According to reports on social media, this type of cloud - which forms due to the presence of strong humid winds in uneven terrain, such as mountains or valleys – appeared in the municipalities of Fundão, Figueiró dos Vinhos, Castanheira de Pêra, Ponte da Barca and Viseu.


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