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

What are Up, Down and Net Fluxes?

Given the vertical layered structure of Earth atmosphere above underlying surfaces, the vertical variability of these fluxes is of particular interest. Hence the term "Up" and "Down" for characterizing the direction of flow of radiative fluxes at a particular level. Moreover, by counting in or out these "Up" and "Down" energy fluxes, one can define a "Net" flux that is ultimately responsible for the net energy loss or gain within any two such layers. This concept is important in defining the radiative heating or cooling of each atmospheric/surface element.