Sources
- NOAA National Data Buoy Center (NDBC): buoys and coastal stations, including NOAA tide stations that NDBC republishes. Read hourly.
- NOAA Tides & Currents (CO-OPS): tide stations not available through NDBC. Read hourly.
- U.S. Geological Survey: water temperature gauges on lakes, reservoirs and bays. Read hourly.
- NASA JPL MUR sea surface temperature: a daily, roughly 1 km global analysis that also covers large lakes. We read the original grid republished by the Australian Antarctic Division. It is a “foundation” temperature (below the daytime-warmed surface layer) and usually lags one to two days.
- National Weather Service: air temperature, forecast and active alerts (rip currents, beach hazards, surf, small craft).
Which reading is the headline
- Beaches: the nearest NOAA tide station or shore station within 12 km (7.5 miles), or a NOAA buoy within 8 km, that reported in the last 6 hours.
- Lakes: a USGS gauge on the same lake (its name must match the lake), so a gauge on a neighboring reservoir is never used.
- Otherwise, the satellite value for the nearest water pixel within 6 km.
Bay, creek and estuary gauges and offshore buoys are shown as extra readings, never as the beach headline, because back-bay and offshore water can differ a lot from the surf.
Quality checks
- Values outside −2°C to 40°C are discarded, along with values flagged by NOAA.
- Satellite readings must be no more than five days old. We verify the analysis date, original grid, temperature units and missing-value markers before accepting a reading.
- A sensor reading that differs from the satellite value by more than 4°C (about 7°F) is treated as suspect and not used as the headline. In a comparison of 71 locations with both, in-water sensors averaged 0.7°C warmer than the satellite and differed by 1.3°C on average, consistent with afternoon warming at the surface.
- Real-time data from all agencies is provisional and can be revised.
Monthly averages
Monthly averages come from NASA JPL’s monthly MUR product for 2016–2025 at the location’s nearest water pixel: the average of the ten monthly means, plus the coldest and warmest of those ten. The “vs. average” comparison uses the current satellite value against that month’s average, so both numbers come from the same instrument.
Coverage
We cover beaches with nearby sensors or satellite coverage, the Great Lakes and larger lakes that the satellite analysis resolves. Many smaller reservoirs have neither a public real-time sensor nor satellite coverage, so they don’t have pages yet.
See also: satellite vs. buoy temperatures and cold water safety.