They look like holes in the sky, but they are, in fact, fallstreak holes or cavums - a rare meteorological phenomenon.

These clouds caught the attention of residents and curious onlookers, at least in those three points of the country, according to the Facebook page Meteo Trás os Montes, which shared some of the images sent by its followers.

Fallstreak clouds appear when the water temperature in the clouds is below zero, during ice formation. The surrounding droplets evaporate and leave a large hole in the cloud.

This cloud occurs, according to MetSul Meteorologia, when a part of the cloud layer freezes into ice crystals and falls, creating a hole in the centre.