Crazy Clouds: 12 Unusual Cloud Formations and What They Mean

Most clouds fit neatly into one of the ten basic categories taught in school: cirrus, cirrocumulus, cirrostratus, altocumulus, altostratus, nimbostratus, cumulus, cumulonimbus, stratocumulus, and stratus. Most do, but not all. Some clouds form in patterns so unusual they get their own classification, their own name, and their own folklore. Here are twelve of the strangest cloud types in the sky, how each one forms, and what each one is telling you about the weather to come.

1. Anvil Clouds

Anvil clouds are a subtype of cumulonimbus, the towering thunderhead that produces severe storms. The flat, anvil-shaped top forms when the rising column of warm air hits the tropopause (the top of the lower atmosphere) and spreads horizontally because it cannot rise any further. The result is the unmistakable mushroom-with-a-flat-top shape. An anvil cloud overhead almost always signals an active or imminent thunderstorm. The “leaning anvil” pattern, where the anvil top spreads predominantly in one direction, points to the direction the storm is moving. If the anvil leans toward you, the storm is heading your way.

2. Cirrus Kelvin-Helmholtz Clouds

Kelvin-Helmholtz clouds look like a row of breaking ocean waves frozen in the sky. They form when two layers of air slide past each other at different speeds, creating a shear instability at the boundary between them. The boundary curls up into a wave pattern (named after the 19th-century physicists Lord Kelvin and Hermann von Helmholtz, who described the underlying fluid mechanics). Kelvin-Helmholtz clouds are short-lived, often lasting only a minute or two before the wave shape dissipates. Pilots use them as visual confirmation of wind shear at altitude, which is why these clouds are sometimes spotted in turbulent air over the Rockies and the Alps.

3. Fallstreak Hole Clouds

Also called hole-punch clouds, fallstreak holes are large round gaps in a uniform cloud layer, often with wispy streaks of falling ice in the middle. They form when supercooled water droplets (water vapor below freezing temperature that has not yet crystallised) suddenly freeze, usually triggered by a passing aircraft or a chunk of ice falling from above. The freezing droplets create ice crystals that fall away, and the surrounding water droplets evaporate to balance the local air saturation. The result is a clean round gap in the cloud, sometimes a quarter mile across.

4. Lenticular Clouds

Lenticular clouds are saucer-shaped, often layered like a stack of dinner plates. They form when stable, moist air flows over a mountain ridge or other terrain feature and gets pushed upward into a wave pattern. The cloud forms at the crest of each wave, where the air cools enough for water vapor to condense, and dissipates at the trough. The result is a stationary, lens-shaped cloud that can hover over the same spot for hours. Lenticulars are most common around the Rockies, the Sierra Nevada, the Cascades, and the Appalachians. They are also the cloud most often mistaken for UFOs in photographs.

5. Mammatus Clouds

Mammatus clouds look like a layer of pendulous bumps hanging down from the underside of a larger cloud, usually a thunderstorm anvil. The shape is unusual because most clouds bulge upward, not downward. Mammatus form when pockets of cooled, moisture-laden air sink below the surrounding air at the base of a cumulonimbus cloud. The negative buoyancy creates the drooping pouch shape. Mammatus clouds are almost always associated with severe thunderstorms and often appear in the wake of a major storm cell. Seeing them overhead means the worst is usually behind you, but the storm itself was strong.

6. Roll Clouds (and Morning Glory)

Roll clouds are long, perfectly cylindrical formations that appear to roll along like a log laid horizontally across the sky. They form along the leading edge of cool air masses, often along coastlines, where cool sea air meets warmer land air. The boundary curls into a tube shape that the cloud condenses inside.

The most famous roll cloud is the Morning Glory of Cape York Peninsula in northern Australia. This particular formation can stretch over 600 miles in length and produces predictable enough weather conditions that glider pilots travel from around the world to surf the leading edge of the cloud. Morning Glory clouds form roughly 10 days a year between September and November, and the tiny town of Burketown has become an unlikely glider destination because of them.

