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The detection of cloudiness is investigated by means of partial and totalcloud amount estimations from pyrgeometer radiation measurements and visibleall-sky imager observations. The measurements have been performed inWesterland, a seaside resort on the North Sea island of Sylt, Germany, duringsummer 2005.  An improvement to previous studies on this subject resulting inthe first time partial cloud amounts (PCAs), defined as cloud amountswithout high clouds calculated from longwave downward radiation (LDR)according to the APCADA algorithm (Dürr and Philipona, 2004), arevalidated against both human observations from the <i>National MeteorologicalServive</i> DWD at the nearby airport of Sylt and digital all-sky imaging. Theaim is to establish the APCADA scheme at a coastal midlatitude site forlongterm observations of cloud cover and to quantify errors resulting from thedifferent methods of detecting cloudiness.  Differences between the resulting total cloud amounts (TCAs), defined ascloud amount for all-cloud situations, derived from the camera imagesand from human observations are within ±1 octa in 72% and within±2 octa in 85% of the cases. Compared to human observations, PCAmeasurements, according to APCADA, underestimate the observed cloud cover in47% of all cases and the differences are within ±1 octa in 60% and±2 octa in 74% of all cases. Since high cirrus clouds can not bederived from LDR, separate comparisons for all cases without high clouds havebeen performed showing an agreement within ±1(2) octa in 73(90)% forPCA and also for camera-derived TCA. For this coastal mid-latitude site underinvestigation, we find similar though slightly smaller agreements to humanobservations as reported by Dürr and Philipona (2004). Though limited todaytime, the cloud cover retrievals from the sky imager are not really affectedby cirrus clouds and provide a more reliable cloud climatology for all-cloudconditions than APCADA

Total and partial cloud amount detection during summer 2005 at Westerland (Sylt, Germany)