The longwave effective cloud fraction is derived from surface longwave measurements by pyrgeometer, interferometer, and cloud base height measurements. Results from two sets of radiance thresholds, one for low clouds and another more sensitive to high clouds, are presented and compared to the opaque total sky cover from a sky imager. With thresholds set for the low clouds there was some agreement with the sky imager for lower clouds, but agreement decreased as cloud base height increased.

When the threshold was set for high clouds there was increased high cloud detection, at the cost of overestimating cloud amounts for middle and low clouds. As with any method for cloud detection from surface longwave measurements, there are two problems. First, clouds are harder to detect as temperature and water vapor amount increases because of masking by increased gaseous emission. Second, multiple cloud layers make it difficult to determine proper threshold values for cloud detection.