The Earth’s atmosphere perpetually forms clouds, ranging from common wispy cirrus to towering cumulonimbus. Clouds, which are visible masses of water droplets or ice crystals suspended in the air, are integral to weather systems and the planet’s climate. While many types are seen daily, others are so infrequent or require such a precise alignment of atmospheric conditions that they become natural rarities. Exploring these unusual formations reveals the extremes of meteorology and the interplay of temperature, altitude, and geography.
Clouds Formed by Extreme Altitude and Temperature
Rare clouds form far beyond the familiar troposphere, where most weather occurs, in the extremely cold, thin air of the upper atmosphere. Polar mesospheric clouds (noctilucent clouds) are the highest clouds on Earth, forming in the mesosphere at 80 to 85 kilometers. Their existence requires temperatures to drop below -120°C, which typically happens during the summer months at high latitudes. These luminous, electric-blue or silvery streaks are composed of tiny ice crystals that form around meteoric dust particles, reflecting sunlight long after the sun has set.
Another category of high-altitude rarities is the polar stratospheric clouds (PSCs), or nacreous clouds, which form in the stratosphere between 15 and 25 kilometers high. These clouds are distinguished by their intense, iridescent, mother-of-pearl colors, caused by the diffraction of sunlight through their small ice crystals. PSCs form only when temperatures plunge below -78°C, restricting their appearance primarily to the polar winter months. The extreme cold and dry conditions of the stratosphere make any cloud formation rare, but these specific ice crystal compositions indicate intense atmospheric cooling.
Rarities Requiring Specific Geography and Fleeting Timeframes
Other rare clouds are defined not by their height, but by the highly specific geographical features and momentary weather patterns needed for their formation. The Morning Glory cloud is a spectacular example, manifesting as a long, tubular roll cloud that can stretch up to 1,000 kilometers. This phenomenon is most reliably observed in the Gulf of Carpentaria region of northeastern Australia, particularly near Burketown, where the unique coastal configuration allows for a predictable collision of sea breezes. The resulting atmospheric wave, or undular bore, generates a series of rolling cylinders of air that form the cloud line at dawn.
The Kelvin-Helmholtz instability clouds, also known as fluctus, are another type of rarity defined by their brief, ephemeral nature. These clouds resemble breaking ocean waves or cresting surf, formed when two distinct layers of air move at different speeds and densities, creating a strong velocity shear. This instability causes the cloud layer to curl into characteristic wave-like patterns, but the conditions are so temporary that the clouds often dissipate within a minute or two of forming. The required atmospheric shear can occur at various altitudes, but the momentary balance needed to make the wave pattern visible is the limiting factor.
Recently Recognized Cloud Types and Unique Morphology
Rarity can also be defined by the complexity of a cloud’s visual structure and the recency of its formal classification. The most prominent example is Asperitas, formerly known as Undulatus Asperitas, which was the first new cloud type officially recognized by the World Meteorological Organization since 1951. This formation is characterized by well-defined, wave-like structures on the underside that appear chaotic and turbulent, like viewing a roughened sea surface from below. Although they often appear ominous, Asperitas clouds typically dissipate without producing precipitation, and their exact formation mechanism is still debated, though they are often linked to the aftermath of convective thunderstorms.
Another morphological rarity is the fallstreak hole, sometimes called a hole-punch cloud, which is not a cloud type itself but a localized gap appearing in mid-level clouds like altocumulus or cirrocumulus. These clouds consist of supercooled water droplets that are below freezing but remain liquid due to a lack of ice-forming nuclei. When an aircraft passes through this layer, the air expands and cools rapidly, triggering the supercooled droplets to freeze instantly and fall out, leaving a striking circular or elliptical void with wispy trails of ice crystals below. While the physics of their formation is understood, the combination of supercooled clouds and a specific disturbance makes a visible fallstreak hole an infrequent sight.