Fairbanks just finished a long run of unusually cold weather, including numerous days colder than -40F (-40C). In the StinkyHouse, we have a Davis Pro Vantage 2 automatic weather station, which is our main inside view on the outside temperature and a super device to have. We also have an old style thermometer off the front porch, and another one in the dog yard. I had been perplexed that while the radio and our favorite online weather site, (the Weather Underground) was reporting temps well below -40F, our thermometer seemed never to to beyond -39.3.
My first theory was that there are just temperature variations, and this is certainly a factor. Due to different elevations and other factors, it is common to see 10 degree differences or larger among weather reporting stations just a few miles apart. I saw that a number of personal stations reporting to Weather Underground were around -40, then a number of other stations (including the official NOAA and FAA stations) were substantially colder.
The real answer is that our weather station stops registering temps colder than -40, plus or minus one degree. I checked the manual to verify this, and asked some of my professional weather pals (in the National Weather Service) and other weather geeks. It looks like lots of people in Fairbanks have the same unit, including at least one that is operated by the University. The Weather Service did an experiment a few years ago, and found that the same Davis units would get very flaky below -50 or -60, yielding sporadic results. A firmware upgrade, which was applied to later units like the one I bought, limited the lower range to -40F.
The moral of the story is that if it’s colder than -40F, trust to the stations that are reporting lower temps (or, consult your analog thermometer!), rather than the large group clustered right around -40. Free advice to Davis Instruments: rather than reporting a number at the bottom of a range, a message like “out of range” would be more useful.