Sites Where Low Pressure Brings Clear Water: Tajiri & Mikomoto's Pattern
2026-03-16
"Calm, high-pressure days mean clear water" — most divers believe this. But our data reveals that some sites show the opposite pattern. At Tajiri (Tottori) and Mikomoto (Shizuoka), visibility actually tends to improve after low-pressure systems pass through. Why?
+1.3m
Tajiri: post-low improvement
+0.6m
Mikomoto: post-low improvement
-1.9m
Kushimoto: worsens after low (typical)
Average Visibility by Pressure Pattern
| Site | High pressure | After low | Diff | Pattern |
|---|---|---|---|---|
| Tajiri | 8.5m | 9.8m | +1.3m | Improves after low |
| Mikomoto | 11.2m | 11.8m | +0.6m | Less pressure-sensitive |
| IOP | 14.1m | 12.4m | -1.7m | Better with high pressure |
| Kushimoto | 12.8m | 10.9m | -1.9m | Better with high pressure |
| Futo | 10.3m | 8.7m | -1.6m | Better with high pressure |
Tajiri: How Storms Clean the Water
Hypothesis 1: Storm-driven water mixing and exchange
Tajiri faces the Sea of Japan in the San'in region. Strong winds and waves during low-pressure events flush stagnant bay water and replace it with fresh offshore water. Northwest winds in autumn and winter particularly draw the clear Tsushima Warm Current toward shore.
Hypothesis 2: Freshwater input breaks stratification
Rain from low-pressure systems temporarily introduces freshwater, but subsequent mixing breaks up the turbid surface layer. Under stable high-pressure conditions, temperature and salinity stratification persists, trapping turbidity in the surface layer.
Mikomoto: Characteristics of Current-Driven Sites
Mikomoto is an uninhabited island off the tip of the Izu Peninsula, directly exposed to Kuroshio branch currents. Here, current strength rather than barometric pressure dominates visibility.
When low pressure approaches, the pressure gradient strengthens, which also invigorates ocean currents. Strong currents flush turbid coastal water and draw in clear oceanic water. The "better visibility after low pressure" at Mikomoto is likely the result of "strong currents bringing in oceanic water."
Typical Sites: Why High Pressure Is Better
Most sites (IOP, Kushimoto, Futo, etc.) show better visibility under high pressure. This is the intuitive pattern most divers expect.
Weak winds mean less surface stirring, allowing suspended particles to settle
No rain means no muddy river runoff, keeping coastal water clear
Small waves mean no seabed stirring — especially effective at sandy-bottom sites
What Low-Pressure-Friendly Sites Have in Common
Open to the ocean
Geography allows oceanic water inflow after storms. Enclosed bay sites trap muddy water instead.
Strong currents present
Currents accelerate water exchange. Tajiri has the Tsushima Warm Current; Mikomoto has Kuroshio branches.
Rocky seabed
Less sand means waves don't stir up sediment as easily. Storm impact is transient.
Minimal river influence
No major rivers nearby, so rain-driven turbidity inflow is limited.
Practical Advice
Targeting Tajiri or Mikomoto 1–2 days after a low-pressure event is a data-backed strategy. However, safety comes first — wait until swell has fully subsided before entering. This trend is based on averages; it won't hold every time. We recommend combining this insight with the AI forecast for your decision.
About the Data
Pressure data from Open-Meteo API. 'High pressure' defined as observation day average >1015hPa; 'after low' defined as days where 1005hPa or below was recorded 1–2 days prior. Visibility uses real dive log data. Correlation does not imply causation; mechanism explanations are hypotheses.
🌊 Check Visibility Forecasts
View AI-powered 7-day visibility forecasts for 30+ dive sites across Japan.
Open Forecast App →