Image (IMAGE) University of Tsukuba Caption Sea of clouds (SOC) formation was observed over three years in Nagano Prefecture. Large-scale SOCs were commonly observed in early morning, while small-scale SOCs in the eastern valley corresponded with low-level orographic clouds ascending mountain slopes. On days of wide-ranging SOCs, 67% were associated with a subsidence inversion layer from a synoptic-scale high pressure system, indicating that along with nocturnal radiative cooling, the subsidence inversion layer plays an important role in forming large-scale SOCs. Credit University of Tsukuba Usage Restrictions Unauthorized use is prohibited License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.