Thursday, January 3, 2013

Accessing Deep Cold in Fairbanks

Reader Gary asked about quantifying deep cold in Fairbanks, i.e. when it's cold enough to require extra effort by hardly Fairbanks folk to stay warm and keep things running. Monthly mean temperatures only hint at the question, since means, by definition, smooth out the day to day variation. So here's an effort at addressing this question.

I start by calculating a "Bitter Degree Day" (BDD) statistic for each day. I've chosen a daily mean of -30F as the threshold, based on my own experience: daily means colder than -30f require some extra work. Recall that the daily mean is simply the average of the high and low, so a day with a high of- 30F and a low of -40F would have a mean of -35F. Like most degree day calculations, values outside of the threshold are set to zero: there are no negative degree days. So for each day in the winter, if the mean temperature is ≥ -30F, the BDD is zero. If the mean is less than -30F then I subtract -30 from that value and the remainder is the BDD.
Doing this uncovers some interesting nuggets. For example, this past December, there were 73 BDD, a smidgen more than the 70 BDD in December 1980, even though the monthly mean last month was nearly 7 degrees warmer than December 1980. This reflects the fact that the coldest days this past month were colder than in December 1980 AND the cold snap broke after Solstice last month, but not til the 30th in 1980. In fact, the January correlation between monthly mean temperature and BDD is a less than might be expected 0.69: good, but not great (the correlation of, say, mean temperature and conventional Heating Degrees Days would be 1.00, since the daily mean is always less than 65F in January). 
Winter totals have ranged from none eight times to 570 in 1933-34. In the past 30 years the highest total is 172 in 188-89, all of which were in that memorable January. Here is a plot of the winter (Oct-Mar) totals for each winter 1929-30 through this winter (thus far):   

This plot reveals that almost 20% of winters have very few (or no) BDDs, while very few (and none since 1971) have had more than 200. Except for the outlier high values, there is not much of trend, though the positive PDO phase (1977-2007), with many very BDD totals, is evident.

Since 1929-30, the monthly BDD median and maximum values are:

Nov:  0, 18 in 1994
Dec: 6, 237 in 1961
Jan: 24, 369 in 1934
Feb: 0, 84 in 1950
Mar: 0, 8 in 1956 (only March with >0 BDD)

Now the daily BDD is a measure of the mean temperature on the coldest days, and there is no doubt that the growth of the Fairbanks urban area and modern transportation have impacted very cold temperatures (mostly by providing water vapor for ice fog development). So instead of total BDD, how about number of days with a daily mean of <-30f: p="p">
The correlation between number of days and the seasonal total is quite good: 0.91 for the whole period, and still 0.88 since 1977, which can arguably be considerable the modern era of Fairbanks urban influences on temperatures.

So, this little exercise demonstrates that a focused statistic (like BDD) can get context specific information: climatology has a lot more to offer the informed user than just averages and normals.


  1. Excellent Rick..I knew you could offer a good Cold Weather/BDD analysis. The CC's above are good. Time now to mine the databases for more.

    Does Negative PDO have a denial coefficient?

    Thank you.


    1. OK, no noun modifiers allowed here...PDO Coefficient Of Denial.

      It stems from realizing we're now in a cool phase of the PDO, but living in denial that the fine relatively warm years of a positive PDO (1976-7 to about mid 2000's) may have lapsed.

      Who wants years more of cold? = denial. How we allow that fact to run our lives might yield a correlation coefficient (ooops, another noun modifier slipped in). (;-)



  2. Rick, This is a fascinating, creative analysis. So December 2012 was almost in the top 10 coldest on record according to the degree of "bitter cold", though it was much less remarkable from the perspective of mean temperature.

    The PDO phases show up really well in these statistics. It seems that 18 of the 20 most negative PDO winters had above-normal BDD, whereas 18 of the 20 most positive PDO winters had below-normal BDD. So the BDD distribution is strongly influenced by PDO phase. Would be interesting to examine whether BDD's are more sensitive to PDO than just the mean temperature, i.e. how does the PDO affect the tails of the temperature distribution.


  3. After some thought and sleep, I agree with the -30F criteria for breaking the "cold barrier" and BDD. It's a subjective value, yet over my last 48 years in Alaska it becomes a real challenge to operate and run equipment normally at low temps when they fall below -30F, Chill factor included. For example, the local International Airport maintenance crew minimizes their activity at that temp due to increased breakdowns in equipment.

    Snow machining, flying, travel, vehicles, house heating/fuel expenses, whatever takes on a new dimension below -30F. Even my dog recognizes the chill.

    Good show to see what the rest of the winter brings.