Lenticular clouds are the smooth, lens-shaped clouds that seem to park themselves over a mountain and stay there for hours. They form when moist, stable air flows over high ground and sets up standing waves downwind, and their saucer outline has made them one of the most common explanations for UFO sightings around the world.
How lenticular clouds form over mountains
When wind crosses a mountain range in stable air, the flow does not simply rush down the far side. It bounces, setting up a train of standing waves downstream, much like the ripples that form in a river behind a boulder. The UK Met Office uses exactly that comparison.
As air rises toward the crest of each wave it cools. If it cools below its dew point, its water vapour condenses and a cloud appears. On the descending side of the wave the air warms again and the droplets evaporate. The result is a cloud that sits at the wave crest, often aligned with the wind and strung out in a line along successive crests. Meteorologists call these strings wave clouds.
The waves themselves can be present on clear days too. The cloud only shows up when there is enough moisture at the right height to make the invisible airflow visible.
Why a lenticular cloud seems to stand still
A lenticular cloud is called stationary, yet the wind blowing through it can be very strong. The cloud is constantly re-forming on its upwind edge, where air rises and condenses, and dissipating on its downwind edge, where air sinks and dries out. As long as the wind stays steady, the shape stays put while the air pours through it.
The US National Weather Service in Honolulu documented a striking example on 25 November 2003, when southwest winds of 40 to 60 mph blew above 10,000 feet over the Big Island of Hawaii. Mauna Loa and Mauna Kea, both close to 13,000 feet high, forced the flow up and over their summits. Huge lenticular clouds formed downwind, stayed in place all day and were large enough to show up on high-resolution satellite images, with tops reaching about 40,000 feet at times. They were visible from the Mauna Loa Observatory and from the Kona coast about 40 miles away.
Types of lenticular clouds and their heights
Lenticular clouds are classified by the height at which they form above the ground. The three main types are:
- Stratocumulus standing lenticular (SCSL), the lowest of the three.
- Altocumulus standing lenticular (ACSL), the mid-level form.
- Cirrocumulus standing lenticular (CCSL), high, thin ice-crystal versions.
They can form at altitudes up to about 12,000 metres (40,000 feet). When several moist layers are stacked in the airflow, the wave can produce a pile of lenses one above another, which is what gives some formations their “stack of plates” or flying-saucer look. Low sun can also light up the edges in pastel colours, an effect known as iridescence.
A lenticular cloud sitting directly on top of a peak is often called a cap cloud. Because they need high ground to form, these clouds are rare over flat country, and many people never see one.
Lenticular clouds around the world
Any mountain range with strong winds and moist, stable layers can produce them. Lenticulars are common over the western United States because of the Rockies, and they are relatively rare in Hawaii. In the British Isles they are unusual but do appear from time to time downwind of hills.
The photographs below come from four very different mountain settings: a towering stack near El Calafate in Argentine Patagonia, a cap cloud on a mountain in Oregon, a row of lenses over Mount Hotaka in Japan and a thin orange lens near Pāhala on Hawaii’s Big Island at dusk.
What lenticular clouds mean for pilots and glider flyers
For pilots, a lenticular cloud is a visible warning sign of mountain waves. Powered aircraft generally avoid them because of the turbulence and sinking air on the trailing side, and the Met Office notes that very strong, gusty winds can blow in one spot while the air a few hundred metres away is calm.
Glider pilots see the same clouds as an opportunity. The rising air on the leading edge of a wave is often smooth and powerful, and the cloud’s position shows exactly where that lift is. Wave soaring has carried gliders on flights of more than 3,000 kilometres and to altitudes above 22,000 metres.
For skiers and mountaineers, a lens forming over the summit is a useful sign that the wind is strong up high, even when the valley feels calm. Plan summit days with that in mind: check the upper-level wind forecast before heading for exposed ridges.
Related reading
- pouches and waves on other cloud surfaces
- viewing wind at different atmospheric levels
- how a Grand Canyon cloud inversion fills the canyon with fog
- Planet Earth: A Short Landscape Montage
- Strokkur Geyser: From a Blue Dome to a Column of Spray
- Tateyama’s Snow Corridor and the Alpine Route
- Lightning in Slow Motion: Branches and Bright Pulses
- Magic Flashlights: Knife Show’s Mount Hood Snowboard Film
- Abraham Lake’s Ice Bubbles: Layers beneath the Surface
Sources
- Lenticular cloud – Wikipedia
- Unusual cloud formations – Met Office
- Lenticular clouds over the Big Island – NWS Honolulu
- Diagrams: Premier Flight Center, “Standing Lenticular Clouds”. Photographs: lenticular-cloud collection.
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