wed, 14-jan-2009, 19:07

The cold snap of 2008/2009 has finally broken. The plot below comes from my Ninety day weather summary plot for the Fairbanks Airport weather station (PAFA). That web page has a description of all the variables shown on the plot, but here I’m interested in the vertical red bars, which represent the high and low temperatures for the day. You’ll notice that from December 27th through January 11th, the temperature was far below normal (the 30-year normal high and low temperature are the solid blue, mostly horizontal lines. Record high and low temperatures are the green x marks. We didn’t come close to breaking any low temperature records.

Ninety day weather summary

So how does this cold snap compare to those in the past? Most people cite the cold snap of 1989 as the one they remember. Was the current period comparable?

My first attempt at answering this question was to generate a list of consecutive days where the low temperature for the day was below -40. It’s a fairly complex query because we need to compare a table against itself, and we need to report the results from both rows that match. Here’s the SQL:

```SELECT a.dt, a.min_temp, a.temp, a.max_temp
FROM isd_daily AS a INNER JOIN isd_daily AS b
ON a.isd_id=b.isd_id AND a.dt=b.dt - INTERVAL ’1 DAY’
WHERE a.isd_id=’702610-26411’ AND a.min_temp<=-40 AND b.min_temp<=-40
UNION
SELECT b.dt, b.min_temp, b.temp, b.max_temp
FROM isd_daily AS b INNER JOIN isd_daily AS a
ON a.isd_id=b.isd_id AND a.dt=b.dt - INTERVAL ’1 DAY’
WHERE b.isd_id=’702610-26411’ AND a.min_temp<=-40 AND b.min_temp<=-40
ORDER BY DT
```

The output is a list of consecutive dates and temperatures. From there, I wrote a Python script to read these periods and calculate the length, average temperature, average minimum, and the cumulative degree days below freezing. Using this metric, our recent cold snap was the coldest since the cold snap of 1989. Here are the two compared:

```For period between 1989-01-19 and 1989-02-01
Days = 14
Average temp = -42.9
Average minimum = -48.0
Cumulative degrees below freezing = 1048.7

For period between 2009-01-02 and 2009-01-11
Days = 10
Average temp = -40.8
Average minimum = -44.8
Cumulative degrees below freezing =  728.0
```

You can see that this doesn’t include the first portion of the cold snap, because the low temperature on New Year’s Day was a balmy -38°F. So it doesn’t seem like it really captures the full length of the recent event.

Getting the mail at -53°F

The other approach I tried was to examine the entire historical record, calculating the average temperature for all periods of various lengths. It turned out that a window of 16 days resulted in the highest ranking. But even so, it was the 62nd coldest 16 day period between 1946 and the present. You can see the reason why the ranking is so low if you look at the results:

```rank      Date interval     avg temp (°F)
1  1947-01-17 - 1947-02-01  -47.19
2  1947-01-18 - 1947-02-02  -47.06
3  1947-01-19 - 1947-02-03  -46.84
4  1947-01-16 - 1947-01-31  -46.82

29  1989-01-17 - 1989-02-01  -41.43
30  1989-01-18 - 1989-02-02  -41.27

32  1989-01-16 - 1989-01-31  -40.96

35  1989-01-19 - 1989-02-03  -40.58

47  1989-01-20 - 1989-02-04  -39.56
48  1989-01-15 - 1989-01-30  -39.53

62  2008-12-27 - 2009-01-11  -38.07
```

The top four coldest 16-day periods were all part of the same, longer cold snap, that lasted from January 16, 1947 through February 3rd of that year. Because the entire period was so cold, it yielded the top four coldest 16-day periods. I also included the intervening 1989 cold periods for reference. You can see that the 1989 event was also much longer than the 14-day interval identified by the consecutive days below -40 technique. The average temperatures over the 1947 events are quite impressive when compared to the recent average and the 1989 event. As bad as it seemed, we don’t really have much too complain about.

tags: Fairbanks  weather
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