According to a report by NM, despite their unusual shape, it was not a demonstration of extraterrestrial life, but instead lenticular clouds.

According to experts, the phenomenon was due to the winds in the local area near Lisbon.

This type of cloud forms due to the presence of strong wet winds in rough terrain, such as mountains or valleys.

According to meteorologist Derek Van Dam, speaking to the British newspaper Telegraph - during a similar appearance in South Africa - these clouds are relatively common, although special conditions are necessary for them to form.

According to Van Dam, clouds form when air moving along the Earth's surface hits an obstacle, such as a mountain. The moving air thus alters its course, causing a whirlpool, and causing the water vapour to condense to generate these shapes.