"Devil Cloud" Covers Jeju After Seoul... Meteorological Agency Explains the Phenomenon

Formed by the Collision of Cold and Warm Air: Undulatus Asperatus Clouds
Also Known as "Rough Wave Clouds"... Added to the International Cloud Atlas in 2017

In Jeju, clouds resembling large waves covered the sky, drawing public attention.

On the afternoon of the 5th, Children's Day, a strange multilayer cloud was seen above Hallasan in Samyangdong, Jeju City. It is called undulatus asperatus or rough wave cloud. Yonhap News

On the 5th, unusually shaped dark clouds completely blanketed the sky over Jeju City. These clouds, which stretched widely above the northern part of Hallasan and Jeju City, appeared as if large waves were rolling, with a bumpy and undulating texture. Some citizens reacted by saying, "Is this a sign of an impending earthquake?" "The sky looks terrifying," and "It feels cursed."

However, an official from the Jeju Regional Meteorological Administration explained, "When the Korea Meteorological Administration classifies clouds, there are three types of high-level clouds, three types of mid-level clouds, and four types of low-level clouds. Today's clouds appear to be mid-level clouds," adding, "Among mid-level clouds, they seem to be undulatus asperatus." The official explained that such clouds are formed when cold air and warm air collide roughly, causing atmospheric instability and resulting in clouds with a textured appearance.

Clouds of this shape are also called "Asperatus" or "rough wave clouds." The name comes from the Latin word "asperitas," meaning "rough" or "uneven." The term "Asperatus" was coined in 2009. In June 2009, a photo of clouds resembling the surface of a stormy sea was posted as the "Cloud of the Month" by the Cloud Appreciation Society. Initially, these clouds were nicknamed "Jacques Cousteau cloud" after the legendary French diver and ecologist Jacques Cousteau, but soon, similar photos were reported from around the world. Eventually, the society combined the Latin words for "wave" and "rough" to name them "undulatus asperatus." Later, in 2017, the term was included in the International Cloud Atlas and has since been abbreviated as "Asperatus." This was the first new addition to the International Cloud Atlas in 66 years since cirrus clouds (feather clouds) were added in 1951.

Morning sky over Chungmuro as seen from the Asia Economy building on the 1st. Photo by Lee Kyungho

Previously, on the 1st, similar clouds appeared in the sky over Seoul, prompting citizens to react with comments such as "devil cloud," "doomsday," and "the last day."