Clouds have always been an endless source of fascination to humans all over the world. Their ever-changing shapes and marvelous ability to capture and reveal hidden colors are wondrous. These fluffy weather phenomena are the ultimate secret keepers. To grasp the complexities of cloud systems, scientists have developed a method to classify clouds by type.
Cloud types are classified according to their genera, species, variety, and supplemental features. A cloud may only belong to one genus and species but may have multiple varieties and supplemental features. Common, special, accessory, and mother clouds form the four broad groupings of clouds.
The study of clouds has left scientists scratching their heads for a long time. To effectively study and understands clouds, it is essential to have a working knowledge of our entire atmosphere. Advances in cloud science have been achieved by using modern technology such as NASA’s Terra, Aqua, Aura, CALIPSO, CloudSat, and other satellites.
It is vital to study clouds, as this provides valuable data on both the current complexities and predictions for climate change. Thankfully, you and I do not need NASA-level knowledge to learn about the different cloud types and ways they are formed!
What Is A Cloud?
Clouds are hydrometers formed when air containing water vapor is forced into a low-temperature, low-pressure atmospheric condition by convection currents, low-pressure weather systems, or colliding weather fronts. As the moisture-laden air is forced upward, the air pressure drops, causing the air particles to move further apart. The gaseous expansion allows the atmospheric gaseous water and air molecules to cool until the dew-point is reached.
Air can hold more water under high-temperature, high-pressure conditions than low-temperature, low-pressure conditions. As the air cools and air pressure drops, the air’s maximum saturation point is lowered. When saturation is reached, the gaseous water vapor will turn into liquid water droplets (Condensation) or tiny ice crystals (Deposition).
Condensation and deposition occur more quickly if there are solid non-aqueous particles like dust, ash or pollen, for the liquid droplets and ice crystals to form on. A considerable accumulation of these droplets and ice-crystals form visible clouds.
Principles Of Cloud Classification
Clouds can form in empty skies or seamlessly transition from one cloud type to another. They seem to be in an ever-changing pattern of luminance, color, and shape. Human’s like to classify things, and clouds are no exception. Like the classification for plants and animals, clouds are classified according to their genera, species, and variety with Latin names.
Clouds are broadly categorized as common, special, and upper stratospheric clouds, accessory clouds, and mother clouds. There are ten common cloud genera, six special cloud genera, and three upper stratospheric cloud genera. Each cloud type belongs to only one genus.
Cloud types are then differentiated into species based on their shape and internal structure. After species, clouds are divided into varieties based on their relative transparency/opacity and differing arrangements of the visible elements.
Common Types of Clouds
Although clouds are often known for their transitory nature, common cloud genera are characterized by their relatively stable characteristics. Common cloud genera frequently occur in multiple weather systems across the globe and often exist as the dominant or co-dominant cloud system.
A cloud may temporarily exist in a transitory form, but these are usually of little interest to scientists. Transitory clouds are unstable and have no significant influence on atmospheric conditions.
Cloud Level: High
Name Meaning: Cirrus – curl.
Altitude: 16,000ft – 49,000ft
The ubiquitous Cirrus clouds have a distinctive easy-to-identify appearance. Primarily made of ice-crystals, these clouds have a much flatter, fibrous appearance than other cloud systems. Their wispy, delicate, lace-like appearance allows plenty of sunshine to get through the cloud cover.
While these clouds are often the forerunners of precipitation, they have zero precipitation potential. Occasionally these clouds can be associated with a fallstreak. The deluge of a fallstreak evaporates before touching the earth’s surface.
Cloud Level: High
Name Meaning: Cirro- curl and cumulo- heap.
Altitude: 16,000ft – 49,000ft
Cirrocumulus occurs at the same altitude as cirrus and cirrostratus clouds; these three cloud systems often exist simultaneously. Cirrocumulus clouds are the only clouds to appear at this altitude and still display cloud-heap characteristics. They are not fibrous clouds and will often form thin cloud patches that have a grainy appearance, like rice.
