What Are the Clouds Telling Us?
The skies of Arizona have been painted, photographed, and admired countless times for their wide-open vistas and dramatic cloud displays. You may have gazed at the sky and noticed familiar shapes or imagined entire narratives for animals in the sky. But these clouds are more than just beautiful backdrops for the rising and setting sun; they are constantly telling a meteorological story about conditions throughout the atmosphere. The type and shape of clouds can inform us about our constantly changing atmospheric conditions.
The formation of clouds requires water vapor, CCNs, and the right temperature conditions to form. When the air temperature cools to the dew point, water vapor condenses on CCNs and forms clouds. How and where in the atmosphere the temperature cools to below the dew point to form clouds can provide clues as to what is happening to the air around us, as it is constantly in motion. With the constant motion of air moving both horizontally and vertically, clouds are simultaneously forming and dissipating, appear and disappear, grow and shrink, and change their shape so we can imagine new narratives, forecast incoming weather, or rapidly challenge our cloud identification skills.
Much like the Linnean system for identifying living organisms (Kingdom, Phylum, Class, etc.), clouds have a hierarchical method to identify clouds. There are four core types of clouds based on form and appearance, and ten basic cloud genera based partly on altitude that include features of two or more categories. From the ten basic clouds (genera), they can be further described by cloud species (Table 1), variety (Table 2), and accessory clouds (Table 3). Being able to read the constantly changing clouds can tell us how the atmosphere is behaving and what we may expect for weather in the next hours, days, or seasonally.
How are clouds formed?
There are a few ingredients required to form clouds: water vapor, tiny particles called cloud condensation nuclei (CCN), and the right temperature conditions. Clouds are formed when water vapor (gas) condenses into water droplets or ice crystals on objects called CCNs. Water molecules are too small to bond together and require a surface or an object of at least one micron (1/1,000,000th m) on which the molecules can condense on the CCNs (NOAA, 2023). CCNs are abundant small particles in the atmosphere that may come from smoke, fire, dust, or salt crystals from ocean spray. For example, when warm, moist air cools as night-time temperatures fall, water molecules slow down and need a place to condense, such as grass or other surfaces, resulting in dew. CCNs are places where these molecules can collect. These microscopic particles are hygroscopic, meaning they attract water molecules for which the water vapor can condense.
Temperature also plays a role in cloud formation. First, the air temperature must cool to its dew point. The dew point is the temperature the air needs to be cooled in order to produce a relative humidity of 100% (saturation), or when evaporation equals condensation. Air can reach the saturation point in a variety of ways, the most common being warm air rising from the surface convectively through the atmosphere and cooling. As an air parcel rises, it expands in the lower pressure at higher elevations and cools. Before the air reaches saturation, it cools at about 5.5 °F per 1,000 feet, known as the dry adiabatic lapse rate. After the air reaches its dew point and condensation begins, it cools more slowly at the saturated adiabatic lapse rate. These rates describe the cooling of rising air parcels, while the environmental lapse rate describes how the surrounding air temperature changes with height.
When the air temperature cools to the dew point (or saturation), water molecules will condense on CCNs, resulting in cloud formation. However, the atmosphere is in constant motion with air moving both horizontally and vertically. Warm air continues to rise and get mixed with cooler air, and both condensation and evaporation are occurring simultaneously. As a result, cloud droplets are constantly forming and dissipating. When more water vapor condenses on CCNs than are evaporating, clouds form and grow. Conversely, when water molecules on CCNs are evaporating faster than water condensing, clouds dissipate. This is why clouds appear and disappear, grow and shrink, and change their shape as we have all seen in the sky.
Cloud classification
Classification systems are developed for most things; for example, the Linnean system used to classify living things (Kingdom, Phylum, Class, Order, Family, Genus, Species) is one of the more notable classification systems. In fact, Luke Howard (1772-1864), the "father of meteorology," proposed a consistent and science-based system of referring to cloud types with Latin terms similar to those used for plants and animals, and his system largely remains the official system used worldwide (Cloud Appreciation Society, 2022). While many critics insisted clouds were too chaotic and ephemeral to classify, Howard's system primarily categorizes clouds into four cloud types that describe structure and appearance. Further, as Howard observed, clouds often have features of two or more categories, and he suggested combinations based on altitude modifications between the four core types of clouds. Basically, when the shape or form of a cloud has been identified, altitude can help refine the classification.
