One day many years ago at Purdue - late 90's - they shut down the campus for a day because it was forecast to be -40 with wind chill. But it was only overcast, the air and ground were clear. In the afternoon, for some reason the notion took me to bundle up and go out on my bike. I had two hard falls on black-iced concrete between my dorm and wherever I was going. I locked it up and walked back from there.