How we measure sargassum

What we measure

Every beach has an index from 0 to 100. An engine that runs on its own several times a day works it out from these sources:

• Satellite. The AFAI index, which picks out floating seaweed in satellite images of the whole western Atlantic, in a seven-day composite.
• Wind. The National Weather Service forecast for each beach: wind blowing toward the sand pushes sargassum onto the shore.
• Currents. Surface currents from the Copernicus Marine Service, which move the seaweed mats offshore.
• Tides and water. Tides and water temperature from NOAA stations.
• The community. Reports from people on the sand. A recent report weighs more than an old one and never fully replaces the satellite.

The index cut-offs

The number turns into a word so it reads at a glance.

How far the forecast reaches

Sargassum cannot be forecast as reliably as the weather. The mats move with wind and currents, and clouds can hide them from the satellite for days. So:

• Today and the next three days are a forecast.
• Days four to six show a trend, not a firm forecast. They are drawn with a dashed border and a ~ before the number.
• We show nothing beyond seven days.

Sources and attributions

• AFAI: University of South Florida / NOAA AOML Physical Oceanography Division (CoastWatch Caribbean & Gulf of America Node).
• Generated using E.U. Copernicus Marine Service Information; https://doi.org/10.48670/moi-00016.
• Weather data: NOAA National Weather Service. Tides and water temperature: NOAA NOS CO-OPS.
• Map: OpenFreeMap © OpenMapTiles Data from OpenStreetMap.

How we check accuracy

Every forecast we publish is stored as it was. When its day comes, we compare it with what actually happened at that beach. For each lead time we measure two things: how often we got the word right (clean, moderate or heavy sargassum) and how many points we were off on average.