The core four types of clouds
In his essay, Modifications of Clouds (1803), Howard describes the shape and form of four core types of clouds which provide the foundation for the World Meteorological Organization's (WMO) International Cloud Atlas, the worldwide standard for clouds.
Cirrus (Ci)
Latin prefix: cirro, meaning tendril or curl
Cirrus clouds are wispy, hair-like clouds, almost as if they have been penciled in the sky (Photo 1). They are the least dense and have the greatest altitude, which can range from 15,000 ft. to 45,000 ft. in the mid-latitudes. Cirrus clouds form when warm air with low amounts of moisture rises, forming ice crystals on nuclei. Cirrus clouds are often the first to appear in advance of a low-pressure area, indicating a front or storm may be approaching. At lower latitudes (like Arizona), cirrus clouds can also be the remnants of tropical convection that formed over ocean areas far away. However, though they can be a sign that rain may be arriving, these clouds do not produce rain. Rather, when the ice crystals fall, they dissipate, melt, and evaporate as they fall through the warmer and drier air and never reach the ground. This is why cirrus clouds tend to be short-lived and may appear and disappear readily. Cirrus clouds often form as the upper-level outflow from large clusters of thunderstorms, particularly in the tropics. These expansive shields of cirrus generated by tropical convection near the equator can stream off the Pacific Ocean and drift into the Southwest U.S. at any time of year.
Photo 1. Cirrus clouds are wispy, hair-like clouds, almost as if they have been penciled in the sky. They are the least dense and have the greatest altitude, which can range from 15,000 ft. – 45,000 ft. in the mid-latitudes.
Andrew Brischke
Cumulus (Cu)
Latin prefix: cumulo, meaning pile or heap
Cumulus clouds are white, puffy clouds that look like a pile of cotton balls. They have a flat base, sharp outlines, and appear to be dense (Photo 2). They are generally low-level clouds ranging from 1,000–6,500 ft. and can be isolated or organized into lines or clusters. Cumulus clouds are indicative of convection or thermally driven instability that causes rising air in the atmosphere and can be the precursors to precipitation if sufficient surface moisture and instability are present. They are formed as rising parcels of air cool to their saturation point (flat, bottom of cloud) and then release additional heat as water vapor condenses into liquid water drops or deposits onto ice crystals. This additional heat causes more instability and pushes the parcel to continue to rise, furthering the condensation and deposition processes. Cumulus clouds are often called "fair weather" clouds and rarely produce precipitation by themselves but can grow and transform into other types of rain-producing clouds.
The puffy shape of a cumulus cloud reflects how rising air interacts with the surrounding atmosphere and the strength of convection. On fair-weather days, when vertical motion is weak, cumulus clouds appear soft and diffuse. When surface instability is high and rising air is vigorous, the clouds develop sharper, more defined edges and can grow into towering thunderstorm clouds (cumulonimbus) as the day goes on.
Photo 2. Cumulus clouds are white, puffy clouds that look like a pile of cotton balls. They have a flat base, sharp outlines, and appear to be dense. They are generally low-level clouds ranging from 1,000 – 6,500 ft. and can be isolated or organized into lines or clusters.
Michael Crimmins
Stratus (St)
Latin prefix: strato, meaning layer or sheet
Stratus clouds are usually broad and widespread clouds that are relatively featureless and can best be described as a white or grey overcast layer, or at other times appear as mist or fog (Photo 3). They are average in density and are the lowest clouds, ranging in altitude from the surface to around 1,500 ft. Stratus clouds can form in several ways. One way is when gradual daytime warming lifts morning fog off the ground, spreading it into a uniform, low cloud layer. Another way is when moist air moves over a colder surface, such as snow or a cold body of water, cooling the air from below and causing it to condense into a broad, low cloud layer. Both processes occur in calm, stable conditions, meaning the air resists rising and instead spreads horizontally. This is why stratus clouds typically appear as flat, featureless layers rather than building upward.
