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 t hat of the total cloud cover. Analysis of the occurrences of cloud types su ggests that the decrease in low-level cloud cover was primarily due to the decrease in stratiform clouds. The presence of stratiform clouds is general ly related to atmospheric static stability and/or frontal system passages. Thus, the reported decrease in occurrence of these clouds could be caused b y weakened atmospheric stability and/or circulation associated with the inc rease in surface air temperature and reduction of the meridional temperatur e gradient. A spectacular increase in cumulus clouds over the FUSSR has bee n discovered over the past several decades. This increase (bring supported by a significant increase of cirrus clouds) is a manifestation of intensifi ed convective processes over Northern Eurasia in the second half of the 20t h century, and is consistent with the observed increase in heavy precipitat ion. While over the European part of the FUSSR the increase in heavy precip itation (above 20 mm/day) is a part of the general increase in annual and s ummer precipitation totals over the past century and/or past 50 years, a st atistically significant increase in heavy precipitation in Siberia has occu rred 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 particu larly 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 availa ble. Copyright (C) 2000 Royal Meteorological Society.