Differences in the emission efficiency (emissivity) of surfaces are greatest at the lower frequencies (such as AMSU 23 and 31 GHz, SSM/I 19 GHz channels). Notice in the individual channel imagery the large differences in brightness temperatures between ocean and land. Ocean has a very low and uniform emissivity (0.40 - 0.50 vs ~0.9 for land), making oceanic areas ideal for the retrieval of cloud and precipitation properties.

Also evident over the oceans are relatively bright (warm) features associated with emission by water vapor and water clouds. These emissions represent a net increase in the radiance to space by the ocean surface and therefore result in observed brightness temperatures that are notably larger.