When Powell, Wyoming, resident Dave Varkony stepped outside at sunrise Sunday, he saw what he described as a “cool weather phenomenon” stretched across the sky.
A thin, gently undulating cloud was hanging motionless over Park County, and he photographed the “rope cloud” as it hovered over Heart Mountain.
“It was a sunrise snapshot of Heart Mountain's fresh snow and ridiculously cool rope cloud,” he told Cowboy State Daily.
Straight And Standing
Cowboy State Daily meteorologist Don Day said Varkony saw an altocumulus standing lenticular cloud, aka rope cloud, which is common in the Rocky Mountain Region.
“If you were to pick up a garden and snap your wrist, you would see that wave traveling right along the garden hose. That’s what's going on,” Day said, explaining how rope clouds appear in the sky.
Lenticular clouds are a sign of strong winds aloft at altitudes between 10,000 and 30,000 feet. When those winds blow at an angle perpendicular to a mountain range, the result is long, thin, motionless clouds.
“The mountains west of Cody generally trend north-south,” Day said. “When the winds aloft are coming in from a westerly direction and encounter those north-south mountain ridges, a wave function forms in the wind and rises east of those ridges. If the atmosphere is moist enough as that air rises, a lenticular cloud will form.”
Day said lenticular clouds tend to be much wider than the one seen over Heart Mountain. Also, unlike other clouds, they don’t appear to move because they constantly reconstruct themselves.
“The standing part of altocumulus standing lenticular cloud refers to how those clouds tend to stay in place,” he said. “They don't move because they only form at the top of that wave, and the cloud is forming within that feature the whole time.”
Clear Air Turbulence
Anyone who’s ever flown into a mountainous airport might have experienced the wrath of a rope cloud. Day said they are a well-known source of turbulence during flights into airports adjacent to mountain ranges like those in Cody, Jackson and Denver.
“As you make your descent into those airports, you get that clear air turbulence,” he said. “The cloud forms in the lee of the mountains, but further downstream, those waves continue downwind of the mountains but may not climb high enough to make a cloud.”
While lenticular clouds tend not to form too far from the mountains, the wave function that leads to their creation continues far beyond them.
Day said that the cloud Varkony observed over Heart Mountain was probably jostling jet passengers as they arrived at Yellowstone Regional Airport that morning.
“Pilots know that when you see those clouds, you will encounter a lot of turbulence,” he said.
Andrew Rossi can be reached at arossi@cowboystatedaily.com.
