Actuallly, the zones are based on averages, not lowest ever temps. And the
#1 thing you should learn looking at it the map is that latitude is not
helpful.  Montana and Seattle have very different climates.  The same is
true of Las Vegas and PahDump (high desert) versus San Francisco (sea level
rains all the time).

Our part of SE TN is zone 7a (fringe 6b). The low temp shown is 0-10, our
average lowe winter temp.  Our record low of 1983 is -24, with temps below
zero for several days.  A nice weather break in AK, but a disaster here,
causing several elderly deaths.  The same latitude includes areas of the
country in zone 7, 6, 5,4 and 10 (CA, where it doesn't even freeze in
winter).  The frost/freeze maps show similar variations.  They are useful
for figuring the number of average days that no flowers will remain for
forage, but don't tell you much else. Again, our average last frost is Mar
15, but I've been caught in snowstorms in early may (and sometimes it is in
the 70's for half of december).  We are currently right at our first frost
dayte, but nothing below upper 30's yet on in the extended forcasts. Last
year the aster/goldenrod bloom was all killed by early frosts in Sept.

-Karen