Review of Past Studies
There have been a large number of studies from specific countries or regions documenting spatial and temporal variations and trends in cloud cover (Henderson-Sellers 1992; Sun and Groisman 2000; Kaiser 2000; Groisman et al. 2004; Dai et al. 2006; Warren et al. 2007). These studies have focused mostly on long-term trends for Europe (Henderson-Sellers 1986; Warren et al. 2007; Sanchez-Lorenzo et al. 2012), North America (Henderson-Sellers 1989; Milewska 2004; Warren et al. 2007), Australia (Jones and Henderson-Sellers 1992; Jovanovic et al. 2011), the United States (Sun 2003; Sun and Groisman 2004; Groisman et al. 2004; Dai et al. 2006) and the former Soviet Union (Sun and Groisman 2000; Sun et al. 2001). Analyses of cloud data suggest increased total cloud cover over the United States (Karl and Steurer 1990; Sun 2003; Groisman et al. 2004; Dai et al. 2006; Warren et al. 2007), Canada (Milewska 2004), the former Soviet Union (Sun and Groisman 2000; Sun et al. 2001), Western Europe, mid-latitude Canada, Russia (Chernokulsky et al. 2011; Sanchez-Lorenzo et al. 2012) and Australia (Jones and Henderson-Sellers 1992; Jovanovic et al. 2011). In contrast, a decreasing trend in the total cloud cover has been revealed over much of China during 1951-1994 (Kaiser 1998, 2000) and 1954-2005 (Xia 2010), over the United States since the 1980s (Sun 2003; Sun and Groisman 2004), over India during 1961-2007 (Jaswal 2010), most of South Africa during 1960-2005 (Kruger 2007) and Italy during 1951-1996 (Maugeri et al. 2001). On the other hand, total cloud cover trends are also period-specific over some regions. Several authors have found significant positive trends in the United States total cloud for periods beginning in or after 1947 and ending before 1996 (e.g., Angell 1990; Plantico et al. 1990; Sun et al. 2001), but Elliott and Angell (1997) found no significant trend in cloudiness for 1973-1993. Sun (2003) indicates increasing trends from about 1950-1990 but declining cloud cover from the 1980s to 2000, suggesting that the increasing trend might be limited to the period from about 1950 to the 1980s. On continent scale, Warren et al. (2007) found a large decrease for South America, small decreases for Eurasia and Africa and no trend for North America in total cloud cover during 1971-1996. Warren et al. (2007) have attributed decreasing trends in cloud cover on many continents, and that some of these trends are linked to ENSO variations. However, cloud feedback remains an uncertain source in global climate change (IPCC 2007).
Published studies about long-term trends in total cloud cover over India have reported decreasing trends (Rao et al. 2004; Warren et al. 2007; Biggs et al. 2007; Jaswal 2010). Rao et al. (2004) studied long-term trends in annual total cloud amount over 15 stations in India and found decreasing trends at 11 stations. Biggs et al. (2007) have reported decrease in annual total cloud amount by 0.09 % per year during 1952-1997. Decreasing trends in total cloud cover over India during 1961-2007 were reported by Jaswal (2010). While on monthly scale, statistically significant decrease has occurred during 1961-2007 for April (3 % per decade), June to September (2 % per decade) and December (5 % per decade); decline in total cloud cover is significant for annual, summer and monsoon (Jaswal 2010). However, Roy and Balling (2005) have found significant increase in cloud cover during summer over Jammu and Kashmir. In our neighbouring country China, total cloud cover decreased up to the middle of 1990s and then levelled off at much of synoptic stations (Xia 2010; Zong et al. 2012).