If stratus clouds form thick layers, it is possible for them to produce light drizzle, mist, or light rain, especially during cooler seasons. However, they typically do not produce heavy rain because of the lack of vertical movement. Thick stratus clouds can also help reduce daytime temperatures by blocking the sun; inversely, the cloud layers will trap outgoing heat and keep nighttime temperatures warmer.
Photo 3. Stratus clouds are usually broad and widespread clouds that are relatively featureless and can best be described as a white or grey overcast layer, or at other times appear as mist or fog. They are average in density and are the lowest clouds, ranging in altitude from the surface to around 1,500 ft.
Andrew Brischke
Nimbus
Latin prefix: nimbo, meaning rain or storm
Howard also designated a special rain cloud category which may combine two or more of the core clouds and their variable range of altitudes. The overwhelming majority of rain occurs from nimbus-type clouds, making them the densest of the clouds, often blocking out the sun entirely. While the term nimbus indicates the presence of rain-bearing clouds, it is often used as a modifier to other genera or species of clouds.
The ten basic clouds
Howard observed that clouds often have characteristics of two or more categories of the four core clouds. For example, clouds can have familiar features of cirrus and stratus, or cumulus and nimbus, amongst over 100 other different combinations. As Howard framed his cloud classification system using the Linnean system as a template, it should not come as a surprise that his system includes genera and species of clouds. The genera of clouds, or the ten basic clouds, are organized by height.
High-level clouds
There are three basic clouds in the high-level category: cirrus, cirrocumulus, and cirrostratus.
Cirrocumulus (Cc)
Cirrocumulus clouds are thin, white clouds high in the atmosphere. They appear as patches or layers of cloudlets (small, isolated clouds) arranged into sand-like ripples across the sky. The cloudlets must appear no larger than the width of a finger, held at arm's length. Cirrocumulus clouds are predominantly made of ice crystals and are often short-lived and soon freeze into other high clouds such as cirrus or cirrostratus. They are generally an indication of continued fair weather or improving weather after a storm.
Cirrostratus (Cs)
Cirrostratus clouds are transparent, white, veil-like clouds similar to a sheer curtain high in the atmosphere. They form a delicate layer of ice crystals and can be very extensive, covering the entire sky. Cirrostratus clouds are known for producing colored arcs, rings, and points of light known as halo phenomena. Sunlight shining through a layer of cirrostratus clouds is refracted and reflected by the ice crystals, each of which behaves like a tiny prism. Cirrostratus often indicates increasing moisture high in the atmosphere. If the cloud layer thickens and lowers over time, it may signal an approaching front or storm system.
Figure 1. The ten basic genera of clouds, arranged by height.
Valentin de Bruyn, from Wikimedia Commons (Cloud_types_en.svg), CC BY-SA 4.0
Photo 4. Cirrocumulus clouds are thin, white clouds high in the atmosphere. They appear as patches or layers of cloudlets (small, isolated clouds) arranged into sand-like ripples across the sky.
Andrew Brischke
Photo 5. Cirrostratus clouds are transparent, white, veil-like clouds similar to a sheer curtain high in the atmosphere. They form a delicate layer of ice crystals and can be very extensive, covering the entire sky.
Robin Cannon
Mid-level clouds
There are three basic clouds in the mid-level category. Altocumulus, altostratus, and nimbostratus are generally composed of water droplets but may be formed by ice crystals when temperatures are low enough.
Alto: from the Latin prefix altum, meaning high. However, alto-clouds are classified as mid-level clouds and are distinguished from higher-level clouds from their low-level liquid-based cousins, stratus, and cumulus clouds (NOAA, 2023c).
