Cold Revisited

Now that the weather has calmed down, it's a good time to look back at a couple of aspects of the cold spell in Fairbanks.

First, I've been meaning to compare the cold to previous episodes on various time scales.  From a historical standpoint, the most significant aspect of the cold snap was arguably its long duration, rather than its extreme intensity at any given time.  The widely-cited headline statistic is certainly the most notable: the temperature at the airport stayed below 0°F for 32 consecutive days, the most in over a century.  The record is 41 such days in 1917-18.

In terms of individual days, the daily maximum temperature of -46°F at the airport on January 4th was the lowest since 1977; even the cold snaps of 1989 and 1999-2000 didn't have a day that cold.

A lot more statistics could be compiled, but Rick Thoman saved me a lot of work with this summary (click to enlarge);

A curious aspect of the cold spell was that the international airport was colder than any other reporting site in the Fairbanks area except for North Pole.  This is unusual: the two longstanding co-op sites in the Goldstream Valley ("Ester 5NE" and "Goldstream Creek") are typically significantly colder than the airport.  The discrepancy was large enough that a question has been nagging at me: is it possible that the airport thermometer has been running too cold, so that the cold snap wasn't actually quite as severe as reported?

To illustrate the issue, here's a look at the joint distribution of daily mean temperatures from Fairbanks airport and Goldstream Creek since 2012, for the date window from December 1 through January 15:

Based on data from 2012-2024, Goldstream Creek is typically colder than the airport on more than 75% of days in December and January, but the chart shows that most of the coldest days in the recent cold spell were colder at the airport.  The airport's cold "advantage" emerged in mid-December as the cold snap deepened:

I decided to dig into this further by calculating the 2012-2024 December average temperature difference between the airport and several other valley-level sites around the area, and then examining how these differences vary from year to year.  The idea is to see if last month's cold anomaly at the airport is outside the bounds of usual variability.

The following figure shows the result of my calculations.  Points above the horizontal zero line indicate Decembers that were relatively warmer at the airport compared to the usual difference for the site indicated; for example, in December 2021 the airport was warmer than would normally be expected based on any of the other sites (yet that month wasn't particularly warm overall).  And in 2025, the airport was colder than expected.

The result confirms the unusual situation last month, but it also reveals that there's quite a lot of variability from year to year (e.g. 2021), so the fact that the airport was colder doesn't necessarily indicate a problem with the thermometer.

Interestingly, the temperature differences between the airport and Goldstream Creek show the least variability from year to year between 2012 and 2024.  In other words, Goldstream Creek has the most similar temperature variations to the airport, and the most consistent temperature differences.  However, last month the difference was way off, and this is what drew my attention to the topic.

Honing in on Goldstream Creek, then, the same calculation with temperature differences relative to other sites shows that December 2025 stands out as an apparently significant outlier - more so than using the airport as the reference point.

My conclusion is that if something is awry with the temperature measurements, then it's more likely at the Goldstream Creek co-op than at the airport; Goldstream may have been running too warm.  Consider that since 2012, the Goldstream Creek has never had fewer days reaching -40° than the airport in a winter; but this winter the airport has 21 such days, while Goldstream has only 13.

Of course, it's also possible that the Goldstream valley was actually less cold than would normally be expected for legitimate meteorological reasons.  Perhaps drainage flows from the hills into that valley kept the winds up a little bit, and this would explain why the nearby Ester 5NE site was also warmer than would be expected.