To see this cloud formation is a privilege because of its rarity. Occasionally these clouds, adopt an almost fish-scale pattern to produce an effect known as a mackerel sky. A mackerel sky at sunset is genuinely wonderous, like the earth is slowly donning an iridescent coat of colors.
Cloud Level: High
Name Meaning: Cirro- curl and Stratus- layer.
Altitude: 20,000ft – 43,000ft
These clouds exist as an almost transport layer high up in the troposphere. Ironically the most distinctive feature about Cirrostratus is their lack of features. There are only two species (Cirrostratus fibrates and Cirrostratus nebulosus) and two varieties (Cirrostratus duplicatus and Cirrostratus undulatus) associated with this genus.
As a relatively common cloud, most people will be familiar with these clouds’ halo-effect. When viewing a light source, like the sun or moon, through the pale, veil-like lens of these clouds, a halo effect is produced. These clouds can create a 22° halo around the source of light.
Cloud Level: Middle
Name Meaning: Alto- high and Stratus- layer.
Altitude: 7,000ft – 23,000ft
Altostratus is the acknowledged shape-shifters of the cloud world. Sharing the Cirrocumulus’ ability to produce a mackerel sky, they can also form tube-like shapes, haughty castle towers and, very cool UFO-type shapes. Altostratus is found a little lower in the troposphere than the Cirrocumulus but higher than the Cumulus and Stratocumulus clouds.
Like the preceding three cloud species, Altostratus has almost zero precipitation potential. The clouds vary from moderate to light cloud cover with a white to mid-grey appearance. This is a prevalent cloud type, and most people will be familiar with the gently rolling appearance of these cloud systems.
Cloud Level: A multilevel cloud that extends from relatively close to the ground up towards the middle stratosphere
Name Meaning: Nimbo- rain and stratus- layer
Altitude: 2,000ft – 8,000ft
Have you ever looked outside and seen the heavy laden dense, and soupy-looking cloud? The kind that almost guarantees rain or snow. The cloud was most likely a Nimbostratus.
Both Nimbostratus and Cumulonimbus clouds are consistent precipitation producers. Without seeing either cloud, you can tell which cloud is involved with the rain by the type of rain falling. Dramatic, high-stakes weather is usually caused by Cumulonimbus clouds, while steady, unrelenting rain is the work of Nimbostratus.
These clouds are purists and demonstrate no species nor varieties. They have two supplementary features: Nimbostratus praecipitatio (rain that reaches the ground) and Nimbostratus virga (evaporating rain). These common clouds range from mid-grey to dark grey and offer moderate to dense cloud cover with a high, almost guaranteed chance of precipitation.
Cloud Level: Low
Name Meaning: Strato- layer, and Cumulus- heap.
Altitude: 2,000ft – 7,000ft
The name of the relatively common Stratocumulus is the perfect example of the mid-range characteristics of these clouds. They exist as both a layer and cloud heaps. Small cloud heaps are connected by filaments and conjoining of adjacent clouds. As mentioned, these clouds excel at mid-range inoffensive characteristics, with a moderate chance of precipitation, light to dark grey color, and mostly cloudy to mostly sunny cloud cover.
Like the Altocumulus clouds, the Stratocumulus is associated with multiple species, varieties, and cloud features. Their low level and changeable nature mean that the “cloud game” is most commonly played with these cloud types.
Cloud Level: Low
Name Meaning: Cumulus- low.
Altitude: 0ft – 7,000ft
The Stratus clouds, accused of being bland and featureless, are the long-time favorites of cinematographers. Where others see a dull grey blanket of featureless dreariness, cinematographers see a portal to another world. The kind where ghosts and fairies can step through the veil of time, shrouded by the secretive Stratus. The Stratus clouds are associated with two species, three varieties, and two supplementary features.
The Stratus nebulosus forms a dense cloud layer that can reach down and touch the ground creating fog. As the fog begins to dissipate, the swirling Stratus fractus species develops as the Stratus structure is fragmented into shards of clouds.
The Stratus opacus completely obscures the sun, while the Stratus translucidus allows the viewer to dimly see the sun through a haze. The Stratus undulatus has a wave-like appearance as there are shifting bands of opacity and translucence. Stratus with supplementary features may appear as a Stratus fluctus and Stratus praecipitatio.
