And now for a different topic. I'm sure many readers are aware that daily maximum and minimum temperatures are automatically recorded by the ASOS observing platform at airports across Alaska, and these measurements generally go into the books unaltered as the "official" climate observations. This isn't always true, however; one reason is that sometimes the ASOS report is in error. There happened to be an example of this in Fairbanks this week, when the ASOS reported a 24-hour (midnight-to-midnight) high temperature of 59°F on Thursday, but in reality the high was only 51°F. Oddly the 6-hourly maximum temperatures reported by the ASOS (at 0, 6, 12, and 18 UTC) were correct for Thursday, but somehow the 24-hour maximum wasn't. The error was caught by NWS personnel and the climate record already shows the correct number.
Looking back at the history of ASOS measurements in Fairbanks, it seems that manual adjustments to the ASOS data are not common, but neither are they rare. The charts below show the daily differences between the final "official" numbers and the ASOS reports for June through August. Curiously it seems that the daily low temperature is most often adjusted down by a single degree Fahrenheit. This didn't happen in summer 2016, but in August 2015 it occurred 4 times between August 10 and August 19.
It's strange and a bit unsettling to find that the ASOS temperature reports are apparently erroneous with some frequency, but in Fairbanks the errors are not large or frequent enough to cause significant differences in the long-term temperature averages; and in any case presumably the vast majority of the errors are noticed and corrected. The problem is intriguing, but it doesn't make a big difference in the grand scheme of things.
However, the same is not true of the data situation at Bettles - see the charts below. The maximum temperature differences show rather more frequent and significant changes, and the minimum chart is frankly shocking, with very frequent and mostly downward adjustments to the ASOS data, especially in recent years. Remarkably, in June-August of this year, the daily low temperatures were adjusted downward on 32 of 92 days, by an average of 3.3°F. In summer 2014, adjustments averaging 2.9°F were performed on 70 of 92 days. The question arises immediately as to whether this is an appropriate fix to a very bad ASOS problem, or whether the adjustments themselves are the problem.
The overall impact of the low temperature adjustments at Bettles is significant, as demonstrated in the chart below. In summer 2014, when the adjustments were greatest, the seasonal mean of daily minimum temperatures changed from 46.4°F to 44.2°F; this difference is comparable to the typical magnitude of changes from year to year.
So what is going on at Bettles? Well, I've learned that the FAA contract observer believes that the ASOS minimum temperatures are generally too high (at least in summer) and regularly adjusts them down, purportedly based on other thermometers. The adjustment is different from day to day and is often zero. From a scientific standpoint, this is difficult to accept, because a systematic error at the ASOS thermometer would show up all the time, not some of the time, and it would presumably affect daily maximum temperatures as well. NWS personnel have inspected the ASOS temperatures repeatedly on-site and have not uncovered a problem. Unfortunately there is nothing that can be done about the issue, because the FAA manages the observing program, and the accuracy of climate measurements are not a high priority for that agency.
Additional evidence indicating that the Bettles low temperature data have been corrupted can be found in a comparison of the official (airport) temperatures to the nearby SnoTel site at Bettles - see below. The SnoTel site runs much colder for overnight lows in summer, but the difference is consistent and the seasonal means are highly correlated from year to year. Notice, however, the trend towards smaller differences over time, as the official low temperatures have been adjusted downwards more often in recent years, and especially in 2014 and 2016. Remarkably, the differences between the two sites were smaller in every year from 2010-2016 than in any year from 2003-2009; there isn't much chance that this could happen at random, and accordingly the trend in the differences is statistically significant.
Here's what the chart would look like if the ASOS temperatures were left alone. There is still a slight trend towards smaller differences over time, but it's not statistically significant.
For additional context on ASOS errors, the daily temperature adjustments are shown below for McGrath. The situation there looks a lot more like Fairbanks, which suggests that this kind of frequency and magnitude of errors are typical. In contrast, the data adjustments at Bettles are highly atypical and (in my view) clearly erroneous; and this is quite unfortunate for the integrity of the long-term climate record in Alaska.