A Shift in Winter Season Timing in the Northern Plains of the Usa as Indicated by Temporal Analysis of Heating Degree Days

The temporal distribution of monthly heating degree days (HDD) approximates a bell-shaped curve, usually with a January maximum. The first moment, or centroid, of this distribution can be taken as the midpoint of the heating season. Analysis of seasonal HDD centroids reveals significant inter-decadal trends in the Northern Plains. From 1950 to 1990, the timing of the HDD centroid is observed to have advanced at a rate of 1.6 days per decade, a trend largely explained by cooling in October and warming in March and April. A trend of comparable magnitude (2.2 days per decade), but opposite direction, is observed for the period 1925–1950, and is again primarily associated with temperature trends in autumn and spring. The results suggest that changes in seasonal timing could be a feature of natural climate variability, and that it may be premature to infer an unprecedented climate change from a subtle shift in the timing of the winter season. Copyright © 2000 Royal Meteorological Society.