Recent Trends in Cloudiness over the United States: A Tale of Monitoring Inadequacies

Automated Surface Observation Systems (ASOS) were widely introduced to replace manned weather stations around the mid- 1990s over North America and other parts of the world. While laser beam ceilometers of the ASOS in North America measure overhead clouds within the lower 3.6 km of the atmosphere, they do not contain cloud-type and opacity information and are not comparable with previous cloud records. However, a network of 124 U.S. military weather stations with continuous human observations provides useful information of total cloud cover over the contiguous United States, thus lessening the disruption caused by the ASOS. Analyses of the military cloud data suggest an increasing trend (~1.4% of the sky cover per decade) in U.S. total cloud cover from 1976 to 2004, with increases over most of the country except the Northwest, although large uncertainties exist because of sparse spatial sampling. Thus, inadequacies exist in surface observations of global cloud amounts and types, especially over the oceans, Canada, and the United States since the mid- 1990s. The problem is compounded by inhomogeneities in satellite cloud data. Reprocessing of satellite data has the potential for improvements if priority is given to the improved continuity of records.