Methodology
Satellite clarity proxy: what Kd490 can — and cannot — tell us
SnorkelForecast uses NOAA ocean-colour data to add an independent optical-water signal to Maui and Oahu. The source variable is Kd490, the diffuse attenuation coefficient of downwelling light at 490 nm. Lower Kd490 means light is attenuated more slowly and is generally associated with optically clearer water; higher values mean stronger attenuation.
Source
The first cohort uses NOAA CoastWatch dataset noaacwNPPN20S3AkdSCIDINEOF2kmDaily: a science-quality, global ~2 km, daily, gap-filled product combining VIIRS S-NPP, VIIRS NOAA-20 and Sentinel-3A OLCI ocean-colour observations. The page module uses the newest 30 valid daily values available at the nearest model grid cell and reports the median plus the 25th–75th percentile range.
NOAA CoastWatch dataset · NOAA ocean-colour background
What we publish
- Kd490 median (m⁻¹): the recent median diffuse attenuation coefficient. Lower is optically clearer.
- Kd490 range: the 25th–75th percentile of the newest 30 valid daily values. This shows recent variability rather than pretending one point estimate is exact.
- Optical attenuation depth:
1 / Kd490. At this depth, downwelling 490 nm light has fallen to about 37% of its surface value. We report the corresponding interquartile range by inverting the Kd490 bounds. - Observation dates: the actual source-data window, so satellite latency is visible instead of being hidden.
What this is not
The optical-depth number is not snorkeler visibility in metres.
Human underwater visibility depends on contrast, particles, bottom type, sun angle, surface glare, depth, local runoff, waves, bubbles and the direction you are looking. A satellite grid cell also cannot resolve every sheltered cove or near-shore plume.
We therefore call this a modelled satellite clarity proxy. We do not convert Kd490 into a claimed Secchi depth or diver-visibility distance. If a future source provides a validated Secchi-depth product directly, that would remain a separate modelled optical variable rather than an in-water observation.
Why gap-filled data?
Clouds and sun-glint can leave gaps in raw daily ocean-colour imagery. NOAA's DINEOF product fills missing grid cells statistically using multi-sensor information. That makes a recent range more stable, but it also means the result is partly reconstructed rather than a direct satellite retrieval every day. The page labels the product as modelled/gap-filled for that reason.
How to use it
Treat the clarity proxy as one extra signal alongside waves, wind, currents, rainfall/runoff context and local guidance. A low Kd490 does not make rough water safe, and a high Kd490 does not mean snorkeling is impossible at every nearby beach.