1. Lenticular Clouds
Lenticular clouds (Altocumulus lenticularis) are stationary lens-shaped clouds that form at high altitudes, normally aligned perpendicular to the wind direction. Lenticular clouds can be separated into altocumulus standing lenticularis (ACSL), stratocumulus standing lenticular (SCSL), and cirrocumulus standing lenticular (CCSL). Due to their shape, they have been offered as an explanation for some Unidentified Flying Object (UFO) sightings.
Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing waves may form on the downwind side. If the temperature at the crest of the wave drops to the dew point, moisture in the air may condense to form lenticular clouds. As the moist air moves back down into the trough of the wave, the cloud may evaporate back into vapor. [Source: Wikipedia]
2. Undulatus asperatus
Undulatus asperatus (or alternately, asperatus) is a cloud formation, proposed in 2009 as a separate cloud classification by the founder of the Cloud Appreciation Society. If successful it will be the first cloud formation added since cirrus intortus in 1951 to the International Cloud Atlas of the World Meteorological Organization. The name translates approximately as roughened or agitated waves
The clouds are most closely related to undulatus clouds. Although they appear dark and storm-like, they tend to dissipate without a storm forming. The ominous-looking clouds have been particularly common in the Plains states of the United States, often during the morning or midday hours following convective thunderstorm activity. [Source: Wikipedia]
3. Noctilucent (night) clouds
Night clouds or noctilucent clouds are tenuous cloud-like phenomena that are the “ragged-edge” of a much brighter and pervasive polar cloud layer called polar mesospheric clouds in the upper atmosphere, visible in a deep twilight. They are made of crystals of water ice. The name means roughly night shining in Latin. They are most commonly observed in the summer months at latitudes between 50° and 70° north and south of the equator.
They are the highest clouds in the Earth’s atmosphere, located in the mesosphere at altitudes of around 76 to 85 kilometres (47 to 53 mi). They are normally too faint to be seen, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in the Earth’s shadow. Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon; there is no record of their observation before 1885.
Noctilucent clouds can form only under very restrictive conditions; their occurrence can be used as a sensitive guide to changes in the upper atmosphere. Since they are a relatively recent classification, the occurrence of noctilucent clouds appears to be increasing in frequency, brightness and extent. It is theorized that this increase is connected to climate change. [Source: Wikipedia]
4. Fallstreak Hole or Hole Punch Cloud
A fallstreak hole, also known as a hole punch cloud, punch hole cloud, canal cloud or cloud hole, is a large circular gap that can appear in cirrocumulus or altocumulus clouds. Such holes are formed when the water temperature in the clouds is below freezing but the water has not frozen yet due to the lack of ice nucleation particles.
When a portion of the water does start to freeze it will set off a domino effect, due to the Bergeron process, causing the water vapor around it to freeze and fall to the earth as well. This leaves a large, often circular, hole in the cloud. It is believed that a disruption in the stability of the cloud layer, such as that caused by a passing jet, may induce the domino process of evaporation which creates the hole. Such clouds are not unique to any one geographic area and have been photographed from the United States to Russia. Because of their rarity and unusual appearance, fallstreak holes are often mistaken for or attributed to unidentified flying objects. [Source: Wikipedia]
5. Mammatus clouds
Mammatus, also known as mammatocumulus (meaning “mammary cloud” or “breast cloud”), is a meteorological term applied to a cellular pattern of pouches hanging underneath the base of a cloud. The name mammatus, derived from the Latin mamma (meaning “udder” or “breast”), refers to a resemblance between the characteristic shape of these clouds and the breast of a woman.
Mammatus are most often associated with the anvil cloud and also severe thunderstorms. They often extend from the base of a cumulonimbus, but may also be found under altocumulus, altostratus, stratocumulus, and cirrus clouds, as well as volcanic ash clouds. [Source: Wikipedia]
6. Wave Clouds
A wave cloud is a cloud form created by atmospheric internal waves. The atmospheric internal waves that form wave clouds are created as stable air flows over a raised land feature such as a mountain range, and can form either directly above or in the lee of the feature. As an air mass travels through the wave, it undergoes repeated uplift and descent. If there is enough moisture in the atmosphere, clouds will form at the cooled crests of these waves. In the descending part of the wave, those clouds will evaporate due to adiabatic heating, leading to the characteristic clouded and clear bands. The cloud base on the leeward side is higher than on the windward side, because precipitation on the windward side removes water from the air.
