Your record high should be around 103, record low -21. Your ground probably stays frozen maybe three months of the year? Heat kills more chickens than cold.
I don’t know if your coop is elevated or on the ground, how big it is, how many chickens you plan to have in there, or how high it is. I don’t know how it is situated, whether it is in the open or in a protected from wind area. Is it in the shade in the summer? Which direction are your prevalent winds from? I don’t know how often you’ll clean it out or if you plan to use a droppings board. Each coop is going to be unique. It’s hard to get real specific when you don’t know what conditions you are looking at. Pat was reluctant to give numbers because of all these differences so she gave a very conservative number that should cover practically everyone on this forum, which means it can be overkill for a lot of us. That’s pretty common for most guidelines on this forum. Still, I agree with her that it’s really hard to provide too much ventilation at any time as long as direct breezes do not hit the chickens in colder parts of winter.
I’ve seen single-combed chickens sleep in trees with the temperatures below zero degrees Fahrenheit with no problems and no frostbite. They were not on the dead limb of a tree overlooking a bluff squawking defiantly in the teeth of a blizzard. They were in a thicket in a protected valley with the ability to reposition themselves behind a tree trunk if the wind changed directions. They were roosting on tree branches and when they were cold they’d fluff up their feathers. Fluffing up the feathers traps insulating air plus it covers their feet on those tree branches. They really can handle cold quite well with just a little protection, but in our coops they have limited ability to move to get out of a breeze so we need to help them a bit. If a breeze strong enough to ruffle their feathers much hits them, it can release that trapped air. Also think of wind chill.
Ammonia is lighter than air. Warm air rises and holds more moisture than cold air. Ammonia comes from the poop decomposing. Moisture comes from their breath and their poop. Ammonia can damage their lungs. In winter excess moisture can lead to frostbite (think dew point and condensation). In the summer, excess moisture in the coop can speed the decomposition of the manure. This causes it to smell and releases ammonia. It also gives certain potentially harmful bugs a good home. Lots of ventilation in the summer (and some turning) will help the bedding dry out.
You need to get rid of both ammonia and moisture year around, though if the poop is frozen it is not giving off ammonia or moisture but look out for when it thaws if it has built up. Openings up high year around are important. In the summer, openings down low are important. In a coop with reasonable height you’d be surprised how much of a draft you can create just from cooler air coming in from the bottom and warmer air going out the top. That can take a lot of moisture with it. If your lower air intake can come from a cooler shaded spot (like the north shaded side) you are even better off. In the summer chickens really enjoy a nice breeze.
How much ventilation do you need? I don’t know. I’d certainly leave as much area under any overhangs open as I reasonably could. My rafters are 2x6’s because of the span so I have 5-1/2” open on two sides, covered with hardware cloth. The end that the prevalent wind and most rain comes from is inside a roofed shed so I left a large opening and covered it with hardware cloth. On the normally down-wind side I also left a hardware cloth covered opening. When I get a rain from that direction the inside of my coop gets wet, but with enough ventilation it dries out pretty quickly. If you toss a little scratch on the bedding the chickens will turn it for you. You might want to use a gable vent or shutters to keep moisture out. A roof vent will remove a whole lot of air, whether a turbine or just something like a cupola. Since you get snow, I’d not depend on a ridge vent. It could get blocked.
Windows are really nice. You need some light in there anyway so you and the chickens can see what you are doing. If you have traditional windows, cover the outside with hardware cloth and leave them open in the summer, even if they are at or below the chickens’ roosting level. Or you can make your own windows by framing an opening, cover it with hardware cloth for year around, and have a removable panel of Plexiglas for the winter months. Other hardware covered openings at or near ground level are really great for summer. Just have a flap or something you can put on there in the winter.
If your coop is fairly small, another option is to build a three sided coop with the fourth side wire. That will create a cul-de-sac for them to sleep in where they are protected from storms. Then in the winter, have a panel that covers that entire side.
There are a lot of different ways to do these things. What works best for you depends on what your coop looks like and how it is situated among many other things. An 8x12 walk-in coop on the ground will have different requirements than a 4x4 elevated coop. Civilization as we know it will not end if you don’t do it any one specific way. Try to figure out what is easiest to provide passive ventilation for your unique situation. You don’t have to follow a formula and be real precise with this stuff, just try to err on the side of providing too much. Try to not overthink it, though I tend to do just that and probably gave you enough information to do just that.
Good luck with it. From your posts I’m confident you’ll be successful.