Cloud Level: Low but can extend vertically into the upper cloud layers.
Name Meaning: Cumulus- heap.
Altitude: 2,000ft – 7,000ft
Beloved by cartoonists, Cumulus clouds are easily recognizable by their characteristic cotton-ball appearance. These popcorn-shaped clouds are commonly associated with four species: Cumulus fractus, Cumulus mediocris, Cumulus congestus, and Cumulus humilis.
Although primarily a low-level cloud, the species, Cumulus congestus can extend vertically into the middle troposphere layer. Cumulus humilis is wider than it is tall, the opposite of Cumulus congestus, which is taller than its wide. Like a gentleman wearing a tall hat, the height of Cumulus congestus may be further extended by a Pileus cloud cap.
The neutral Cumulus mediocris tries very hard to be square and make its height equal to its width. Cumulus fractus is the ragged, broken fragments of other Cumulus clouds as they dissipate in windy conditions.
These clouds are widespread across the globe with a moderate chance of precipitation, a white to light grey color, and they allow sunny or mostly sunny conditions.
Cloud Level: A low-hanging cloud that can extend vertically into the middle and upper cloud layers.
Name Meaning: Cumulo- heap and Nimbo- rain.
Altitude: 2,000ft – 52,000ft
Cumulonimbus has undoubtedly earned the title of a drama queen. These potent clouds are the chief perpetrators of thunderstorms, the kind where nature unleashes all her fury and serves it with a full complement of rain, wind, hail, thunder, and lightning. These clouds are relatively uncommon and show a range of colors from light-grey to an intense mood-setting storm-grey.
Pilots work to actively avoid flying into a Cumulonimbus weather system. Not only is the storm challenging to navigate through, but the internal conditions of a Cumulonimbus cloud have been known to tear light aircraft apart.
Upper Atmospheric Clouds
There are three types of Upper Atmospheric Clouds; Nacreous, Polar Stratospheric, and Noctilucent Clouds. They exist only in the upper stratosphere and mesosphere around specific global regions, primarily the polar vortices surrounding the Arctic and Antarctic regions.
The lowest of these clouds form at altitudes of 50,000ft (15Km). The highest altitude, Upper Atmospheric Clouds can occur at is near or in the mesopause layer. The mesopause layer sits at a whopping 300,000ft (85km) above sea level.
Cloud Level: Upper Stratosphere
Name Meaning: Nacra- an old English word meaning mother of pearl.
Altitude: 68,000ft – 100,000ft
Nacreous clouds are rare clouds occurring only in the Antarctic polar vortex, the Arctic Vortex, and occasionally above Alaska, Scotland, Scandinavia, North Russia, and Canada. They have a similar shape and organizational structure to Cirrus and Altocumulus lenticularis, although Nacreous clouds occur at much higher altitudes than the main cloud types.
Nacreous clouds are most stunning when irisation occurs. These clouds are primarily made up of uniform ice spheres, approximately 10°µm diameters; these tiny spheres form at temperatures below -85°C. Irisation occurs when the ice spheres cause the light waves to undergo interference or diffraction, producing the breath-taking iridescent mother-of-pearl colors of Nacreous clouds.
As the fading sun falls below the horizon, the sun’s rays strike the base of the Nacreous clouds. The rays are bent (diffraction) or overlap with similar waves (interference) to produce stunning shimmery blues, pinks, and pastel colors.
Nacreous clouds most often exist as stationary lenticular waves in the gravity wave, downwind of mountain ranges. Non-lenticular forms exist amongst Nacreous clouds and appear to slowly undulate as the gravity wave pulls them along.
Nitric Acid And Water Polar Stratospheric Clouds
Compared to Nacreous clouds, Nitric acid and water polar stratospheric clouds form at a balmy -78°C (nitric acid trihydrate) and -81°C (supercooled ternary solution). Nitric acid trihydrate is a molecule formed by combining three water molecules with a nitric acid molecule at temperatures below -78°C. If a nucleation barrier exists, a ternary solution (i.e., three-part mixture) of co-condensed water molecules and nitric acid forms on stratospheric sulphuric particles at temperatures below -81°C.
