When the wind is blowing and the snow is flying? I'd recommend sitting in front of the TV with a pair of sweatpants on.
I'm a bit pigeon-toed, which means my feet tend to snag on loose material when I run, so I prefer tights. I've also found that I don't lose much heat off my legs, at least compared to my upper body or even my arms, and unless it drops much below 10 degrees I generally stick with mid-weight tights.
My go-to tights work for both cycling and running. If you both ride and run, my first recommendation is to check out Performance Bicycle's house-brand Radiator Tights ($50). They come without a cycling chamois and are made from a stretchy polyester-Spandex blend that resists wind and wicks moisture. They're warm, but I've used them comfortably in a wide range of temperatures and weather conditions. Otherwise, take a look at the following three options from Nike, Arc'teryx, and Pearl Izumi.
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