Measurements of Cloud Emissivity in the 8–13 μ Waveband

Abstract

To determine the emissivities of clouds, measurements of the zenith radiative intensity in the atmospheric window were made for 20 months. The height of the cloud base was measured by stereophotography for high and medium clouds and by ceilometer for low clouds. Radiosonde soundings were used to find the cloud hue temperature and, together with the known absorption coefficients of water vapor and carbon dioxide, to compute the effects of the intervening atmosphere. The low cloud emissivity was found to be 100 ± 3%. The emissivity of altocumulus was found to range from 30?100%, with a mean of 79 ± 6%. The emissivity of cirrus was found to range from zero to 100% with a mean of 35 ± 5%. This latter value is significantly less than the value of 50% which has previously been used, and has a corresponding effect on calculations of the longwave radiation field in the atmosphere.