At dawn, I went out to say good morning to the starlings in the reed bed. The canal was icing over; as the morning went on, the ice advanced from either side to meet in the middle, before the sun warmed things up enough to melt it away again.
But over to the east I saw row of curlicues in the clouds, like two intertwined sine waves.
It was a Kelvin-Helmholtz wave.You get these when a warm air mass is sitting on top of a colder one, and they're moving at different speeds. Very exciting, if you get excited by that sort of thing.