PNNL

Abstract

We present here an overview of sky imaging, and techniques that may be applied to the analysis of full color sky images to infer cloud macrophysical properties. Details of two different types of sky imaging systems developed by the authors are presented, one of which has been developed into a commercially available instrument. Retrievals of fractional sky cover from automated processing methods are compared to human retrievals, both from direct observations and visual analyses of sky images. Although there exists some uncertainty in fractional sky cover retrievals from sky images, this uncertainty is no greater than that attached to human observations for the commercially available sky imager retrievals, with still acceptable uncertainty with the research system used in Girona, Spain. Thus, the application of automatic digital image processing techniques on sky images is a useful method to complement, or even replace traditional human observations of sky conditions for cloud cover and potentially cloud type. Additionally, the possibilities for inferring other cloud parameters such as cloud brokenness and solar obstruction of benefit for scientific research further enhance the usefulness of sky imagers.