  Theoretical calculations of the cooling potential of radiative cooling
materials are crucial for determining their cooling capability under different
meteorological conditions and evaluating their performance. To enable these
calculations, accurate models of long-wave infrared downwelling atmospheric
irradiance are needed, However, the transmittance-based cosine approximation,
which is widely used to determine radiative cooling potentials, does not
account for the cooling potential arising from heat loss to the colder reaches
of the atmosphere itself. Here, we show that use of the approximation can lead
to > 10% underestimation of the cooling potential relative to MODTRAN 6
outputs. We propose a temperature correction to the transmittance-based
approximation which accounts for heat loss to the cold upper atmosphere, and
significantly reduces this underestimation, while retaining the advantages of
the original model. In light of the widespread and continued use of the
transmittance-based model, our results highlight an important source of
potential errors and a means to correct for them.