Altocumulus (Ac)
Altocumulus clouds are highly variable white to gray clouds arranged in globular masses or sheets composed of popcorn-like clumps or layered rolls. Altocumulus clouds are distinguished from cirrocumulus clouds by their transparency and size; where the cloudlets may provide shade and are 1–3 finger widths wide held at arm's length. These clouds are the most common mid-level clouds and often appear at different levels at the same time and with other cloud types. Altocumulus often signifies elevated instability and convection, or the vertical movement of air. Towering altocumulus (altocumulus castellanus) often signals the development of thunderstorms later in the day due to the instability and convection of air.
Altostratus (As)
Altostratus clouds are rather featureless clouds that range from gray to blue-tinged sheets or layers that form a blanket-like and overcast appearance. They are barely transparent enough to see the sun peek through, but do not form the halo phenomena like cirrostratus. Altostratus are formed when large masses of warm, moist air rise, causing the vapor to condense. They are an indication of an approaching warm front that may arrive with continuous rain or snow. Altostratus can produce annoying mist, drizzle, or light snow, but when it produces more significant precipitation, it generally has developed into a nimbostratus cloud.
Nimbostratus (Ns)
Nimbostratus clouds are another rather featureless cloud that is thick, gray, and full of precipitation. They are thick enough to block out the sun and are one of two clouds, the other being cumulonimbus, that are defined by producing precipitation. Nimbostratus clouds are generally formed by the thickening and lowering of an altostratus cloud, often with lower, ragged clouds on the underside that can merge with the base. They are distinguished from an altostratus cloud by the amount of precipitation released. These dark, ominous clouds can produce moderate to heavy rain for long periods of time over large areas.
Photo 6. Altocumulus clouds are highly variable white to gray clouds arranged in globular masses or sheets composed of popcorn-like clumps or layered rolls.
Flavie Audoin
Photo 7. Altostratus clouds are rather featureless clouds that range from gray to blue-tinged sheets or layers that form a blanket-like and overcast appearance.
Robin Cannon
Photo 8. Nimbostratus clouds are another rather featureless cloud that is thick, gray, and full of precipitation.
Michael Crimmins
Low-level clouds
There are four basic clouds in the low-level category. Cumulus and stratus clouds, which have been discussed above, stratocumulus and cumulonimbus are the other genera of clouds in this category.
Stratocumulus (Sc)
Stratocumulus clouds are one of the more variable genera of clouds. They range from gray to white and can be patchy or form a sheet similar to a honeycomb appearance. Stratocumulus are well-defined, large, rounded masses that usually form in large groups, lines, or waves. They are held in place at a lower altitude due to drier, stable air above them preventing any vertical buildup like the cumulonimbus clouds.
Cumulonimbus (Cb)
Cumulonimbus clouds are deep, vertically developed storm clouds capable of producing heavy precipitation, lightning, strong winds, hail, and flash flooding. They are multi-level, heavy, dense clouds that resemble a mountain or a very tall tower, or as many would know it a thunderstorm cloud. The base is very dark, and as the cloud builds, the top may form the very characteristic blacksmith's anvil shape. Cumulonimbus clouds are typically formed when water vapor condenses and builds upward by strong updrafts in the atmosphere. As air rises higher in a cumulonimbus cloud, liquid droplets may freeze, and water vapor may deposit onto ice particles, helping form snow, graupel, or hail. Graupel forms when supercooled water droplets freeze onto snow crystals or other ice particles. Cumulonimbus have the capability of producing severe weather conditions such as lightning, extreme winds, tornadoes, variable-sized hail, and flash flooding. These clouds can also produce a large range of other cloud species discussed below.
Photo 9. Stratocumulus clouds are one of the more variable genera of clouds. They range from gray to white and can be patchy or form a sheet similar to a honeycomb appearance. Stratocumulus are well-defined, large, rounded masses that usually form in large groups, lines, or waves.
Flavie Audoin
Photo 10. Cumulonimbus is the other cloud defined by producing precipitation. They are multi-level, heavy, dense clouds that resemble a mountain or a very tall tower, or as many would know it – a thunderstorm cloud.
Michael Crimmins
Typical clouds across the seasons in Arizona
The cloud types that characterize each season in the desert Southwest interact closely with the region's varied landscapes and topography. In winter, low-level stratus and mid-level altostratus often accompany cold fronts pushing in from the Pacific Ocean, bringing widespread rain and snow to higher elevations.
