visibility occurs in January and February, both averaging 14.2m. The lowest is July at just 8.4m. That's a gap of roughly 6m between winter and summer.

This "clear in winter, murky in summer" pattern may come as a surprise to many divers, since the common assumption is that the ocean is cleaner in summer. However, the data clearly demonstrates winter's superiority.

Why Is Summer Visibility Lower?

The main factors behind Kumomi's reduced summer visibility are river runoff and plankton growth. Heavy rainfall during the rainy season (tsuyu) washes nutrients from the mountains into the sea, triggering massive plankton blooms. Additionally, thermoclines form more readily in summer, and it is not uncommon for surface and deep-water visibility to differ significantly.

In winter, by contrast, precipitation is low and northwesterly seasonal winds churn the water, effectively resetting water quality and allowing visibility to recover. December's average visibility of 13.2m shows the rapid improvement from autumn into winter.

Yearly Trends: Any Long-Term Changes?

Looking at year-over-year visibility trends from 2013 to 2026, no significant deterioration is evident. There is some variation of 1 to 2m between years, which is likely attributable to large-scale oceanographic factors such as Kuroshio Current meandering and typhoon impacts.

Cave Diving and Visibility

Kumomi's greatest draw is its cave diving, but visibility inside the caves is governed by conditions distinct from the open water. Diver exhaust bubbles and silt kicked up by fin strokes can rapidly reduce visibility within the caves, making good buoyancy and finning skills essential. That said, the higher the open-water visibility at entry, the more stunning the shafts of light inside the caves become — resulting in truly photogenic dives.

Based on the data, our recommendation is the winter months of December through February. With visibility in the 13 to 14m range, this is when the curtains of light streaming into the caves are at their most beautiful. A drysuit is required, but the experience is well worth it. If you visit in summer, one strategy is to target the brief visibility recovery that often follows a typhoon.

Summary

Our analysis of 1,980 days of Kumomi visibility data reveals the following key findings. The best season is January to February (averaging 14.2m), while the worst is July (averaging 8.4m). Winter's superior visibility maximizes the appeal of cave diving.

No significant long-term deterioration has been detected, indicating a stable marine environment. Use this data to plan your visit and experience Kumomi's magnificent underwater caves at their very best.