Why Is My SST Chart Blank?

A blank stretch of ocean on a satellite chart almost always means the sensor was blocked, by cloud, by nightfall on an optical-only instrument, or by the edge of the satellite's swath, not that the app failed to load anything.

The three real causes of a gap

• Cloud. Infrared sensors (VIIRS, MODIS) measure radiation leaving the sea surface, and cloud blocks it almost completely. This is the most common cause by far.
• Daytime-only sensors, no daylight. Chlorophyll passes specifically require sunlight reflecting off the water, so any pass captured at night has no usable reading at all: not a gap, a complete miss for that overflight.
• Swath edge. A satellite only images a fixed-width strip of the Earth per orbit; the region you're looking at may simply have fallen outside that strip on a given pass.

Pass gaps are real; composite gaps are rarer

A raw pass shows gaps often: it's one instrument's one overflight, with no filling. A composite is built specifically to minimize this by blending multiple sensors (see how cloud-free SST works), so a genuinely blank composite pixel means every contributing sensor missed that spot: rare, but it happens.

How the map labels a real gap vs. a bug

Chlorofishy never silently shows stale data as current, and never shows a blank tile without explanation. A satellite pass with no reading at a clicked point reads “No usable data”; a composite gap reads “No data (gap)”. If you see either label, the map is working correctly and telling you the truth about what the satellite saw. See why there are holes for the full mechanics, including how land is masked separately from a real data gap.

If a chart is blank everywhere, not just in patches, something else may be wrong. See troubleshooting. Otherwise, step back a pass or two on the SST map timeline, or switch to the composite for full coverage. Back to Learn.