APCADA was adapted to several longtime series of longwave downward radiation (LDR) measurements worldwide and results were compared to eye observations of cloud amount during day- and nighttime. It is important to note that thin cirrus clouds are not detectable. Hence, APCADA calculates the total cloud amount without high clouds (called partial cloud amount). We found a probability of about 85% that the difference of APCADA estimate and eye observation does not exceed 1 octa. One octa means one-eighth of visible sky surface covered by clouds. The probability even rises over 90% for maximum 2 octas difference.
APCADA participates at the International comparison campaign for Temperature, hUmidity and Cloud profiling (TUC-2003) at Payerne in winter 2003/2004.
I want to thank Rolf Philipona who provided the Infrared Traveling Standard for APCADA to use at the TUC-2003 campaign in Payerne. Many thanks also to Thomas von Allmen for the web interface, and Ursin Soler and Christian Thomann for the setup of the data collection device in Davos.
Automatic cloud amount detection by surface longwave downward radiation measurements, B. Duerr and R. Philipona, Journal of Geophysical Research, Vol. 109, doi:10.1029/2003JD004182, 2004