Water Clarity (Kd490)

More detail

Divide 1 by the number: the result, in meters, is how deep the top layer of water is that the satellite sees into. A reading of 0.02 is about 50 meters (about 165 feet), gin-clear. A reading of 0.2 is about 5 meters (about 16 feet), green and murky.

The layer is made by the Copernicus Marine Service, the European Union's ocean data service, from the satellite's reading of plankton.

It pulls the extremes toward the middle: the clearest water is drawn less clear than it is, and the murkiest less murky. Use it to find where the clean-water edge runs, not as an exact clarity number.

Cloud holes are filled in by estimate, so the layer rarely goes blank. The first version of each day is replaced with a corrected one within about a month, so an older date may not look the way it did. The layer covers the Gulf of America, the East Coast and Baja.

How it relates to the other layers. Water Clarity (Secchi depth) is the same satellite reading turned into a depth you could see down to. Its colors are reversed, so clear water is the same color on both. Both come from the same satellites as the chlorophyll layers, at the same detail as Chlorophyll (GlobColour). Green water on a chlorophyll layer is murky water here.

The numbers

How often it updates	Once a day, one frame per day
How fresh it usually is	About 1 day behind the satellite passes
How detailed it is	About 2.2 nautical miles (4 km) per pixel
Where it covers	Gulf of America, East Coast and Baja
How far back you can look	About 8 days; the provider keeps only a short archive
Goes blank under cloud?	Rarely. Cloud holes are filled in by estimate
Observed or modeled	Calculated from a satellite reading of plankton, then filled
What you see on screen	The Kd490 number, shown as m^-1 (per meter). Surface only

Source: a satellite picture from Copernicus satellite ocean color.