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.
Formally, the 'Tropic Zone' is the belt around the Earth's middle, between the. Tropics of Cancer and Capricorn. From 23.5 degrees south to 23.5 degrees. north latitude, and all longitudes, all the way around.