Tuesday, December 31, 2013

Deep Freeze (Temperatures Aloft)

Last week's post about the first -40° temperature of the winter in Fairbanks provoked a bit of discussion about the causes of the long-term decline in the frequency of very cold days in Fairbanks.  As reader Gary noted, there are numerous candidates for possible drivers of the long-term change, including changes in wind, cloud cover, and air mass characteristics; we also can't exclude the possibility of an urban heat island effect or subtle station siting issues.  As a simple first step at tackling this, I looked at the 850 mb temperatures from Fairbanks balloon soundings since 1948; the chart below shows the annual (cold season) number of days with 850 mb temperatures below -26 and -32 °C; these thresholds were chosen because the long-term frequencies of occurrence are close to those for -40 and -50 °F at the surface, respectively.  Last week's chart of -40 and -50 °F frequency is also reproduced below, except with an adjusted time scale for ease of comparison between the charts.

It's plain to see that the frequency of very cold temperatures aloft has also shown a notable decrease since the mid-1970's, although the warming appears less pronounced than at the surface.  Temperatures of -32 °C or lower at 850 mb have become rare in the past decade or so, just like -50 °F temperatures at the surface.  From a first glance, then, it seems that warming aloft explains at least a significant part of the dwindling of extreme cold frequency in Fairbanks.

Viewing the data in summary form, here are the 30-year counts of the extreme temperature categories, at the surface and aloft, for 1951-1980 and 1981-2010.

-40 °F or Lower434224-48%
-50 °F or Lower10615-86%

850 mb1951-19801981-2010Difference
-26 °C or Lower401257-36%
-32 °C or Lower8941-54%

Looking at the December-February mean conditions, temperatures at 850 mb have increased substantially, and only modestly less than at the surface:

DJF mean (°F)1951-19801981-2010Difference
850 mb8.311.9+3.6

As a final note, and as food for thought, the surface and upper-air data from McGrath show broadly similar trends, although the drop-off in extremely cold days is less pronounced at the surface than for Fairbanks (especially for -50 °F and below); see charts below.  Note that I used different 850 mb temperature thresholds to correspond to the long-term frequencies of the respective categories.  As an aside, it is also interesting that McGrath has a higher frequency of very cold days than Fairbanks (much higher in the modern era) despite being warmer in the mean, both at the surface and aloft.

Here are the 30-year counts and DJF means:

-40 °F or Lower481304-37%
-50 °F or Lower12783-35%

850 mb1951-19801981-2010Difference
-26 °C or Lower441341-23%
-32 °C or Lower11664-45%

DJF mean (°F)1951-19801981-2010Difference
850 mb11.413.3+1.9

Update: here's the McGrath 850 mb chart for the same temperature thresholds as Fairbanks:


  1. Nice job! I suspect that if you look also at 500 and 300 mb that you would see a similar pattern. Warmer airmasses should stretch to the tropopause and it would help flush out inversions. What does McGrath's 850 look like if you use the same thresholds as Fairbanks?

    1. Eric, I added the 850 mb chart for McGrath with the same thresholds as Fairbanks.

  2. Fascinating presentation Richard. You're spoiling us.

    Rick's earlier analysis of McGrath and Fairbanks winter WX:

    We have the warming Tanana Jet, McGrath less so I believe based upon conversations with residents years ago that noted it was cold there but not as windy. McGrath also has numerous river drainages focusing cold air on that location.

    Our Urban Heat Island has expanded to higher elevations in the hills surrounding Fairbanks. Not sure of that effect if any. McGrath as noted is what it has been for years occupation wise.

    The '50's and '60's were cold. Less so after the mid-'70's on warm PDO.


    1. Gary,

      Thanks - this kind of investigation is always tremendously interesting to me. Many thanks for pointing me to Rick's presentation.

      You suggest that the Fairbanks UHI is affecting higher elevations near Fairbanks, not just the airport and valley locations - is that someone else's conclusion or have you seen data pointing in that direction?

      Happy New Year!

    2. Hello Richard. No conclusions or data, just speculation based upon the never ending horizontal and vertical growth of Fairbanks.

      How much the expansion of sources that typically influence UHI have altered the local climate I haven't documented. But if temperature and pollution are linked to settlement, it surely has had an effect.

      For reference, here's some links to UHI studies in the Fairbanks area:

      Search: EPA 600478027 (Fairbanks Heat Island 1978)




      http://dec.alaska.gov/air/doc/EAFB_att7.pdf (interesting obs on wind)


    3. Gary, Thank you for looking these up. Lots to learn.

  3. Richard, thanks for the added graph. I wanted to get a feeling on how temperatures affected the distributions. You can see the general pattern for 850 McGrath is still there but somewhat muted. Picking the "biggest amplitude" was a good idea. You also see how much variability there is per year. I think some formal stats on many stations would be helpful if a more formal study was wanted. I haven't read any of the posted papers.

    Especially during an inversion downtown Fairbanks is consistently 10 F warmer than the airport. How much of this is location and how much is UTI? I think it's more difficult to pull the UTI signal in Fairbanks than other places due to lots of variablity. Again I haven't read any papers.

    Keep up these simple but awesome posts!