Low-Level Cloudiness in the Appalachian Region

Abstract

Low-level (<2 km) cloud frequencies have been derived for the Appalachian Mountain region for the period 1985?88 based on in situ measurements by optical cloud and relative humidity sensors, and regional analyses incorporating the U.S. Air Force Real-Time Nephanalysis (RTNEPH) database. Statistics include cloud frequency as a function of elevation, season and time of day. The in situ results reveal that the higher Appalachian peaks (>1400 m) were in cloud an average of 29%?37% during the period, while peaks near 1000 m experienced cloud 11%?19% of all hours. RTNEPH regional results indicate that low-level cloud was most frequent between 900 m and 1300 m with a maximum at 1100 m. Orographic effects are probably responsible for the difference in these findings. Drought conditions during the period reduced overall cloudiness in the southern portions of the Appalachians, while more normal amounts were observed in northern areas. Cloud was found to be more abundant at night over the mountains in contrast to trends observed at regional airport sites.