Sea Temperature by Satellite, 5 km Grid

The source is NOAA's Geo-polar Blended analysis: several satellites, geostationary and polar-orbiting, merged into one daily field on a 0.05° grid — about five kilometres. Our server asks for a small box around your place rather than a single point, and then keeps the nearest cell that actually carries sea. That box costs roughly 1.1 kilobytes and four tenths of a second, and it is the whole difference between a page that answers and a page that shrugs.

Why a box at all? Because a coordinate taken from a map sits on land. Tested on twenty European bathing beaches, asking for the exact point on a twenty-five-kilometre grid returned nothing at all nine times out of twenty — Brighton and Faro among them — and when it did answer, the pixel was 8.1 kilometres away in the median case. The five-kilometre grid with a box answered twenty times out of twenty, at 2.2 kilometres in the median. Raising the resolution never removes the problem on its own: even on a one-kilometre product, the exact pixel at Biarritz is empty.

• You are choosing between two stretches of coast for a holiday and want the real difference, not the brochure's: eighteen degrees in the Channel against twenty-six in the Balearics is the kind of gap this answers straight away.
• You swim or dive through the year at the same spot and want to see the season turn — the water lags the air by weeks, and a page that shows the date lets you follow that lag honestly.
• What it cannot do is tell you the temperature of the water at the end of the pier this afternoon. It is a satellite analysis of the surface, a day old, averaged over twenty-five square kilometres. For an actual swim, the only honest instrument is a thermometer.

• Nice, 18 September 2026: 25.8 °C. Fourteen of the twenty-five pixels in the box carried sea; the other eleven were the Alpes-Maritimes. On the coarser twenty-five-kilometre product the same request gave 25.84 °C — indistinguishable, and taken 11.6 kilometres out to sea.
• Lisbon, the same day: 16.4 °C — nine degrees colder than Nice, at almost the same latitude. That is the Atlantic upwelling, and it is the single most useful thing this page tells a traveller who assumes southern means warm.
• Palma, on the same product: twenty-three of the thirty pixels in the window were masked, and the two nearest valid ones gave 26.73 and 26.39 °C. A third of a degree hangs on which pixel is picked, which is why the choice is made by distance and settled the same way every time rather than by whichever line the file happens to list first.

• It is not today's temperature. The analysis is published once a day and runs a day behind, and the cut-over slides through the afternoon. The page shows the date the service itself stamps on the data, and never writes "today".
• It is the surface, and only the surface. A metre down the water can be a degree colder; in a thermocline, several. The product describes the skin the satellite sees, not the column you swim through.
• The swimming bands are ours, not the agency's. NOAA publishes a temperature; it does not say at what point water becomes pleasant. Those thresholds are an editorial guide, and the page labels them as such rather than lending them an authority they do not have.
• Inland, there is nothing to read, and the page says so rather than reaching for the nearest coast. Measured from Paris, finding any sea at all means widening the search to about two hundred kilometres — which would be an answer, but not the right one.