You just know I was going to jump into this thread. :-)
I went riding at Jay Peak when it was -58C with the wind-chill. It was something like -28C without the wind-chill. I believe I also had similar temps on one of my outings to Tremblant.
I find temps colder than -20C without the wind-chill to be the best ones to ride in. The snow stays fresh since it's too cold for it to melt, the sky is clear and the sun is always out, and most people stay away from the slopes because they find it too cold.
You just have to make sure you don't have any skin exposed, you're wearing enough layers and your boots aren't too tight -- in general, nothing is restricting your circulation. Once you have that done, you'll have an awesome day riding toasty warm on hero snow with almost the entire place to yourself.
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