• Total cloud cover from various methods is in 65-85% within ±1 okta • Sky camera overestimates cloudiness with respect to other automatic methods • Clouds can be correctly classified in 50-90% of cases using sky cameras • Figure S1 • Movie S1 Abstract We present observations of total cloud cover and cloud type classification results from a sky camera network comprising four stations in Switzerland. In a comprehensive intercomparison study, records of total cloud cover from the sky camera, long-wave radiation observations, Meteosat, ceilometer, and visual observations were compared. Total cloud cover from the sky camera was in 65-85% of cases within ±1 okta with respect to the other methods. The sky camera overestimates cloudiness with respect to the other automatic techniques on average by up to 1.1 ± 2.8 oktas but underestimates it by 0.8 ± 1.9 oktas compared to the human observer. However, the bias depends on the cloudiness and therefore needs to be considered when records from various observational techniques are being homogenized. Cloud type classification was conducted using the k-Nearest Neighbor classifier in combination with a set of color and textural features. In addition, a radiative feature was introduced which improved the discrimination by up to 10%. The performance of the algorithm mainly depends on the atmospheric conditions, site-specific characteristics, the randomness of the selected images, and possible visual misclassifications: The mean success rate was 80-90% when the image only contained a single cloud class but dropped to 50-70% if the test images were completely randomly selected and multiple cloud classes occurred in the images. In recent years, the use of hemispherical sky cameras has been steadily increasing. While the first camera systems, a series of Whole Sky Imagers developed by the Scripps Institution of Oceanography at the University of California [e.g., Johnson et al., 1989; Shields et al., 2003 Shields et al., , 2013 and the commercially distributed Total Sky imager [Long and DeLuisi, 1998 ], were relatively expensive due to the high-quality optics and engineering involved, the rapid development of low-cost digital cameras has favored their application for WACKER ET AL. cloud observations. Numerous research groups nowadays use various camera systems developed for their specific needs [e.g., Feister and Shields, 2005; Kalisch and Macke, 2008; Kreuter et al., 2009 ].
Other pixel clustering and segmentation based approaches REF [6] explicitly measure the cloud cover from sky-images in terms of meteorological unit of okta (the number of eighths of the sky occluded by clouds) and cloud type.
Cloud observations in Switzerland using hemispherical sky cameras
Journal of Geophysical Research
Geology