How Do Cloud-Free SST Charts Work?

A cloud-free SST composite exists because no single satellite instrument can see through cloud on its own, so instead of waiting for a clear day, the analysis blends several instruments that fail in different places, filling in what one alone would miss.

Why any one pass has gaps

Infrared sensors (VIIRS, MODIS) measure the thermal radiation leaving the sea surface, and cloud blocks that radiation almost completely. Any single overflight with cloud in the way simply has no reading underneath it. That's what a raw pass looks like on a cloudy day, and it's a real limitation, not a bug.

The fix: a different kind of sensor, plus blending

Microwave sensors like AMSR2 aboard GCOM-W measure at wavelengths that pass through non-raining cloud, at the cost of a much coarser footprint: about 25km, versus roughly 1–2km for the infrared sensors. A cloud-free composite works by combining infrared passes (sharp, but gapped by cloud) with microwave passes (blurry, but gap-free) and prior observations, using a spatial analysis method to produce one complete field. Chlorofishy's SST composite is NASA's GHRSST MUR (Multi-scale Ultra-high Resolution) Level 4 analysis, which JPL's own documentation describes as combining data from AVHRR, MODIS, and AMSR-E/AMSR2 using Multi-Resolution Variational Analysis, a wavelet-based method built specifically to handle the irregular spacing of real satellite measurements.

What “Level 4” means

GHRSST's own product tiers make the distinction explicit: Level 2 and Level 3 products carry only the pixels a sensor actually measured on one pass; a Level 4 “foundation” analysis like MUR is a gap-free, spatially complete estimate produced by combining and modeling multiple sources. Cloud-free doesn't mean every pixel is a direct measurement: it means the analysis fills what a single instrument couldn't see, using real measurements from elsewhere in space and time as its input. See is cloud-free SST real or interpolated for what that means pixel by pixel, and microwave SST for more on the sensor that makes cloud-penetration possible at all.

See the composite for yourself on the SST map for any region, or check data sources for the full sensor and resolution list. See how our named-pass approach compares to other cloud-free composite services on Chlorofishy vs. SatFish. Back to Learn.