The air is crystalline, any water vapor is floating around in frozen super small flecks. Your eyelashes will freeze up. Any wind makes it ultra magnified, I went out with maintenance at -42° (at -40° it doesn't matter if you are using F or C) and they had just enough time to get to the equipment at 40 yards away when they came back into the truck in a slight run to warm up. No outside work got done that night.