It is a lenticular cloud, which has the shape of a lens or a disc and is sometimes mistaken for a UFO.

The images were shared on the Facebook page of Meteo Trás os Montes - Portugal, where it is possible to see this phenomenon, which occurred this morning, Wednesday, December 24th.

These clouds, as explained by the Portuguese Institute of the Sea and the Atmosphere (IPMA) to Notícias ao Minuto two years ago, "are called 'altocumulus lenticularis'" and "are found at an altitude between 3,000 meters and 6,000 meters and are formed by the interaction of wind with mountains, under certain conditions of atmospheric stability".