The Satellites Watching Over Your Beach
Published
Every morning, while most of the Caribbean is still asleep, a European satellite passes overhead at 800 kilometres above the ocean. It scans the water below, records what it sees, and sends the data back to Earth. A few hours later, that data shows up on SeaweedFree as a coloured dot on a map.
Here is the story of how that happens.
Meet Sentinel-3
The satellite doing this work is called Sentinel-3, part of the European Space Agency's Copernicus programme. It was built for one purpose: to monitor the health of Earth's oceans and land surfaces with extraordinary precision.
Sentinel-3 carries an instrument called OLCI, which stands for Ocean and Land Colour Instrument. OLCI measures how light reflects off the ocean surface across 21 different wavelength bands. Sargassum seaweed has a very specific optical signature: it reflects light differently from open water, from clouds, and from other types of algae. OLCI can detect that signature from 800 kilometres up.
The satellite completes a full orbit of Earth roughly every 100 minutes and covers a swath of ocean about 1,270 kilometres wide with each pass. That means it can scan the entire Caribbean basin within a day or two, depending on cloud cover.
When clouds get in the way
On days when cloud cover blocks Sentinel-3's view of a particular area, SeaweedFree switches to a second Copernicus product that processes data from the ABI instrument on the GOES-16 geostationary satellite. Where Sentinel-3 passes over the Caribbean once every day or two, GOES-16 sits fixed above the Americas and provides continuous coverage. The combination means cloud cover is far less likely to leave a beach without any satellite data at all.
What the satellite actually sees
Sentinel-3 does not take a photograph in the traditional sense. It measures light intensity across different wavelengths and builds a data image from those measurements. Scientists use a calculation called the Floating Algae Index to identify pixels where sargassum is likely present on the surface.
The result is a daily map of sargassum density across the open ocean. Patches can range from thin scattered mats to dense aggregations covering hundreds of square kilometres. Tracking how those patches move and grow is exactly what makes it possible to warn beach visitors days in advance.
From open ocean to your beach
Here is the part that most people find surprising: satellites cannot see sargassum right at the shoreline.
The reason is something called the coastal land mask. When a satellite image pixel sits close to land, the signal gets mixed with light reflecting off the coast itself, buildings, vegetation, and shallow water. That interference makes it impossible to reliably detect seaweed in the final few hundred metres before the beach.
This is where SeaweedFree's four-layer model comes in. Sentinel-3 tells us where sargassum is offshore. NOAA's Sargassum Inundation Reports tell us the regional risk level based on oceanographic monitoring. Wind data from the Global Forecast System tells us whether conditions are pushing sargassum toward shore or away from it. And citizen reports from people on the ground tell us what is actually happening at sand level right now.
No single data source is enough on its own. The satellite sees the ocean but not the shoreline. NOAA covers broad regions but not individual beaches. Wind models predict movement but cannot confirm arrival. Local reports are ground truth but only exist when someone submits one.
Together, the four layers give a picture that none of them could provide alone.
Why 2026 matters more than usual
Sargassum has been growing as a phenomenon since around 2011, when a new bloom belt established itself in the central Atlantic. Scientists call it the Great Atlantic Sargassum Belt, and it now produces tens of millions of tonnes of seaweed each year.
2026 is forecast to be a record year. Early satellite readings from the central Atlantic show bloom densities significantly above the 2023 peak, which was itself a record at the time. Warmer sea surface temperatures and nutrient runoff from major river systems are driving the growth.
For Caribbean beaches, that means higher baseline risk across the region from spring through autumn. It also means that the difference between a beach with good conditions and one with poor conditions on any given day is more valuable information than ever.
What SeaweedFree does with the data
Every day, starting at 08:00 UTC, SeaweedFree's pipeline fetches the latest Sentinel-3 readings from the Copernicus Marine Service and checks them against each of the 65 beaches on the map. If cloud cover or the coastal land mask blocks a clear reading, the system falls back to the GOES-16 ABI product, and if that is unavailable too, to the most recent valid reading within the last three days.
NOAA risk data arrives separately and acts as a floor: if NOAA says a region is at elevated risk, SeaweedFree will never show a lower risk level than that, even if the satellite image looks clear. Wind data is checked every six hours and can escalate a reading upward but never downward. Citizen reports are blended in using a time-weighted formula: a report submitted in the last six hours carries full weight, while older reports carry progressively less.
The result is updated once a day and shown on the map as one of four conditions: Clear, Light, Moderate, or Heavy.
Honest about what we cannot do
Satellite data is not magic, and SeaweedFree is not a guarantee.
Sargassum can arrive at a beach within hours, faster than any satellite pass can capture. Cloud cover can block readings for several days in a row. The coastal land mask means the last stretch of ocean before the shore is always an educated estimate rather than a direct measurement.
What SeaweedFree can do is give you the best available picture before you travel, built from the same data sources that oceanographers and researchers use, made accessible in a format that takes three seconds to read.
The satellites do their job every morning. We do ours with the data they send back.
Check current sargassum conditions at seaweedfree.com. Citizen reports can be submitted directly on the map, no account needed.