Figure 1. Last freeze and number of freezes at the Fairbanks International Airport (1948-2014). The 2014 days is through May 15th.
A better metric for measuring freezing conditions, and energy usage, is freezing degree days (FDD). Much like heating degree days and cooling degree days, freezing degree days measure the accumulated degrees below freezing over the course of the season. Figure 2 shows the annual number of freezing degree days for the Fairbanks International Airport.
Figure 2. Annual freezing degree days (FDD) at the Fairbanks International Airport (1948-2014). The 2014 days is through May 15th.
Finally, the date for the final freeze at the Fairbanks International Airport is not especially representative of the area. In fact, with the exception of Eielson Field, every station around Fairbanks has a later last freeze of the season. Figure 3 shows the date of the average last freeze around Fairbanks and Figure 4 is a map of Alaska showing the average last freeze date.
Figure 3. Average date of last freeze in the greater Fairbanks area. All stations in the GHCN (v3) database with at least 10 complete years of temperature data since 1981 were used.
Tie this into the PDO and ENSO and we'll learn something.
ReplyDeleteGary
Gary, I am not sure if you are being sarcastic. There is a lot of very useful information (in my opinion) in this type of data regardless of the teleconnection index. For example, there is a well established relationship between freezing degree days and frost depth. Utility companies might be interested in that. If I was planning on deciding how much wood to have on hand before the winter began, the two lines in Figure 1, the bars in Figure 2, and the map in Figure 3 would each independently cause me to collect a different amount of wood. The map in Figure 3 is particularly useful in determining when my wife would move her plant seedling from the house to the garden.
DeleteThere's no sarcasm implied or intended. The question posed is not only what happened, but perhaps why. That's assuming some external condition forces our weather and long term climate in Interior Alaska.
DeleteThe information is useful as it stands. However, any teleconnections are of a longer term interest, especially as they relate to the number of freezes or freezing degree days. I approach cause and effect as a biologist, perhaps others do not.
As we swing through the various phases of the PDO and ENSO, there may be a relationship that's reflected in your analysis.
Gary
The general trend back up in freezing degree days since 1980 is quite surprising to me. Presumably you calculated the FDD with the minimum temperatures not the daily mean temperatures? I was under the impression that minimum temperatures in recent years were much different from the decades prior to the 1976 PDO shift. Would be interesting to look at the same calculation for stations with less urban influence than the airport.
ReplyDeleteRichard, I am pretty sure that the number of freezing degree days is based on the daily average temperature:
Deletehttp://ccr.aos.wisc.edu/resources/data_scripts/wisconsin_climate/descriptions.php
http://www.glerl.noaa.gov/pubs/fulltext/1980/19800001.pdf
Here is a NWS site with an interactive Freezing Degree Day tool (which interestingly has a different value for Fairbanks): http://aprfc.arh.noaa.gov/frzdd_all_sites.html
Brian, thanks - that matches my understanding of the usual degree day calculation, but for some reason I assumed you had done the calculation with minimum temperatures. Not sure why. The recent trend back up makes more sense with mean temperatures.
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