Water Clarity (Secchi depth)

More detail

The app shows it in feet if your depth unit is feet, and in meters otherwise. About 130 feet (40 meters) is gin-clear blue water. About 16 feet (5 meters) is 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.

Do not read it as a visibility gauge: the provider reports a typical error of about 20 feet (6.3 meters) against real disk readings. Use the layer to find where the clean-water edge runs.

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 (Kd490) is the same satellite reading, shown as the number for how fast light fades going down. Its colors are reversed, so clear water is the same color on both. Use this layer when you want clarity as a depth. 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	Feet if your depth unit is feet, otherwise meters. The default color range is about 16 to 131 feet (5 to 40 meters). Surface only

Source: a satellite picture from Copernicus satellite ocean color.