A model of thermal radiation from partly cloudy and overcast skies

A mathematical model for predicting thermal radiation from partly cloudy and overcast skies was developed. The model uses previous clear sky equations for predicting full‐spectrum sky emittance and 8–14 μm atmospheric window transmittance and then assumes that the cloud contribution to sky thermal radiation must be transmitted to the earth's surface through the atmospheric window. Unlike previous ‘cloud corrections,’ the cloud contribution is computed from surface vapor pressure and temperature, cloud amount, and elevation data, as recorded by National Weather Service observers. The average predicted increase due to 100% cloud cover was 18% for Phoenix in 1978 with a theoretical upper limit of about 40%. Comparisons were made between model predictions and measured values of sky radiation for a wide range of weather conditions. The agreement was excellent with an overall coefficient of determination for fit to a 1:1 line of 0.941 on an hourly basis and of 0.960 on a daily total basis.