Rare Cloud Formations: Eight Types From Lenticular Clouds to Asperitas

Most clouds belong to a handful of everyday types, but a few take shapes that stop people in the street: smooth lenses parked over mountains, breaking waves in the sky, perfect round holes in a layer of cloud. Here are eight of the most striking cloud formations, what the World Meteorological Organization calls them, and how they form.

How clouds are named: from Luke Howard to the WMO Cloud Atlas

The modern cloud names go back to Luke Howard (1772–1864), a British chemist and amateur meteorologist. In 1802 he presented a classification to London's Askesian Society, published in 1803 as his Essay on the Modification of Clouds. He introduced the Latin names cumulus, stratus and cirrus, and combinations such as cirrostratus and cirrocumulus.

Today the reference is the World Meteorological Organization's International Cloud Atlas. Its latest edition, from 2017, sorts clouds into genera, species and varieties, and adds "supplementary features" such as incus, mamma, fluctus, cavum and asperitas for the striking details in this gallery.

Lenticular clouds and anvil clouds

Lenticular clouds form when moist, stable air flows over a mountain and sets up standing waves on the downwind side. Where the air rises to a wave crest and cools below its dew point, a smooth lens of cloud appears. Because the wave stays put, the cloud seems to hover in one place even in a strong wind. Glider pilots seek out the smooth rising air at the leading edge, and the saucer shape has led to many UFO reports.

Anvil clouds are the flattened tops of cumulonimbus thunderstorm clouds. The Atlas calls the feature incus: the upper part of a cumulonimbus spread out in the shape of an anvil, with a smooth, fibrous or striated surface.

Kelvin-Helmholtz waves and roll clouds

When two layers of air slide past each other at different speeds, the boundary can curl into a row of breaking waves, known as Kelvin-Helmholtz waves. In the Atlas the feature is called fluctus: a short-lived wave formation, usually on the top surface of a cloud, seen mostly in cirrus, altocumulus, stratocumulus and stratus. The billows are short-lived, so they are a lucky catch for a photographer.

Roll clouds are low, horizontal tubes of cloud that seem to roll about a horizontal axis. They belong to the arcus family, together with shelf clouds, but a roll cloud is completely detached from its parent cloud; the Atlas name is volutus. The most famous is the Morning Glory over the Gulf of Carpentaria in Queensland, Australia, which can appear on up to four days in ten in October.

Polar stratospheric clouds and fallstreak holes

Polar stratospheric clouds float 15 to 25 kilometres up, far above ordinary weather, and form only when the winter stratosphere drops below about −78 °C. They are best seen in twilight, when the Sun is a few degrees below the horizon. The ice-only type, nacreous or mother-of-pearl cloud, glows with iridescent colours. Other types, containing nitric and sulphuric acid, play a key role in ozone depletion over the poles.

A fallstreak hole, or hole-punch cloud, is a round gap in a thin layer of altocumulus or cirrocumulus made of supercooled water droplets. The Atlas calls it cavum. Wisps of ice crystals or virga usually trail down from the centre, and the hole grows over time. An aircraft passing through the layer can cut a long, straight version, called a dissipation trail.

Mammatus and asperitas: clouds seen from below

Mammatus clouds, called mamma in the Atlas, are pouch-like bulges hanging from the underside of a cloud. They occur with several cloud types, but the most dramatic examples hang beneath the anvils of cumulonimbus storms.

Asperitas, once nicknamed undulatus asperatus, is the newest name in this list. The Atlas describes it as wave-like structures on the underside of a cloud, more chaotic than ordinary undulatus, which can look like a rough sea seen from beneath. It forms mostly with stratocumulus and altocumulus, and low sunlight can turn it into one of the most dramatic skies of all.

More clouds and storms: Twenty Cloud Photographs: Shapes, Light and Atmospheric Features, Lenticular Clouds: How a Still Shape Can Contain Moving Air, storm clouds like a rolling ocean over Nebraska and a supercell thunderstorm in time-lapse.

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