Recent Temperature Changes during Overcast and Clear Skies in the United States

Abstract

A network of 14 nonurban stations was used as a first step toward understanding the character of change of daily temperature between 1948 and 1983 across the United States. Overall, in the eastern two-thirds of the country more stations tend to reflect statistically significant cooler weather than would be expected due to chance alone. In the western one-third of the country the opposite is found. Despite the cooling in the east, air masses with comparable characteristics (dewpoints, cloud cover) affecting these stations have warmed. This finding is in qualitative agreement with the expected impact of rising concentrations of greenhouse gases. Considering all stations regardless of location, a marked increase of daily minimum temperature was found in comparable air masses under overcast skies, which, when combined with the tendency for decreasing maxima, results in a highly significant (at 0.1% level) decrease of an implied diurnal temperature range. Both maxima and minima increased in clear weather after stratification by air mass characteristics. It is highly unlikely that the reported changes are related to boundary layer humidity or direction of air flow since the data were stratified for air mass identification by both surface dew-point temperature and wind direction.