How to Read Diving Visibility from Satellite Data: kd490, Chlorophyll & Real-World Correlation
2026-03-11
NASA and NOAA publish daily ocean color satellite data. The metric kd490 (diffuse attenuation coefficient at 490nm wavelength) measures water turbidity and correlates closely with diving visibility. Matching 46,000+ dive observations with satellite data revealed a clear pattern: days with kd490 < 0.05 average 19.9m visibility; kd490 ≥ 0.20 averages just 7.7m.
What Is kd490?
kd490 (unit: m⁻¹) measures how much blue-green light (490nm wavelength) attenuates per meter of depth in the ocean. Lower values = more light penetrates = clearer water.
- kd490 < 0.05: Very clear (open ocean, Kuroshio)
- kd490 0.05–0.10: Clear (most Pacific coast sites)
- kd490 0.10–0.15: Moderate (IOP during spring bloom)
- kd490 > 0.20: Turbid (unusually high turbidity)
Data source: NOAA ERDDAP (erdSW1chlamday, erdSW1_kd490_8day). 8-day composite data matched to each observation date.
kd490 vs Visibility Correlation (43,992 obs)
| kd490 Value | Obs | Avg Visibility | Clarity Bar |
|---|---|---|---|
| < 0.05 (very clear) | 7,713 | 19.9m | |
| 0.05–0.10 (clear) | 23,648 | 13m | |
| 0.10–0.15 (moderate) | 8,829 | 10.3m | |
| 0.15–0.20 (turbid) | 1,786 | 8.9m | |
| ≥ 0.20 (very turbid) | 1,016 | 7.7m |
Key Finding
Days with kd490 < 0.05 (very clear water) average 19.9m visibility. Days with kd490 ≥ 0.20 (high turbidity) average 7.7m. The satellite-measured optical clarity correlates with a 12.2m difference in actual diving visibility.
Chlorophyll-a Concentration vs Visibility (9,512 obs)
| Chlorophyll-a | Obs | Avg Vis |
|---|---|---|
| < 0.1 mg/m³ | 29 | 16.9m |
| 0.1–0.3 mg/m³ | 3,651 | 14.7m |
| 0.3–0.6 mg/m³ | 4,390 | 12.9m |
| 0.6–1.0 mg/m³ | 1,072 | 11.3m |
| ≥ 1.0 mg/m³ | 370 | 11m |
Chlorophyll < 0.1mg/m³ has only 29 samples — treat as indicative. Satellite chlorophyll has fewer matches than kd490 due to cloud cover and data gaps.
Case Study: IOP Monthly kd490 vs Visibility (3,058 obs)
Spring turbidity (kd490 peak 0.1094 in April) is clearly visible in satellite data. The seasonal pattern of rising kd490 → falling visibility is confirmed.
| Month | kd490 | Visibility | Obs |
|---|---|---|---|
| Jan | 0.0674 | 18.6m | 278 |
| Feb | 0.0621 | 17.9m | 209 |
| Mar | 0.0796 | 13.7m | 240 |
| Apr | 0.1094 | 10.1m | 232 |
| May | 0.1003 | 10.6m | 246 |
| Jun | 0.0853 | 11.3m | 213 |
| Jul | 0.0951 | 11.6m | 270 |
| Aug | 0.0752 | 12.3m | 279 |
| Sep | 0.0797 | 12.7m | 276 |
| Oct | 0.0953 | 13.7m | 270 |
| Nov | 0.0899 | 14.7m | 278 |
| Dec | 0.0949 | 17.3m | 294 |
kd490 Reference Values for Major Sites
| Site | Annual Avg kd490 | Avg Vis | Notes |
|---|---|---|---|
| Yonaguni | 0.035 | 24.5m | On Kuroshio core |
| Hachijojima | 0.058 | 17.7m | Open ocean island |
| Akinohama | 0.066 | 14.3m | Open ocean island |
| IOP | 0.087 | 13.8m | Pacific coast (annual avg) |
| Echizen | 0.079 | 7.5m | Sea of Japan (summer bias) |
Practical Guide: Checking Satellite Data Before You Dive
Use this site's AI forecast (easiest)
This site's AI model automatically fetches kd490 and chlorophyll satellite data and incorporates them into visibility forecasts. Check the homepage for 7-day forecasts for each site.
Check NOAA ERDDAP directly (advanced users)
NOAA ERDDAP allows map-based access to kd490 (dataset: erdSW1_kd490_8day) and chlorophyll (erdSW1chlamday). Enter your site's coordinates to retrieve 8-day composite data. kd490 below 0.05–0.07 suggests high visibility potential.
Limitations of Satellite Data
- Clouds prevent satellite measurement. Data gaps increase during rainy season and typhoon periods
- kd490 measures near-surface optical properties. Deeper visibility may not perfectly match
- 8-day composites miss rapid changes, such as immediate post-typhoon turbidity
- For some sites (enclosed bays like Osezaki), the kd490-visibility correlation is weaker
- kd490 has high Feature Importance in the AI model, but it's one of many factors — not the sole determinant
About the Data
Diving visibility: dive shop daily logs (46,000+ obs, 2015–2026). Satellite kd490: NOAA ERDDAP erdSW1_kd490_8day (SeaWiFS/MODIS). Chlorophyll: erdSW1chlamday. Matched by site coordinates and nearest 8-day composite to observation date. kd490 matches: 43,992 obs; chlorophyll matches: 9,512 obs.
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