1931. International Institute of Applied Systems Analyses, Leemans, Rik, Cramer, Wolfgang P., Rijksinstituut voor Volksgezondheid en Milieuhygi???ne (Netherlands), and National Institute of Public Health and Environmental Protection (Netherlands). This datalayer is a point shapefile that shows long-term (1931-1960) mean monthly values for cloud cover on a global terrestrial grid at 0.5 degree... American Geological Institute.