30 below F. The snow was so sticky, you had to pick your feet up to move in the lift corral.
BTW -40 is the same F and C. I lived in a very cold place as a kid and it would get there regularly. That's when you don't expose your ears or fingers for even a few minutes.
That fun fact doesn't make you feel any better when it's -40!
0F, -17C: Good skiing
-10F, -23C: Brisk. Dry. No tourists.
-20F, -29C: Stings cheeks. Snow squeaks. No uncovered skin.
-25F, -32C: Inhaling the air has a bite.
-30F, -34C: Grease in wheel bearings locks up. Snow noticeably slower.
-35F, -37C: Tires hold the flat spot from being parked all night, and do not roll smooth. Exposure is dangerous.
-40F, -40C: Carrots left outside will shatter like glass. Inhaling burns a little. Skiing is off the list.
That's all just temp, not wind chill. Wind chill just accelerates cooling, it does not change the physical properties like temperature does.
The two recent times I was skiing in severe cold was once on an exposed peak with temps at -18F and winds steady in the 40s. Every stitch in every article of clothing was venting air at a rapid rate. While -18F wasn't normally so bad, with heavy wind it was brutal, and no one in the group was adequately dressed for continual exposure to it. Someone's goggles iced up, (they placed goggles over the facemask) and couldn't see, had to lead them down by hand. Knowing your equipment is important in real cold. The other time was in the mid 20's below F, with winds gusting into the 30's. Skiing was decent, and we were dressed for it, but still there were issues to deal with (toes, fingers, breath ice) The mountain closed early for exposure. It's one thing to ski in it, it's entirely another to contemplate getting stranded on a broken chairlift for an hour in it.
On one of the colder days this year at Killington, temps were at least in the negative teens, and the gondola doors started to fail at a rapid rate, each time shutting the lift down. Each time, they would disable the doors and flag that cabin. Perhaps not a coincidence we're getting all new gondola cabins on K1 this year.