A quantitative evaluation of surface-based observations of oceanic cloudiness

Observations of cloud amount and cloud character are made regularly at ocean locations worldwide, predominantly from civilian ships but reports are also obtained from military vessels, research and rescue ships, voyaging boats, and oil and other drilling platforms. There is a long historical record of these observations of oceanic cloudiness but, to date, very few attempts have been made to try to establish the verity of this cloud climatology. All-sky camera photographs were obtained from voyages of research vessels in the summer of 1986 in the Atlantic and Indian oceans and the Mediterranean Sea. Detailed analysis of this data base generally validates the cloud observations made by the ships officers. Agreement between the synoptic observation and the image analysis was usually within one okta (eighth) of sky cover for total cloud amount. On the other hand the photographs reveal a considerable temporal heterogeneity in oceanic cloud amount. It is not clear whether the synoptic reports sample cloud amount adequately. The all-sky photographs underline the difficulty of identifying cirrus cloud. Occasionally ship reports miss clearly identifiable high cloud.