North American total cloud amount variations this century

Abstract

Records of mean monthly total cloud amount from 143 locations in North America have been assembled. Generally the low and middle latitude station records extend over the period 1900–1984 (U.S.A.) and 1900–1982 (Canada) but few Arctic stations have records before 1930 and some begin recording cloud amount as late as the 1960s. The low and middle latitude station records show a tendency for total cloud amount to increase over this century. Only one of the 77 continental U.S.A. stations does not show an increase. The high latitude stations record increasing total cloud amount in the summer (June, July, August) season but not in the annual mean. The records show the largest increase between about 1930 and 1950. They are temporally consistent but do not exhibit significant spatial coherence. The history of observing and reporting practice has been carefully examined; if any significant effect were to be expected from the changes documented it would be a decrease at the time when the greatest recorded increase occurs. Other factors associated with increased population are possible “explanations”. The most likely cause of increased cloud amount (if the temporal trend is real) is anthropogenerated clouds in the form of jet aircraft condensation trails but the large (1930–1950) increase identified here pre-dates the introduction of widespread commercial airflights even in the U.S.A.