Shimmer
Today I saw something I'd never seen before: heat shimmer in the cold. This morning, sometime after a weak sun had come up, it was about -15°C. Or 14°C. What's the difference, etc. We opened up the balcony door from the office room to look out on the street in front of the house, after it was cold but before it started snowing. Looking up the street, the air danced and waved, like looking through the air above a grill or a blacktop road on a hot summer day.
I refer you back to the temperature. It was not a hot summer day.
So what we were seeing, it took us a moment to realize, that the air coming out from the house—which we keep at a balmy 17°C—was warm enough compared to the outside air to shimmer in front of us. We were seeing something we wouldn't expect to see, and the parts of our brains dedicated to filtering out any potential nonsense coming through our eyes simply said "Nah" for a few moments.