Solar radiation enters the Earth's atmosphere with a portion being scattered by clouds and aerosols.

Processing, archiving and distributing Earth science data
at the NASA Langley Research Center

Why do I sometimes get constant fill values in the L1B2 Terrain product?

For blocks which contain no land, the 14 most significant bits (MSB) of the Terrain product are assigned the fill value 16379, and the actual image data must be obtained from the Ellipsoid product.