Evidence for a Rising Cloud Ceiling in Eastern North America

Data from 24 airport weather stations along the north–south axis (35°–45°N) of the Appalachian Mountains are used to show a significant rising trend in cloud-ceiling height over the past three decades. The mean change in cloud-ceiling height was 4.14 ± 1.03 m yr−1[mean ± 1 SE (standard error),p≤ 0.001] across all stations. The trend was negative (−2.22 ± 0.67 m yr−1) for the six stations south of 37.5°N, but positive (6.26 ± 0.89 m yr−1) for the 18 stations north of this latitude. Mean ceiling height for broken cloud cover was higher and rising faster than mean ceiling height for overcast cloud cover. There were strong seasonal patterns that varied between the northernmost and southernmost stations; differences were most pronounced during the spring and summer months. Some of the potential ecological effects on high-elevation forests, where the transition from deciduous to coniferous forest is thought to be controlled by the height of the cloud base, are discussed.