We have studied the 10 μm temperature of overcast skies, as measured from the ground, in a coastal region of South Australia. The long-term aim of this work is to be able to use measurements of the sky temperature, plus other readily measured ground-based parameters, to recognize the presence, or otherwise, of clouds. Comparisons have been made between the 10 μm sky temperature and cloud height in overcast conditions, measured both with a ceilometer and radiosondes. A low-altitude cloud exhibits a wide range of emissivities, such that clouds with base heights up to 1200 m have emissivities ranging from below 0.2 to close those expected for black bodies at those wavelengths. However, higher clouds exhibit different properties, as their temperatures approach those to be expected for a clear sky with water vapor.
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