These clouds are challenging to see without specialized equipment as they do not have the impressive light display of the Nacreous clouds. They are best viewed at twilight, just after sunset or before sunrise, where they will appear as a thin yellowish haze.
These clouds are particularly influential on the development of the ozone hole. The nitric acid trihydrate and supercooled ternary solution provide convenient surfaces for the catalysis of benign chlorine compounds into reactive chlorine compounds involved in the ozone breakdown.
Noctilucent Clouds (Polar Mesosphere Clouds)
These clouds are undoubtedly one of the most elegantly understated clouds to view. Their serene bluish-silver color is the result of ice-crystals forming on minute dust remnants of cosmic micrometers.
They have a cirrus-type shape and are formed in the mesosphere layer at temperatures below -120°C. Noctilucent clouds can easily be seen in the night sky, under the gentle radiance of the moon. These clouds would not look out of place gracing an elvish sky.
Special clouds do not share the features of the ten common cloud system genera. As special clouds, they infrequently occur due to localized natural or human activity. Despite their relative rarity, they are common enough and with sufficiently stable characteristics to warrant the classification and division of these cloud systems into five sub-types of special clouds based on their origin.
- Flammagenitus: originate from localized superheated structures and phenomena, e.g., forest fires and volcanos
- Homogenitus: These clouds occur as the result of human activity, e.g., factories. Clouds produced as a consequence of human activity are named according to their genus and shape, followed by the word homogenitus.
- Contrails: These are also known as aircraft condensation trails. They have a distinctive ribbon-like appearance and must exist for longer than ten minutes to be classified as a Contrail.
- Homomutatus: These are contrails that persist and begin to grow or spread out under windy conditions.
- Cataractagenitus: Waterfalls produce these clouds when dispersed water is caught in the updraft created by the waterfall.
- Silvagenitus: The clouds are formed over forests due to evaporation and evapotranspiration courtesy of the dense tree canopy found in forests.
Cloud genera are primarily based upon the level and broad characteristics of a specific cloud type. Clouds are grouped into species according to precise shapes and internal structures. Like with genera, each cloud may only belong to one species at any one time unless transformed into another species.
The world meteorological organization recognizes fifteen cloud species. The below table summarizes the characteristics of each species. Cloud species names are abbreviated to three lower case letters.
|Fibratus||fib||They have a hair-like appearance with a thin cloud veil. It is made of wispy strands of straight or irregularly curved lines that do not end or hooks or tufts.||Cirrus|
|Uncinus||unc||Curved strands ending in a hook or tuft type structure||Cirrus|
|Spissatus||spi||Tightly packed, dense cloud structure with a grey appearance.||Cirrus|
|Castellanus||cas||Rising towers or crenelated turrets||Cirrus|
|Floccus||flo||A small tufty heap with a ragged lower edge – it looks a little like a cartoonist would draw a high-speed character.||Cirrus|
|Stratiformis||str||They form an extensive horizontal sheet of rippled cloud shapes. This species is often associated with a mackerel sky effect.||Cirrocumulus|
|Nebulosus||neb||An indistinct veil-like-effect is created with this species.||Cirrostratus|
|Lenticularis||len||A flattened disc or lens shape with defined borders that may occasionally show iridescence to resemble a UFO||Cirrocumulus|
|Fractus||fra||A ragged fractured appearance||Stratus|
|Humilis||hum||They form a horizontal rectangle that is wider than they are tall.||Cumulus|
|Mediocris||med||An approximate square, they are roughly as tall as they are wide with small protuberances and sprouts along the upper edge of the cloud.||Cumulus|
|Congestus||con||A vertical rectangle, these clouds are taller than they are wide, with extensive sprouting along the upper edge resembling a cauliflower.||Congestus|
|Volutus||vol||A low-lying horizontal tube-shaped that can appear to roll about a horizontal axis: these clouds are completely detached from other cloud systems. Their common name of roll-cloud easily identifies them by their characteristic shape.||Stratocumulus|
|Calvus||cal||They resemble a bald cauliflower, with vertical striations/grooves parallel to the airflow.||Cumulonimbus|
|Capillatus||cap||The upper portion of a Cumulonimbus Capillatus has a fibrous structure resembling a bad case of bed head.||Cumulonimbus|
Varieties of Clouds
A cloud can display multiple varieties. Varieties refer to the relative transparency/opacity of a cloud and the arrangement of the different visible elements. The more unusual and captivating cloud forms are typically due to the influence of different cloud varieties. There are nine varieties:
Large cloud systems are often accompanied by smaller accessory clouds, which are dependent on the more extensive cloud system for continued development and existence. These smaller clouds are known as accessory clouds. They may exist separately to the larger cloud system or be connected in some way, like when an older sibling holds the younger sibling’s hand.
