Question about zone hardiness

I have two questions about zone hardiness:
1) why does a plant even care how cold it is once the temperature drops to, say 10 F? Why would a plant be damaged more by -40F than -20F? Is it the depth of the frost, or something else?
2) In the colder zones, does the snow really protect the plant from cold, or just from the wind? I would think that if the air temperature is 5F all day, wouldn't the ground even under the snow also be either 5F or colder? I don't understand how "insulation" can help when there's no heat source: if you wrap an ice cube in a mitten in a snowstorm, the ice cube will still be cold.
Thanks.
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Darryl wrote:

Have you ever been in a room and thought it was too cold, while someone else in the room thought it was warm enough? Or maybe vice versa? Ever see someone walking down the street in shorts and a t-shirt while you've got the heater on in your car? Does your dog look for a jacket before going outside in the winter?
Why is it hard to understand that different plants have different tollerances?

Under the snow there is a heat source. It's the Earth. The temperature at the core is believed to be somewhere around 3000-5000 C. It certainly isn't that warm near the surface, but it still is a heat source. There is also bio activity in the soil that's producing heat as well. There may also be some heat produced by underground utilities. A sanitary sewer, for example, can be pretty darn warm thanks to all the bio activity! There are plenty of heat sources under that snow cover. Enough to melt the snow? In some places, yes. In some places, no.
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Warren H.

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Snow protects in a number of ways. It keeps the plant thinking it's cold. That's important for plants which form next season's buds before the snow. If the snow melts and the plants start growing too early, buds can be damaged. Sometimes the plant dies, or maybe its flowers are ruined. Snow also helps keep the ground from heaving (shifting or lifting) due to repeated thaws and freezes. That can cause mechanical damage to roots, or just expose them to cold, or air, which can dehydrate them. Depends on the plant.
As far as why some plants seem bulletproof while others are fuzzy, someone else will have to explain that. I imagine it has something to do with antifreeze. :-) I mean, how the hell does a tiny crocus put up with as much obscene weather as it does?
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Darryl wrote:

Some plants have more dissolved sugars and nutrients in their sap than others. The greater the concentration of dissolved matter, the lower the freezing point.
Some plants have thicker bark over their sapwood. The more insulation, the less damage from freezing.
etc, etc
A layer of snow keeps colder winds off the plants. That's why some gardeners build cages with tree branches around smaller shrubs, to trap the snow and create an insulating blanket. Under that snow blanket, the temperatures might reach 20F (the temperature when the snow fell) while the air above has dropped to 0F. Also, cold air is more damaging when it is very dry. The snow helps maintain some moisture in the air under it; even if it's not melting, the snow can give up moisture through sublimation (direct transformation from solid to gas).
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