sky_brightness_temperature K [UPKA] The brightness temperature of the sky, or 'sky_temperature' as defined by ISO 9488 section 3.2.39 : "equivalent temperature of a blackbody radiating, on a horizontal surface, the same longwave radiation as the atmosphere".
The brightness temperature of a body is the temperature of a black body which radiates the same power per unit solid angle per unit area. It is strongly recommended that a variable with this standard name should have a units_metadata attribute with one of the values "temperature: on_scale" or "temperature: difference", whichever is appropriate for the data. This distinction is essential for correct unit conversion, because an on-scale temperature is measured relative to the origin of the temperature scale indicated by the units, whereas a temperature difference is independent of that origin. If the distinction cannot be determined, "temperature: unknown" should be used.