frozen fog
I heard that it was going to be warm today. So the first thing, when I steped out of the front door, was the whole front porch was coated with invisible ice so my foot just shot right out from under me. Fortunately my flailing right hand landed on the railing, because otherwise it would have been even worse when I slid and/or stumbled down the first three steps. And even then it was only the fact that the lower steps are concrete that kept me from serious injury; even though they were as icy as the wooden steps above them, the ice was so thin that the texture of the concrete protruded and provided some minimal traction.
What seems to have happened was, the air temperature was above freezing so a considerable quantity of fog formed, even before the sun rose. The ground was still quite cold, however (as were our front steps we find), so all that moisture came to rest and settled in nicely in solid form. The fog didn't contribute to the slippery situation at school, though; there, it was all just snow compacted by the action of hundreds of tiny booted feet into a dense, glacier-like layer, and then polished smooth by the action of warmth on its outer layer. Positively deadly. Who knew that warmth in winter could be so dangerous?! Let's have a good cold snap back again, or we might not all survive until Spring.