There are four main groups of accessory clouds:
- Flumen clouds.
Supplementary Features To Clouds
A supplementary feature may be mistakenly classified as accessory clouds, but this is incorrect. These cloudy accessories are add-on features for existing cloud systems and not separate cloud systems like accessory clouds. A cloud may have multiple supplementary features that can produce interesting-looking clouds and make classification a little trickier. There are eleven supplementary cloud features:
- Arcus: These features are commonly referred to as a shelf cloud or gust collar forming a dark and menacing arch along the lower front edge of specific clouds. They have a dense horizontal shape with ragged edges.
- Asperitas: They have a clearly-defined smooth or dappled wave-like appearance, like the underside of a billowing blanket. They can be similar to undulatus but are more chaotic and have less horizontal organization.
- Cauda: Often attached to a wall cloud (Murus), they resemble a thick tail like the otter tail of a Labrador.
- Cavum: This easily recognized feature is also known as a hole-punch or fallstreak hole as it creates a clearly defined hole in the cloud cover. The gap is most commonly circular but can have a linear shape.
- Fluctus: Also known as Kelvin-Helmholtz waves, they form a curly wave-form on the upper edge of clouds. Enthusiasts need to be quick about seeing these clouds as they have a very short existence.
- Incus: Anvil-shape found above a cumulonimbus
- Mamma: Sac-like balls on the underside of the base of the cloud. Some people claim their appearance resembles a cow’s udders.
- Murus: Associated with supercell storms and multicell storms, MurusMurus is the abrupt lowering of a significant portion of a wall of cloud. Tuba can develop from the vertical motion and rotation of the MurusMurus.
- Praecipitatio: Precipitation involving ice, rain, hail, or snow that succeeds in reaching the earth’s surface.
- Tuba: Sometimes known as a funnel cloud and tornado, they have a narrowly defined inverted cone shape as they sprout from a cloud base.
- Virga: The hazy grey streak is produced by vertical strips of evaporating rain that do not reach the ground.
Clouds often form in a clear sky, but they may also be the product of another cloud system’s processes. Clouds that give rise to other cloud genera are known as mother clouds. The “progeny” of mother clouds are named according to their new genus, followed by the name of the mother cloud’s genus with the attached suffix of either –genitus, or –mutatus .e.g., Stratocumulus cumulogenitus or Cirrus cirrostratomutatus.
The suffix attached to the “progeny” cloud is dependent on the cloud process that gives rise to the new genus. Genitus mother clouds are clouds that produce pronounced extensions. These extensions eventually develop into a new cloud genus. The extension may detach or remain attached to the mother cloud. Mutatus mother clouds are clouds that transform to become a new cloud genus. A mutatus mother cloud will cease to exist once the transformed cloud is established.
Clouds are genuinely fascinating. To study clouds is to embark on a life-long study of shapes, color, light, and various weather systems. Modern cloud science offers a tantalizing glimpse into the secrets held so close by this weather phenomena.
While everyone can indulge in cloud appreciation, the authentic cloud aficionados will know how to classify common clouds according to the ten genera, fifteen species, and nine varieties, plus the four groups of accessory clouds and eleven supplemental features. The truly avid will know the six special clouds, two mother cloud processes, and the three forms of upper atmospheric clouds.
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