WATCH: Weird cloud rolls towards beach looking like massive wave

Watch as this rare weather phenomenon rolls in from the sea at a beach in Portugal.

People relaxing on Praia de Buarcos, 85 miles south of the city of Porto, saw a huge cloud rolling in from over the ocean.

Although the cloud looked like a giant wave heading towards them, it was nothing to worry about for those on the beach.

While some sunbathers got up and moved away, others stayed and were caught in breezy and blustery conditions as the cloud passed overhead.

Porto is a major port city in Portugal

Portugal is in the middle of a heatwave with highs of 42C and the conditions are likely to have been the cause of the strange cloud.

"This is a spectacular example of a roll cloud. It's a type of arcus (curved) cloud that can form near to a powerful thunderstorm," said Sarah Keith-Lucas from BBC Weather.

"Cool moist air from over the ocean rolls in to meet the warm, dry air over land," added Sarah, explaining that the heat it can cause water in the air quickly to turn into tiny droplets which forms a cloud.

"Due to air flowing in different directions above and below the cloud, it can form into the distinctive cigar shape - a long, thin cloud which is most common close to the coast."