Cloudiness variations over the former Soviet Union

In the former Soviet Union (FUSSR), low-level cloud cover has significantly decreased over the period 1936-1990. This change has an opposite sign to that of the total cloud cover. Analysis of the occurrences of cloud types suggests that the decrease in low-level cloud cover was primarily due to the decrease in stratiform clouds. The presence of stratiform clouds is generally related to atmospheric static stability and/or frontal system passages. Thus, the reported decrease in occurrence of these clouds could be caused by weakened atmospheric stability and/or circulation associated with the increase in surface air temperature and reduction of the meridional temperature gradient. A spectacular increase in cumulus clouds over the FUSSR has been discovered over the past several decades. This increase (bring supported by a significant increase of cirrus clouds) is a manifestation of intensified convective processes over Northern Eurasia in the second half of the 20th century, and is consistent with the observed increase in heavy precipitation. While over the European part of the FUSSR the increase in heavy precipitation (above 20 mm/day) is a part of the general increase in annual and summer precipitation totals over the past century and/or past 50 years, a statistically significant increase in heavy precipitation in Siberia has occurred when the precipitation frequency and totals declined over the past 60 years. The present findings (increase in cumulus and decrease in stratiform clouds) explain these phenomena. Changes in total cloud cover, and particularly in its components, indicate that cloud interaction with other climate variables is changing somewhat. Therefore, it would be important to extend this study to other regions of the globe when more related data are available. Copyright (C) 2000 Royal Meteorological Society.