Saturday, January 19, 2013

Seasonal Defintions in Interior Alaska

Continuing on the topic of what are "seasons" in Interior Alaska, I thought it might be useful lay out some definitions that crop up or that I have used on occasion. Feel free to chime in with more.

  • Fixed dates with "Equal" Length Seasons:
    • Traditional: Winter begins at Winter Solstice, Spring begins at Vernal Equinox, etc. I don't know the history of this definition, but it it can't be completely rooted in northwestern European tradition: witness Shakespeare's 'A Midsummer Night's Dream' which takes place around Summer Solstice. The inappropriateness of this definition in Interior Alaska, at least in relation to climate, natural history, etc. is obvious. 
    • Astronomical: Winter is the quarter of the year with the least possible sunshine, i.e. the darkest quarter and summer the quarter of the year with the most possible sunshine. So, Winter is Nov 6-Feb 5, Spring Feb 6-May 5, Summer May 5-Aug 5 Autumn Aug 6 to Nov 5.  This is certainly better than the traditional definition. 
    • Meteorological: by long standing convention, Winter is Dec-Feb, Spring Mar-May, etc.  Mostly commonly used in analysis and certainly captures the core season in Interior Alaska, i.e. winter is the (on average) three coldest calendar months and summer the warmest.  A variation of this would be the two-season definition of winter (cold season) as Oct-Mar and summer (warm season) May-Sep.
  • Fixed Dates with Variable Length Seasons
    • Winter is coldest third of the mean daily temperature variation and summer the warmest third (spring and autumn the middle third), So for Fairbanks, the mean daily temperatures (1981-2010 normals) range from -9F in mid January to 64F for a couple weeks in late June and early July, for a range of 73F. So the coldest third of the temperature range is -9F to +15 F and summer is 40F to 64F. So for Fairbanks this yields a winter season of Oct 26-Mar 24 and a summer season of Apr 26-Sep 25. This is a much better fit to many residents' intuition that spring and autumn are functionally much shorter than winter and summer.
    • A two-season definition with Winter as the Mean Snow Cover Season, for Fairbanks something like Oct 16-Apr 22. Summer is the rest of the year.
  • Phenomena based Yearly Variable Dates (including single season definitions)
    • Winter begins when the first day that the 5 or 7-day running mean temperature is below freezing and ends when the running mean goes above freezing (for Fairbanks typically in Oct and Apr). Summer begins the first day when the 5 or 7-day running mean temperature is above 55F or 60F (depending on application, typically later May to late Aug in Fairbanks). Spring and autumn in between.
    • Winter begins when the permanent winter snow cover is established. For Fairbanks, this almost always in October and ends when the snow is gone (for Fairbanks April or early May). 
    • Autumn starts when deciduous tree leaves turn color (late August or very early September).
    • Summer begins on the date of green-up
    • Summer begins with river ice break-up and winter begins with river freeze-up

1 comment:

  1. Good info Rick. But ugh! This is like going to Walmart and having to choose the least obnoxious product. I guess today I consider winter to be when I have to park my Harley motorcycle in the garage.

    Beyond that selfish definition, I'd choose The Fixed Dates with Variable Length Seasons above.

    One analytical issue used in comparative climatic discussions is what do folks like use to describe "seasons"? I deleted a reply below that bordered on unfair criticism, but basically they publish yet don't appear to offer clear definitions for terms used in their seasonal analyses. I may have missed them if they've done so.

    What was the "winter" of 2012 for example...approximately Jan-Mar and Oct-Dec 2012? It would seem one segment may likely have had little relationship to the other, yet they apparently are grouped when the year's climate is discussed.

    Do they (early and late yearly "winters") tend to correlate over time? If not, then publishing the variability of "winter" over time for Fairbanks may be better served via inter annual analyses.