Winter weather and hardiness zones

We have received more than our normal share of winter precipitation this year.  I have spent my whole life in the same town, and I don't really recall having more than 1 decent snow storm each winter.  This winter, we received a pretty good snowfall on Christmas Eve, measuring 8 inches in the middle of our front yard (away from any drifts).

Christmas Eve blizzard

Then we received an ice storm followed by 6 inches of snow last week.

Very thick icicle hanging from my greenhouse this week

In between those two snow storms our temperatures dipped down below 10 degrees Fahrenheit on three consecutive nights (6F, 6F, 8F).  We're not really used to these temperatures, but I was thinking about our USDA hardiness zone.  I am located in zone 7a, which is rated for winter temperatures between 0 and 5 F.  This is one of those winters that makes our hardiness zone rating seem appropriate.  I would say that in the average winter, our minimum temperature is probably somewhere around 15 degrees F, but the USDA zones aren't set up by average minimum temperatures.  You don't want to plant a tree and expect it to survive in your zone only in the years that are above average.  You want it to survive 50 years or more.  So the USDA zones are set up by using long-term historical climatic minimum temperatures.

I discussed in a previous post how the hardiness zones only tell a small part of the story, but I would like to mention that again here.  The hardiness zones only tell you whether a plant can brave your winter minimum temperatures, not whether they will be happy with your amount of moisture or sunlight or long, hot summer days.  Some efforts have been made to construct other zone maps for variables like humidity and heat index.  Once these maps have been constructed and distributed to plant people, nurseries will need to start to label their plants, in order for them to be useful.  I know that there are a number of plants which can survive our winters, but would not like our heat waves when the temperatures can be above 100 degrees F for a week solid in the middle of the summer.

Hopefully these new maps will be circulated soon and start getting used.  It might seem like a lot of numbers to keep up with, but I can envision a 3 map system, where you just always remember you are a "7-5-8" or something like that.  If the order is kept the same (cold hardiness, heat index, humidity), it will make this new system user friendly and could help a lot of new gardeners.