7. Shelf Clouds

Shelf clouds are wedge-shaped formations that form along the leading edge of a thunderstorm, attached to the parent storm cloud. They appear flat on the bottom, often with a stepped or laddered structure rising back into the storm. Shelf clouds form when cool downdraft air from the storm sinks toward the ground and pushes warmer surface air upward, where it condenses into the visible shelf. A shelf cloud rolling overhead is a clear visual warning that the storm itself is minutes away. They are most common in the Plains and Midwest, where summer thunderstorms produce them frequently.

8. Asperitas Clouds

Asperitas clouds (formerly undulatus asperatus) look like a violent ocean seen from below. They are wave-like, dramatic, and often photographed for their menacing appearance. Despite the look, asperitas clouds rarely produce severe weather. They form when warm and cold air layers meet and create turbulence at their boundary, but the turbulence stays aloft rather than dropping to the surface.

Asperitas was officially recognised by the World Meteorological Organization in 2017 as the first new cloud classification added to the International Cloud Atlas in 67 years. The push for recognition came largely from the Cloud Appreciation Society, a UK-based group that had collected enough photographic evidence of the formation to convince the WMO that the pattern was genuinely distinct from existing categories.

9. Pyrocumulus and Pyrocumulonimbus

Pyrocumulus clouds form above wildfires, volcanic eruptions, and large industrial accidents. The intense heat from the surface event drives a column of warm moist air upward, which condenses into a cumulus-like cloud above the heat source. When the column has enough moisture and updraft, it can develop into a full pyrocumulonimbus, a fire-driven thunderstorm that can produce its own lightning and even tornadoes. Pyrocumulonimbus clouds were a defining feature of the 2003 Canberra firestorm and the 2020 Australian Black Summer, both of which produced large fire-driven storm cells. They are increasingly common as wildfires intensify across the western United States and Canada.

10. Noctilucent Clouds

Noctilucent clouds are the highest clouds in the atmosphere, forming at altitudes of around 50 miles up. They are made of ice crystals on the edge of space, in the mesosphere, and they shine with a pale blue or silver glow against a dark twilight sky. They are visible only from latitudes between about 50 and 70 degrees, during summer months, in the hour or two after sunset or before sunrise. They are most easily seen across the northern US, Canada, the UK, Scandinavia, and northern Europe. Noctilucent clouds were once rare but have become more frequent and visible in recent decades, possibly tied to changes in upper-atmospheric humidity.

11. Undulatus and Fluctus Clouds

Undulatus clouds appear as parallel wave patterns in the sky, formed by wind shear between two atmospheric layers. The variant called fluctus (named in 2017 alongside asperitas) is the modern name for the breaking-wave shape that was previously called Kelvin-Helmholtz. Both undulatus and fluctus formations are short-lived, sometimes appearing for only a minute or two before the wave pattern dissipates. They are most commonly photographed in mountain valleys and along coastlines, where wind-shear conditions are most favorable.

12. Wall Clouds

Wall clouds are dense, lower clouds that drop down from the underside of a supercell thunderstorm. They look like a hanging block of cloud, often rotating, and they are the formation most associated with tornado development. A rotating wall cloud is one of the clearest visual signals of imminent tornado risk, which is why storm chasers track them and the National Weather Service uses them as a key tornado-warning trigger. Wall clouds are most common in Tornado Alley (the central US) during late spring and early summer, but they appear anywhere supercell thunderstorms develop.

How to Read Clouds for the Forecast

The unusual cloud formations are most useful when paired with the more common ones. Use the standard ten cloud types as the base layer, and watch for the unusual formations as alerts:

• Mackerel sky: rain or wind within 24 hours.
• Mare’s tail (cirrus): rain or wind within 24 hours.
• Anvil cloud: active or imminent thunderstorm.
• Mammatus: recent or ongoing severe storm.
• Shelf cloud: thunderstorm arriving in minutes.
• Wall cloud: potential tornado, take shelter.
• Lenticular: stable atmospheric conditions, often clear weather.
• Asperitas: dramatic look, but rarely produces severe weather.

For the more common cloud types and their forecasting value, see our pieces on weird weather terms and animal weather folklore.

Watch the sky. Read the shape. The forecast is overhead.

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Caleb Weatherbee

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Caleb Weatherbee is the official forecaster for the Farmers' Almanac. His name is actually a pseudonym that has been passed down through generations of Almanac prognosticators and has been used to conceal the true identity of the men and women behind our predictions.

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