Deceptive clouds above Mount Rainier spark questions on social media

NEAR MT. RAINIER, Wash. — Once again, some persuasive pictures of Mount Rainier have led a few to wonder if the volcano was beginning to erupt earlier this month. Spoiler alert: no, Mount Rainier was not actually venting. But what was pictured here?

Many of the commenters on this post shared with KOMO News were correct to infer that the picture is in fact capturing a cloud, not volcanic ash (or blowing snow). A critical question remains, though: what type of cloud produced this illusion?

One commenter suggested that a cap cloud was to blame. Cap clouds, more commonly known as pileus clouds but also referred to as scarf clouds, are small, dome-shaped lenticular clouds. These particular clouds form when dry air around a tall cloud or mountain rises so quickly into an upper region of moist air that it is forced to rise and condense, according to definitions given by the American Meteorological Society and the Department of Atmospheric Science at the University of Illinois.

Pileus clouds most commonly known to form above cumulus or cumulonimbus clouds. Occasionally, though, they are spotted atop mountains or even, funnily enough, clouds of volcanic ash.

While cap clouds can form above an isolated mountain (like our beloved volcano), the term is generally restricted to a smooth cloud that is centered atop the associated mountain or cloud tower, and that appears to hug the top in cap-like fashion. The cloud in question does not look like a dome encompassing the peak below, so it seems unlikely to be a cap cloud.

However, this commenter’s theory is not exactly wrong. Pileus clouds are a form of lenticular clouds.

Lenticular is the overarching name given to those lens-like, stationary clouds that form because of some sort of obstruction, such as a mountain, to a moist airflow. Furthermore, lenticular clouds, as a whole, are generally situated to the side of the peak that the air flows over, such as seen in these images.

Lenticular clouds are a rightfully famous accessory to the beautiful view that is Mount Rainier. While a pilot might be inclined to avoid them, cloud enthusiasts and many Washington residents alike would agree that lenticular clouds are among the most beautiful.

So, unless you are a pilot, maybe keep them in mind for the next time you are prompted to share your favorite type of cloud.