It is possible that simple convection from mountain summits can also form wave clouds. This occurs as the convection forces a wave or lenticular wave cloud into the more stable air above. [Source: Wikipedia]
7. Cloud iridescence
Cloud iridescence is the occurrence of colors in a cloud similar to those seen in oil films on puddles, and is similar to irisation. It is a fairly uncommon phenomenon, most often observed in altocumulus, cirrocumulus and lenticular clouds, and very rarely in Cirrus clouds. The colors are usually pastel, but can be very vivid. Iridescence is generally produced near the sun, with the sun’s glare masking it, so it is more easily seen by hiding the sun behind a tree or building. Other aids are dark glasses, or observing the sky reflected in a convex mirror or in a pool of water.
Iridescent clouds are a diffraction phenomenon cause by small water droplets or small ice crystals individually scattering light. Larger ice crystals produce halos, which are a refraction phenomena rather than iridescence. Iridescence should similarly be distinguished from the refraction in larger raindrops that makes a rainbow.
If parts of clouds have small droplets or crystals of similar size, their cumulative effect is seen as colors. The cloud must be optically thin, so that most rays encounter only a single droplet. Iridescence is therefore mostly seen at cloud edges or in semi-transparent clouds, and newly forming clouds produce the brightest and most colorful iridescence. When a thin cloud has droplets of similar size over a large extent, the iridescence takes on the structured form of a corona, a central bright disk around the sun or moon surrounded by one or more colored rings. [Source: Wikipedia]
8. Roll clouds
A roll cloud is a low, horizontal, tube-shaped, and relatively rare type of arcus cloud. They differ from shelf clouds by being completely detached from other cloud features. Roll clouds usually appear to be “rolling” about a horizontal axis. They are a solitary wave called a soliton, which is a wave that has a single crest and moves without changing speed or shape.
One of the most famous frequent occurrences is the Morning Glory cloud in Queensland, Australia. One of the main causes of the Morning Glory cloud is the mesoscale circulation associated with sea breezes that develop over the Cape York peninsula and the Gulf of Carpentaria. However, similar features can be created by downdrafts from thunderstorms and are not exclusively associated with coastal regions.
Coastal roll clouds have been seen over California, the English Channel, Shetland Islands, Lithuania, Eastern Russia, other maritime regions of Australia, off the Mexican coast in the Sea of Cortez, Uruguay, in the Canadian provinces of Nova Scotia and Ontario, and Campos dos Goytacazes and Coronel Vivida bay in Brazil. [Source: Wikipedia]
9. Shelf clouds
A shelf cloud is a low, horizontal, wedge-shaped arcus cloud. A shelf cloud is attached to the base of the parent cloud, which is usually a thunderstorm. Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent and wind-torn. Cool, sinking air from a storm cloud’s downdraft spreads out across the land surface, with the leading edge called a gust front. This outflow cuts under warm air being drawn into the storm’s updraft. As the lower cooler air lifts the warm moist air, its water condenses, creating a cloud which often rolls with the different winds above and below (wind shear).
People seeing a shelf cloud may believe they have seen a wall cloud. This is a likely mistake, since an approaching shelf cloud appears to form a wall made of cloud. A shelf cloud usually appears on the leading edge of a storm, and a wall cloud will usually be at the rear of the storm. [Source: Wikipedia]
10. Pyrocumulus cloud
A pyrocumulus, or fire cloud, is a dense cumuliform cloud associated with fire or volcanic activity. A pyrocumulus is similar dynamically in some ways to a firestorm, and the two phenomena may occur in conjunction with each other. However, one may occur without the other. A pyrocumulus cloud is produced by the intense heating of the air from the surface.
The intense heat induces convection which causes the air mass to rise to a point of stability, usually in the presence of moisture. Phenomena such as volcanic eruptions, forest fires, and occasionally industrial activities can induce formation of this cloud. The detonation of a nuclear weapon in the atmosphere will also produce a pyrocumulus in the form of a mushroom cloud which is made by the same mechanism. The presence of a low level jet stream can enhance its formation. Condensation of ambient moisture (moisture already present in the atmosphere) as well as moisture evaporated from burnt vegetation or volcanic outgassing occurs readily on particles of ash. [Source: Wikipedia]
11. Wave windows
In meteorology, lee waves are atmospheric standing waves. Both lee waves and the rotor may be indicated by specific wave cloud formations if there is sufficient moisture in the atmosphere, and sufficient vertical displacement to cool the air to the dew point. Waves may also form in dry air without cloud markers. Wave clouds do not move downwind as clouds usually do, but remain fixed in position relative to the obstruction that forms them.
