These are called lenticular clouds. They usually - but not always - form where stable moist air flows over a mountain or range of mountains. They can form rapidly and then disappear again just as fast.
Giuseppe Pappa in Sicily, Italy, wrote on December 27, 2018: "The dust and the gas from Mount Etna gave us today this fantastic view during sunset time. The sun became very red due to volcanic ash in the sky."
A beautiful drama is taking place low in the east before sunup this week. Giant planet Jupiter and innermost planet Mercury have swept close, with dazzling Venus above. Photos here from the EarthSky community.
If you see the moon tonight, it'll look nearly full, illuminating the sky nearly all night on this longest night of the year for the Northern Hemisphere. Full moon comes on December 22.
The shower peaked late last week, but, with all the comet excitement going on, people didn't start submitting their best Geminid images until the weekend.