Satellite lake observations

A satellite reading of how green the water looked from space on the date shown: an index that rises with chlorophyll and floating or suspended plant material. It is not a test for harmful algae or toxins, and it is not swimming advice.

What it does not tell you

harmful, toxic or blue-green algae (cyanobacteria)
algae-free or clear water
a chlorophyll concentration
swimming advice or safety
a forecast

For algae at a bathing water, the Environment Agency's own checks are on each spot's page. Look at the water and the signs before you swim.

How the readings are made

From Sentinel-2 images (Collection 1, Level-2A surface reflectance), May to September of each year read; at a location read again later, newer images too, each outside May to September labelled experimental. Around each site, the water within 300 m is found from the images' own scene classification; a reading is made only where at least 9 of its 20 m pixels are open water with open water all round them, and only from an image in which at least 80% of those pixels are clear of cloud, haze and shadow. The reading is the median of the normalised difference chlorophyll index, NDCI = (B5 − B4) ÷ (B5 + B4), over those clear pixels. Each reading keeps its image's own date and time.

Two tests compared the index with what was found in the water. Against laboratory chlorophyll at 38 lakes, it followed the measurements (correlation about 0.5) but missed its stated bar, so it is called promising, not validated. Against the samplers' visual algae checks and reported incidents, it was weak. So no reading here is turned into a chlorophyll figure or a warning.

Sources

Contains modified Copernicus Sentinel data [2020-2026]. Lab chlorophyll: Environment Agency Water Quality Archive, Open Government Licence v3.0. Images read from Earth Search's Sentinel-2 Collection 1 Level-2A archive and processed by SwimSignal. Data credits.