Recent changes in cloud-type frequency and inferred increases in convection over the United States and the former USSR

Significant changes and a general redistribution in the frequencies of vari ous cloud types have been observed during the past 40-50 years over the mid latitude land areas of the Northern Hemisphere. This is evident for North A merica and northern Eurasia in the daytime synoptic data of the United Stat es and the former Soviet Union (FUSSR). An abrupt increase prior to the 196 0s largely contributed to the upward trend in the frequency of convective c louds over both regions, particularly in the warm season. However, over bot h regions during the intermediate seasons and during the winter season over the FUSSR, the frequencies of convective clouds still showed gradual incre ase after the 1960s. The increase in the frequency of convective clouds has been accompanied by increases in the frequency of observation of high-leve l cloudiness (at elevations above 6 km) and heavy precipitation. Low cloudi ness (stratiform types) has decreased over the FUSSR but increased over the contiguous United States. The latter increase was due to an increase in th e frequency of stratocumulus clouds, while the frequency of stratus clouds has decreased. Generally, it appears that during the post-World War II peri od over the FUSSR high cloud-type frequencies increased and low cloudiness decreased with a relatively small change (increase) in total cloud cover, w hile over the United States cloud cover has increased at both low and high levels. The analyses of cloudiness information from the United States and t he FUSSR reveal noticeable differences in definitions and observational pra ctices that affect the estimates of climatology and interpretation of the r esults presented here in terms of changes of convective activity and its re lation to precipitation in these two regions of Eurasia and North America.