Shelf clouds are often associated with squall lines.
A beautiful video of a massive shelf cloud that covered Brazilian city of Caxias do Sul has gone viral on social media. Shelf clouds, which have a round shape and appear to have layers, usually appear before storms. The clip was recorded by a local on Saturday morning and posted on Instagram and other social media platforms. The timelapse video shows clouds advancing over the buildings. They appeared because of the weather in the Brazilian city, which experienced thunderstorms, lightning and wind gusts, as per a report in G1.
Watch the video:
The outlet further said that winds coming from the ocean and instability associated with low pressure in Paraguay were responsible for the extreme weather throughout Rio Grande do Sul.
Temperature also remained low in areas around the city, the outlet said.
The local weather office has forecast rain throughout the day in the Brazilian state.
What are shelf clouds?
According to National Oceanic and Atmospheric Administration (NOAA), shelf clouds are often associated with squall lines, and are many times reported as wall clouds, funnel clouds, or rotation.
Also called Arcs clouds, they are usually associated with a solid line of storms - the wind comes first with rain following behind it. It may appear to rotate on a horizontal axis, said NOAA.
These clouds are sometimes seen beneath cumulonimbus clouds, the dense, towering vertical cloud that cause intense rain.
How are shelf clouds formed?
According to UK Met Office, when a cold downdraft from a cumulonimbus cloud reaches the ground, the cold air may spread rapidly along the ground, pushing existing warm moist air upwards. As this air rises, water vapour condenses into the patterns associated with shelf clouds.
The new cloud may roll if it experiences different wind directions above and below, the agency further said.
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