Recent Trends in Cloudiness over the United States

From space, Earth is a blue planet marked by stark white cloud structures that distinguish it from other planets. By ref lecting sunlight, blocking outgoing longwave radiation, and producing precipitation, clouds have an enormous impact on Earth’s weather and climate. The single largest source of uncertainty in global climate models has always been the response of clouds, especially low clouds, to climate change (Houghton et al. 2001), and it is still a challenge to simulate the observed climatologies of clouds in models (e.g., Dai and Trenberth 2004). It is therefore important to monitor changes in Earth’s cloud cover and other properties, such as their vertical structures (Wang et al. 2000) and optical thickness. Traditionally, clouds have been observed visually by trained technicians at weather stations and onboard ships around the world (often in units of eighths, or oktas), following the general rules outlined by the World Meteorological Organization (WMO: 1975). These cloud data, together with other synoptic observations, are transmitted through the Global Telecommunication System (GTS) in real time to weather centers around the world. In the United States, the National Oceanic and Atmospheric Administration’s (NOAA’s) National Centers for Environmental Prediction (NCEP) receive and process the real-time GTS data, which are then archived at the National Center for Atmospheric Research (NCAR) and at NOAA’s National Climatic Data Center (NCDC). Many countries, such as the United States, also build their own national archives of weather and climate using the data collected from national stations. Individual visual observations of clouds by trained humans are subjective measures of sky cover (albeit RECENT TRENDS IN CLOUDINESS OVER THE UNITED STATES A Tale of Monitoring Inadequacies