How quickly sunlight fades into the ocean (VIIRS, NOAA-21)
What it measures. A measure of how quickly blue-green light fades as it travels down through seawater, which tells you how clear or murky the water is and how deep sunlight reaches. Lower numbers mean clearer water where light penetrates further.
How it's made. Calculated from ocean-color measurements made by the VIIRS instrument on the NOAA-21 satellite, then mapped onto a global grid using an algorithm called KD2.
How & where you'd use it. Helps track coastal water quality and murkiness, estimate the sunlit depth where ocean life can grow, and feed ocean ecosystem and chemistry models. Readings are less reliable in cloudy coastal or inland waters.
What's measured
Coverage & cadence
- Time span2022-11-10 → ongoing
- Measured byNOAA-21 (VIIRS)
- Processing levelLevel 3
- Spatial extent-180, -90, 180, 90
- FormatsnetCDF-4
- StatusACTIVE
What you can do with it
- Watch sea-surface temperature and marine heatwaves
- Spot algal blooms and ocean-colour shifts
- Support fisheries and coastal monitoring
Official description
The KD data suite provides diffuse attenuation coefficient of downwelling irradiance at 490 nm, Kd_490, which quantifies how quickly blue-green light diminishes with depth—an indicator of water clarity and the depth to which sunlight penetrates. Kd_490 is generated from ocean-color reflectance using the KD2 algorithm. Lower values indicate clearer water (deeper light penetration); users commonly apply Kd_490 to estimate euphotic depth, track coastal water quality and turbidity, interpret bloom/light environments, and supply inputs to biogeochemical and ecosystem models. As with all ocean-color products, uncertainties can be larger in optically complex coastal or inland waters; refer to the file metadata and algorithm documentation for quality flags and usage notes. Geophysical variables in this suite include: - Kd_490 — Diffuse attenuation coefficient of downwelling irradiance at 490 nm, KD2 algorithm (m⁻¹)
Get the data
import earthaccess
earthaccess.login(strategy="netrc") # free Earthdata Login
results = earthaccess.search_data(
short_name="VIIRSJ2_L3m_KD",
version="2025.0",
bounding_box=(-122.5, 37.2, -121.8, 37.9), # your area (W,S,E,N)
temporal=("2024-01-01", "2024-12-31"), # your dates
)
files = earthaccess.open(results) # stream straight from OB_CLOUDBrowsing CMR needs no login. Downloading or streaming bytes needs a free Earthdata Login + the earthaccess package. Official links
- OPeNDAP Site for NOAA-21 VIIRS Standard Mapped Image (SMI) Product USE SERVICE API
- Earthdata Search allows users to search, discover, visualize, refine, and access NASA Earth Observation data. GET DATA
- View the version history of this product. VIEW RELATED INFORMATION
- NASA's Ocean Color Web - Data Distribution Site GET DATA
- NOAA-21 Homepage VIEW RELATED INFORMATION
- VIIRS Homepage VIEW RELATED INFORMATION
- Satellite data procesing can be difficult. We're here to help! Visit the Oceandata Help Hub to navigate learning content for accessing visualizing, and analyzing data products. VIEW RELATED INFORMATION
- Algorithm Theoretical Basis Document for Diffuse Attenuation Coefficient of Downwelling Irradiance (KD) VIEW RELATED INFORMATION