Methodology, sources and limitations

Every temperature on this site is measured, not modelled. This page says where it comes from and — more usefully — what it cannot tell you.

The data

NOAA's Optimum Interpolation Sea Surface Temperature (OISST v2, AVHRR): a quarter-degree daily satellite analysis, retrieved through NCEI ERDDAP, sampled every five days and averaged into monthly means. Fetched once and cached, so nothing is queried when you load a page.

Whether a city counts as coastal is decided by the data itself. OISST returns no value over land, so the nearest cell with a reading within 25 km becomes that city's sea point. Cities without one get no page. No coastline dataset is involved — the land mask is a by-product of the measurement.

The swimming season

The season is calculated as the longest unbroken run of months above the threshold, wrapping through December. That matters in the southern hemisphere, where a naive reading would report a summer season as "January to December".

What the numbers cannot tell you

• Open water, not your beach. A quarter-degree cell is about 28 km across. Sheltered bays, lagoons and estuaries run warmer; exposed coasts with upwelling run colder, sometimes by several degrees within a few kilometres.
• Surface only. In summer a warm surface layer often sits above much colder water, and you find the boundary by swimming into it.
• A six-year average. The record used here spans about six years, not the thirty a climate normal requires. The ocean's thermal inertia keeps year-to-year variation small, so this is close — but it is not a normal, and any individual year differs.
• Monthly means hide everything interesting. An average says nothing about the week a cold current arrived, and nothing at all about waves, tides, currents or water quality — which is what actually decides whether a given swim is a good idea.

This is not safety advice

Knowing the average temperature is not knowing the conditions. Cold shock, rip currents, tides and boat traffic kill strong swimmers every year. Check local conditions and local advice, tell someone where you are going, and do not swim alone.