In recent days, the northern coast of Portugal has been the scene of an atmospheric spectacle as mesmerizing as it is disconcerting. While thermometers registered temperatures typical of an intense heatwave, an unusual phenomenon has swept across the Portuguese beaches, capturing the attention of residents, tourists, and experts alike: the so-called 'cloud tsunami' . Its appearance, reminiscent of a gigantic ocean wave, has given rise to dozens of viral videos on social media and numerous questions about its origin and potential dangers.
This extraordinary cloud, also known as a roll cloud or volutus cloud , moved horizontally at a low altitude over the sea, slowly approaching the shore. Despite its impressive presence, the phenomenon posed no risk to people : it was an atmospheric formation that, although rare, is fully identified and monitored by meteorological services.
What exactly is a tsunami cloud?
The tsunami cloud owes its popular name to its visual resemblance to a giant wave about to break on the coast. However, its scientific name is volutus cloud or roll cloud . According to the Portuguese Institute of the Sea and the Atmosphere (IPMA) , it is a horizontal, tubular structure that can travel great distances along the coastline, moving thanks to complex interactions between different air masses.
These clouds form when there is a strong contrast between the warm air that accumulates over the land during very hot days and the sudden arrival of cooler sea breezes . When the two air masses meet, the warm air rises and is replaced by the cooler air, causing very dense condensation. The result is that tube-like cloud that moves along the surface of the sea, like a wave, but it doesn't actually carry water or cause a real tsunami.
Affected cities and development of the phenomenon
This meteorological spectacle was especially visible on Sunday afternoon, when various coastal towns such as Póvoa de Varzim, Figueira da Foz, Vila do Conde, Esposende, Peniche, and Nazaré witnessed the cloud's advance over the sea. The phenomenon began to form offshore around 15:30 p.m. and approached the coast between 17:00 and 18:00 p.m.
The arrival of the cloud not only surprised and unsettled beachgoers, but was also accompanied by a sharp drop in temperature —in some places, the difference was as much as eight degrees in just one hour—and a sudden increase in wind speed . These abrupt changes led many people to leave the beach or seek temporary shelter. All of this occurred under mostly clear skies, at the height of the heatwave affecting Portugal and much of Europe.
Are these types of clouds dangerous?
Although these clouds can appear intimidating and cause some fear among first-time viewers, authorities insist they pose no direct threat to people. They are not tsunamis, dangerous storms, or violent storm fronts . In fact, the phenomenon typically lasts only a few hours and disappears as quickly as it appeared.
A volutus cloud can signal a sudden change in the weather, such as the arrival of strong winds, localized rainfall, or even small tornadoes if the conditions are right. Therefore, although these clouds themselves are not harmful, it is advisable to monitor the weather if you see one.
Recognition and rarity of the phenomenon
The genus *volutus* was officially recognized in the World Meteorological Organization 's atlas in 2017. These formations are usually associated with altocumulus or stratocumulus clouds and, more rarely, with cumulonimbus. The presence of these clouds in Portugal is relatively rare, although there are parts of the world, such as Australia and Argentina , where both experts and weather enthusiasts have documented them more frequently.
Portuguese meteorologists point out that the Atlantic coast offers ideal conditions for this type of situation to occur on days of intense heat waves and temperature contrasts, although it is rare. On some occasions, heavy rain, gusts of wind, and even small hail have been recorded behind the volutus cloud.
Can it happen in other regions?
Roll-up clouds are not exclusive to the Portuguese coast. There are recent records of this type of formation in Argentina (Pico Truncado, Santa Cruz, Mar del Plata), and they are relatively common in northern Australia . In all cases, the formation pattern is very similar: it requires temperature differences, contrasting air currents, and sufficient ambient humidity—elements that are not so easy to find together.
Rolled clouds are not unique to the Portuguese coast. Discover other strange and amazing weather phenomena in different parts of the world where similar formations have also been reported.
