Does Wave Period Affect Visibility? Short vs Long Period Wave Analysis
2026-03-16
Divers watch wave height, but wave period also significantly affects visibility. Wave period is the time (in seconds) between successive wave crests. Short-period waves are local wind chop that stirs up sediment, while long-period swells carry clean oceanic water from far offshore. We analyzed this relationship using data from IOP, Mikomoto, and Kashiwajima.
+0.7m
IOP long vs short period
+3.6m
Mikomoto long vs short
-1.3m
Kashiwajima mid vs short
Average Visibility by Wave Period
| Wave Period | IOP (m) | Mikomoto (m) | Kashiwajima (m) |
|---|---|---|---|
| Short (<6s) | 13.2m | 9.2m | 14.4m |
| Mid (6–10s) | 13.5m | 11.3m | 13.1m |
| Long (>10s) | 13.9m | 12.8m | Insufficient data |
Why Long-Period Swells Improve Visibility
Long-period swell (>10s): Carries clean oceanic water
Swells with 10+ second periods originate hundreds to thousands of kilometers offshore. They arrive carrying clean oceanic water masses. IOP showed +0.7m and Mikomoto +3.6m improvement. The effect is especially pronounced at offshore sites like Mikomoto.
Short-period wind waves (<6s): Local stirring
Waves under 6 seconds are local wind chop created by nearby winds. They disturb shallow seabeds, stirring up sand and sediment. Energy concentrates in shallow layers, directly affecting diving depths. Mikomoto drops to 9.2m during short-period conditions.
The Exception: Kashiwajima Is Clearer with Short Periods
Kashiwajima shows 14.4m with short periods but 13.1m with mid periods — the reverse of the general pattern. This likely reflects Kashiwajima's unique geography. Located in a bay-like formation sheltered by Shikoku's southwestern peninsula, short wind waves are blocked by the peninsula while mid-period swells can wrap into the bay and stir sediment. Additionally, the wind conditions that create short-period waves (northwest wind) may push clean offshore water toward this site.
Practical Advice
Check wave period, not just height. A 1.5m swell with 12-second period may have less impact on visibility than a 1m wave with 4-second period.
Open-ocean sites (Mikomoto, IOP) benefit most from long-period swells. When wind dies but short-period chop persists, visibility tends to stay poor.
Sheltered sites (like Kashiwajima) have unique patterns. Some sites get murky when swell enters, so understanding site-specific characteristics is key.
About the Data
Wave period data is from the Open-Meteo Marine API, matched with same-day visibility observations. Period classification follows standard meteorological criteria (short <6s, mid 6–10s, long >10s). Kashiwajima's long-period data was excluded due to insufficient observations.
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