[NAFEX] Accurate well defined hardiness map.

Megrette Hammond megdanh at comcast.net
Thu Dec 22 21:03:37 EST 2005

Hi All,

    Here's my 2 cents worth.  I'm guessing that these maps are accurate and mean exactly what they say they mean.  They just don't mean what we want them to mean.

   The Hardiness maps divide the country into 10 degree zones based on the average lowest temperature encountered over the winter.  I'm in zone 5 so my average lowest temperate is -10 to -20.  That means one year of a -30 lowest followed by a succeeding year with a -10 lowest temperature keeps me exactly at zone 5.  If I had 10, 20, or 30 years of encountering -30 lows with few offsetting "warm" winters the average would start pushing me to zone 4.

   To get some idea of the lowest temperature you are likely to encounter for a winter (which is what I believe most people think the maps mean) you would have to know the standard deviation.  That is a measure of how much of difference from the average (the mean) is contained in the data set in question.  This will tell you if the bell curve around the mean looks like a pointy Christmas tree (very little variation) or like a wave in the middle of the ocean (a lot of variation). You would also have to assume that the standard deviation looking backward is the same as that looking forward (global warming anyone?).  Finally you have to determine your confidence interval, that is how much of the bell curve you want to chop off.  95% is a common confidence interval.  That means you can say that you have a 95% chance of having the lowest encountered temperature within X degrees of the mean.  That doesn't tell you that next year won't be one of the 2.5% years that are extra cold or extra warm.  I don't think this standard deviation data is readily available.

   Maybe the climate zone map people publish exactly how they do their computations.  If so find it and become friendly with an actuary, stats professor, or some other professional who help you decipher what it will really mean to you.
   Good luck growing.

                                                            Dan Hammond
  ----- Original Message ----- 
  From: Jwlehman at aol.com 
  To: nafex at lists.ibiblio.org 
  Sent: Wednesday, December 21, 2005 9:53 PM
  Subject: [NAFEX] Accurate well defined hardiness map.

  There is no such thing. 

  If you found one that divides down to the county level it will be meaningless. Each zone has a 10 degree difference, and just because a line might be drawn on a county line, those counties will not have average temps 10 degrees warmer or colder than the other. Also those temp ranges aren't absolute.  My zone 5b (-15 to -5, my extrapolation) regularly sees -20 and the record is -32. 

  Conclusion: A well defined map is meaningless. Close counts in horse shoes (the game), hand grenades and USDA Hardiness Zones.  

  Opinion time: Add at least one zone to what most nursery catalogs list their material to be hardy too.  If they say zone 5, if might survive zone 6 over a long time period.  

  Stephen is correct when he writes:  Even in a very large area of a single zone, one might live in a microclimate; 
  I might add, those microclimates can be much colder as well as warmer.  



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