As a round, orange-tinted cloud hovered Thursday over Bursa, Turkey, it might have looked like a flying saucer was about to land. But it was just a lenticular cloud — not uncommon in the vicinity of tall mountains.
Bursa — where the cloud was seen — is about 50 miles south of Istanbul, across the Sea of Marmara. It’s home to about 2 million people.
How did it form?
Integral to our meteorological detective work is knowledge of the surrounding area. In this case, a quick bit of online research finds that Bursa is nestled in the foothills around 8,343-foot Mount Uludag just to the south.
The lenticular cloud in photographs is a textbook example. It resembles a stack of pancakes or hockey pucks in the sky.
Lenticular clouds form in linearly stratified environments — or those characterized by a perfectly layered atmosphere. (Picture “settled” salad dressing that has separated into layers based on the density of ingredients. The atmosphere does the same thing; we just can’t see it.)
Under ordinary circumstances, those layers remain separated. But if an obstruction or obstacle (like a mountain) spans multiple layers, air from below can be forced upward, interrupting the otherwise perfectly layered environment. This is especially true when winds closer to the ground push air masses toward rising terrain, so they have no option but to rise as well.
Because air near the ground ordinarily holds more moisture than the air above it, that pocket of near-surface air winds up moister than the surrounding environment. And since air temperatures cool with height, that air parcel may be chilled down to its dew point as it ascends. When that happens, the air becomes saturated — and forms a cloud.
But the influence of the mountain doesn’t last forever. In fact, once the bunched-up air has passed over the mountain or obstruction, it descends to its original level — warming up, drying out and eroding the cloud. Thus, the cloud is present only over the top of the mountain and just downwind, forming a hat-like cap cloud that is often circular.
Even though lenticular clouds appear to remain stationary over the top of the mountain, they’re actually formed in very windy environments. Remember — they’re born from a stream of air forced up and then back down, so there’s a constant channel of air flowing through them. On Thursday, strong winds from the south were blowing over western Turkey because of low pressure over northern Italy.
Other notable examples
What made the cloud in Bursa especially aesthetically pleasing was the time of day it formed — shortly before sunrise. Its altitude, probably around 10,000 to 20,000 feet, allowed it to catch sunlight and be illuminated before the sun actually poked over the horizon and bathed the city in amber warmth.
A similar photo from Weed, Calif., went viral in February 2020.
In the United States, lenticular clouds are common in western areas, where Pacific moisture is forced over the high terrain of the Rockies. The greater the variations in topography, the more prominent the resulting lenticular. They’re frequently spotted perched above Mount Rainier in Washington state.
They can form in the eastern United States, too, including in the Northeast and Mid-Atlantic. One example from last spring depicts an invasion of shallow lenticular clouds in the D.C. area.
In Gibraltar, an ever-present lenticular cloud known as the “Levanter” is a staple of the city’s skyscape.