It might be an alien invasion. Or, you might just be in Weed, Calif.
Lenticular clouds, or “lennies,” as many meteorologists refer to them, often resemble flying saucers, hockey pucks or heaping stacks of pancakes. They form by the drove in mountainous environments with chaotic wind patterns.
Lenticular clouds form when comparatively moist air rides up a mountaintop, forced into a cooler layer of air above. If the cool-down is significantly large, the air parcel can become chilled to saturation, forming a cloud. Downwind of the mountain, the air eventually sinks lower in the atmosphere, drying out and eroding any visible cloud. As such, wind-sculpted lenticular clouds are usually local.
They are most readily visible when the atmosphere as a whole is stable and layered. In other words, there’s no organic upward motion to create clouds elsewhere. Just the localized forcing of air upward by a mountain peak — in this case, Mount Shasta.
That’s exactly the otherworldly scene that Fire Management Officer Paul Zerr of the Shasta-Trinity National Forest happened upon during an early morning drive Wednesday. The photo was captured just north of Exit 747 on the southbound side of the Cascade Wonderland Highway near the Weed Store. It appears Zerr was looking southbound, meaning the lenticular cloud was to the south-southwest of Mount Shasta.
A look at weather at the time of the photo reveals a textbook lenticular-manufacturing process at work.
Sunrise was slated at just after 7 a.m. Wednesday; the lenticular cloud, probably between 18,000 and 25,000 feet high, would have caught the first amber peaks of sunlight even before the ground was illuminated by the dawn.
Temperatures in town nearby were in the mid-30s, but with a dew point of 25 degrees, the air’s relative humidity was 64 percent. That means as soon as the air is chilled to 25 degrees, the water vapor it contains will rapidly condense. That’s what happened in a narrow channel downwind of Mount Shasta.
We also know based on weather balloons that a strong inversion may have been in place in some areas, helping to keep the environment stable and inhibit the growth of any other clouds — affording picturesque views of the ornate cloud. Northerly winds also were responsible for the position of the cloud south of the peak.
Lenticular clouds aren’t relegated to the West Coast. In fact, they’re quite common even in the vicinity of the Appalachians. Sometimes, lenticular clouds actually form on the mountain peak itself, becoming a “cap” cloud.
Similarly, a lenticular-like phenomenon called a pileus cloud frequently forms over towering thunderclouds, resulting when a turret of warmth, moist air is shoved upward into cooler air by a powerful updraft below.
As for lenticular clouds themselves, they’re usually an indicator of mostly fair weather. So the next time you spot one, no need to fear. It’s not an alien invasion. It’s ordinary science with an extraordinary view.