Temperature Trends

In last week's post I mentioned that the abundant cloud cover over Fairbanks in August produced much lower than normal diurnal temperature variations; the average day/night temperature difference was almost the smallest on record.  Below is a chart (click to enlarge) showing the average diurnal temperature range in August, beginning in 1930 for the university's Experiment Farm and 1952 for the official Fairbanks climate record (when observations moved to the airport).  The downward trend is highly statistically significant at both locations; late summer now tends to see less difference between daily high and low temperatures than in the past.

It's of interest to see how the trends in daily high and low temperatures contribute to the diurnal range trend, and to see how this varies through the year.  Accordingly, the chart below shows smoothed daily values of the 1930-2017 linear trends in each quantity at the Experiment Station.  The relative positions of the blue and red lines show that daily low temperatures have warmed more than daily high temperatures at all times of the year, and therefore the diurnal range has been reduced throughout the year.  Note that both high and low temperatures have cooled slightly in mid-autumn, but the diurnal range has still been reduced.

The largest change in diurnal range has occurred in mid-summer.  Daily low temperatures have risen by more than +0.7 °F/decade from mid-May through the end of July, but high temperatures have not changed significantly in high summer, and so the diurnal range trend peaks at a rather remarkable -0.85 °F/decade (8.5 °F/century!) around July 1st.  It's clear therefore that the reduction in diurnal range in summer is attributable to nighttime warming rather than daytime cooling; as a simple example, the Experiment Farm has seen only four days with a freeze in June in the past 30 years, but a summer freeze was not uncommon in the earlier years (31 freezes in June and July of the 1930s and 1940s).

Using Fairbanks airport data since 1952, there are some differences as we would expect, but some of the same signals are evident: the winter peak in overall warming, the autumn lack of warming, and the summer trend (albeit less pronounced) towards smaller diurnal range.

If we re-do the calculations for Experiment Station over the shorter time period, there is a lot more similarity between the results for the two sites.

Finally, a chart of the June-July diurnal temperature range illustrates the changes at the two sites in the high summer months.  I'm not sure what was going on at the airport in the 1990s, and one does wonder if the transition to ASOS instruments in 1998 could have affected the numbers.  But the long history of consistent data from the Experiment Farm tells a clear story of remarkable change in this aspect of Fairbanks climate.