Increased cloudiness in the United States during the first half of the Twentieth Century: Fact or fiction?

Abstract

A new source of cloud and sunshine data, in conjunction with an inspection of the observing practices, instruments, and daily temperature range over the United States, are used to confirm or contradict the large increases of cloudiness previously reported. This analysis indicates that changes in observing practices are primarily responsible for the reported increase of cloudiness during the first half of the this Century. The response of the sunshine instruments and the surface daily temperature range over the United States are not consistent with the reported increases during the first half of this Century. Since the late 1940s however, the increase of cloud cover is confirmed by an inspection of the sunshine data, daily temperature range, changes in instruments, and the instructions to the observers. The types of changes in the cloud observing practices during the late 1930s and 1940s would tend to lead to increases in reported cloud amounts. Although we have not yet produced a combined sunshine/cloudiness index of cloud cover over the Twentieth Century, some cogent inferences can be made at this time: Monotonic trends of cloudiness on the Century time‐scale do not exist in the United States. Fewer clouds were present during the 1930s and early 1950s. There has been a tendency toward increased since 1948.