Along this coast the wind blows from the northwest for much of spring and summer. It pushes the warm surface water out to sea, and cold water from deep in the Monterey Canyon rises to take its place. That is called upwelling, and it is why a California beach in July can be 54 °F while the air is 80.

In late summer and fall the upwelling winds ease. The sun has been heating the surface for months and the ocean current shifts, so the water in the bay warms up and often peaks in September and October, when the beach water is at its friendliest.

“vs normal” compares each spot with the average for the same date over 2003–2014. Red means warmer than that average, blue means colder. A warm anomaly can bring unusual fish and sea life, and a cold one usually means upwelling is active.

The map uses MUR, a NASA product that blends several satellites and buoys into one gap-free picture of the sea surface each day, at about 1 km detail. It runs about one day behind, and it is the temperature of the top skin of the water. Near shore it can differ by a few degrees from what you feel in the surf.