Acetylcholine. in CNS and PNS: generally excitatory but is inhibitory to some organs and glands; an organic, polyatomic ion that acts as a neurotransmitter in both the peripheral nervous system (PNS) and central nervous system (CNS) in many organisms including humans.
So Zone 3 is more typical of North Dakota, while Zone 9 is lovely Florida weather. Each hardiness zone differs by 10 degrees F, so the difference in low winter temperatures between zones 6 and 7 is 10 degrees; between zones 6 and 8, 20 degrees; between zones 6 and 9, 30 degrees, and so on.
The result is a map that more accurately reflects true plant hardiness and some of the changes are quite interesting. The Plant Hardiness Zone Map divides the United States into numbered zones based on the coldest average yearly winter temperature within that zone. Each numbered zone represents a 10 degree interval, with zone 1 being the coldest zone and zone 13 being the warmest.