Updated daily · Satellite analysis of one day, about two days old (NASA JPL MUR)
Sea temperature
The colour of the sea is its surface temperature, measured from space by several satellites and checked against ships and buoys. Warm water drives the weather above it: it feeds rain and typhoons, and it keeps coastal winters mild.
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How it is measured
Satellites measure the heat given off by the sea surface. Infrared cameras see fine detail but not through clouds; microwave sensors see through clouds but in less detail. NASA's Multi-scale Ultra-high Resolution analysis (MUR) blends both with readings from ships and drifting buoys into one map for each day, with points about 1 km apart. It shows the temperature just below the thin top layer that the afternoon sun warms, so the map does not change between day and night.
Warm pools and cold coasts
The warmest open water, often above 29 °C, lies in the western Pacific and the eastern Indian Ocean: the Warm Pool. Off Peru, California and south-west Africa the sea is surprisingly cold, because the wind pushes the surface water away from the coast and cold water rises from below to replace it. That rising water is rich in nutrients, which is why these coasts have some of the richest fisheries in the world.
Currents carry heat
Warm currents such as the Kuroshio, which flows north past the east coast of Taiwan, and the Gulf Stream in the Atlantic show as warm tongues reaching towards the poles. Zoom in to see their swirls and eddies, tens to hundreds of kilometres across.
The 26.5 °C line
Typhoons and hurricanes take their energy from warm sea water. They usually form only over water of about 26.5 °C or warmer, and they weaken quickly over cooler water or land. In late summer the line reaches well north of Taiwan; in winter it retreats to the south. Warm water alone is not enough: a storm also needs moist air and light winds high above it.
Change from normal
The anomaly map compares the sea with the average for the same day of the year over 2003–2014. Red is warmer than usual, blue cooler. It reveals El Niño and marine heatwaves, long spells of unusually warm water that can bleach coral reefs. Much of the ocean now runs warmer than this baseline, because the ocean has absorbed about nine-tenths of the extra heat trapped by greenhouse gases.
Did you know?
The top three metres or so of the ocean can hold as much heat as the whole atmosphere above it. That is why the sea warms and cools so slowly, and why cities by the sea have milder seasons than cities inland.