Adiabatic compression heating in the trough of each wave oscillation may evaporate cumulus or stratus clouds in the airmass, creating a “wave window” or “Foehn gap” [Source: Wikipedia].
12. Actinoform clouds
An actinoform or actiniform cloud is a collection of marine low clouds that takes a distinct shape. They are named after the Greek word for “ray” due to their radial structure. Actinoform clouds can spread out over 300 kilometers (190 mi) across and thus cannot be easily seen with the naked eye. In addition, actinoform clouds can form “trains” that are up to six times the length of the original cloud field, yet they maintain their own, distinct identity.
In a satellite image, they look like distinct leaf-like or spokes-on-a-wheel patterns that stand out from the rest of the low-lying cloud field. However, why they have this shape or how they are formed is not known, but recent evidence suggests that the interaction of both radiation and precipitation may help to organize them on the mesoscale. The individual convective cells that collectively make up an actinoform cloud are quite shallow, with heights generally less than 2 km, and would be classified as stratocumulus clouds by an observer on the ground. Given the extensive mesoscale organization of these clouds, it is appropriate to describe them as mesoscale stratocumulus cloud systems (MSCS) in analogy with their deeper counterpart, the Mesoscale convective system. [Source: Wikipedia]
13. Polar stratospheric (nacerous) clouds
Polar stratospheric clouds or PSCs, also known as nacreous clouds, are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 meters (49,000–82,000 ft). They are implicated in the formation of ozone holes; their effects on ozone depletion arise because they support chemical reactions that produce active chlorine which catalyzes ozone destruction, and also because they remove gaseous nitric acid, perturbing nitrogen and chlorine cycles in a way which increases ozone destruction.
The stratosphere is very dry; unlike the troposphere, it rarely allows clouds to form. In the extreme cold of the polar winter, however, stratospheric clouds of different types may form, which are classified according to their physical state and chemical composition. [Source: Wikipedia]
14. Pileus (scarf/cap) clouds
A pileus, also called scarf cloud or cap cloud, is a small, horizontal, altostratus cloud that can appear above a cumulus or cumulonimbus cloud, giving the parent cloud a characteristic “hoodlike” appearance. They are formed by strong updrafts acting upon moist air at lower altitudes, causing the air to cool to its dew point. As such, they are usually indicators of severe weather, and a pileus found atop a cumulus cloud often foreshadows transformation into a cumulonimbus cloud, as it indicates a strong updraft within the cloud.
Clouds that are attached to pilei are often given the suffix “pileus” or “with pileus”. For example, a cumulonimbus cloud with a pileus attached to it would be called “cumulonimbus with pileus”. Pilei can also form above ash clouds and pyrocumulus clouds from erupting volcanoes (see the image above). They can also form above some mushroom clouds of high-yield nuclear detonations; in that context they are called ice caps. [Source: Wikipedia]
15. Morning Glory Clouds
The Morning Glory cloud is a rare meteorological phenomenon occasionally observed in different locations around the world. The southern part of Northern Australia’s Gulf of Carpentaria is the only known location where it can be predicted and observed on a more or less regular basis. The settlement of Burketown attracts glider pilots intent on riding this phenomenon.
Morning Glory clouds can most often be observed in Burketown in September to mid-November, when the chance to see it early in the morning is approximately 40%. A Morning Glory cloud is a roll cloud that can be up to 1,000 kilometres (620 mi) long, 1 to 2 kilometres (0.62 to 1.2 mi) high, often only 100 to 200 metres (330 to 660 ft) above the ground and can move at speeds up to 60 kilometres (37 mi) per hour. Sometimes there is only one cloud, sometimes there are up to eight consecutive roll clouds.
The Morning Glory is often accompanied by sudden wind squalls, intense low-level wind shear, a rapid increase in the vertical displacement of air parcels, and a sharp pressure jump at the surface. In the front of the cloud, there is strong vertical motion that transports air up through the cloud and creates the rolling appearance, while the air in the middle and rear of the cloud becomes turbulent and sinks. The cloud can also be described as a solitary wave or a soliton, which is a wave that has a single crest and moves without changing speed or shape. [Source: Wikipedia]