Spring typically has the least cloud cover as strong atmospheric high pressure settles over the region. However, thin shields of high-level cirrus clouds can still be seen streaming from the west, often originating from tropical convection in the equatorial Pacific.
With the dramatic increase in low-level moisture during the summer monsoon, towering cumulus and cumulonimbus clouds form almost daily—first over the mountains, then spreading into the lower deserts as afternoon storms build and organize.
In fall, declining moisture levels bring a return of cirrus and stratocumulus clouds, often signaling the approach of cooler, calmer weather.
Figure 2. Common Arizona cloud types by season.
Graphic generated using OpenAI DALL·E, ChatGPT, January 2026; content reviewed and adapted by the authors
Cloud species and varieties
The ten genera of clouds can be further subdivided, much like plants and animals, into species and varieties based on the shape or the internal structure of the cloud. The variety of clouds describes the different arrangements of the visible elements of clouds and the varying degrees of transparency (International Cloud Atlas, 2017).
There are 15 species that describe the shape and structure of clouds. However, not every species can be associated with every genus of clouds. Below is a list of the 15 species of clouds and a brief description for each and the genera of clouds they may be associated with (Table 1).
| Cloud species | Abbreviation | Description | Associated genera |
|---|---|---|---|
| calvus | cal | Rough tops of Cb becoming smooth | Cb |
| capillatus | cap | Tops of Cb forming a plume or an anvil shape | Cb |
| castellanus | cas | Distinct columns or turrets connected by a common base | Sc, Ac, Cc |
| congestus | con | High vertical extent with rapid growth and cauliflower-like tops | Cu |
| fibratus | fib | Long, fine hair-like strands without hooks | Ci, Cs |
| floccus | flo | Small tufts or clumps with soft, fluffy edges (Photo 11) | Ac, Cc, Ci |
| fractus | fra | Rapidly changing small or broken fragments with very ragged edges (Photo 12) | Cu, St |
| humilis | hum | Small vertical extent and flattened | Cu |
| lenticularis | len | Lens-shaped | Sc, Ac, Cc |
| mediocris | med | Moderate vertical extent with small bulging tops | Cu |
| nebulosus | neb | Thin veil with no distinct detail | St, Cs |
| spissatus | spi | Patchy and dense enough to appear grey towards the sun | Ci |
| stratiformis | str | Extensive horizontal sheet, sometimes with breaks | Sc, Ac, Cc |
| uncinus | unc | Hair-like streaks terminating with hooks or comma-shaped (Photo 13) | Ci |
| volutus | vol | Long, typically low, detached, horizontal tube-shaped roll cloud | Sc, Ac |
Photo 11. Cirrus floccus with small tufts or clumps with soft, fluffy edges.
Robin Cannon
Photo 12. Cumulus fractus with small broken fragments and ragged edges.
Robin Cannon
Photo 13. Cirrus uncinus with hair-like streaks terminating with hooks or comma-shaped.
Michael Crimmins
There are nine types of cloud varieties. Varieties describe the transparency and arrangement of the cloud elements:
| Cloud variety | Abbreviation | Description | Associated genera |
|---|---|---|---|
| duplicatus | du | Sheets arranged at multiple different levels (Photo 14) | Ci, Cs, Ac, As, Sc |
| intortus | in | Irregular, curved, or tangled hair-like strands | Ci |
| lacunosus | la | Sheet with small regularly spaced honeycomb or net-like holes | Cc, Ac, Sc |
| opacus | op | Sheet that completely masks sun or moon | Ac, As, Sc, St |
| perlucidus | pe | Sheet where small gaps allow blue sky, sun, or other clouds to be seen | Ac, Sc |
| radiatus | ra | Broad parallel bands converging towards one or two points based on perspective (Photo 15) | Ci, Ac, As, Sc, Cu |
| translucidus | tr | Sheet translucent enough to permit the sun or moon to be seen | Ac, As, Sc, St |
| undulatus | un | Sheets of parallel undulating waves or ripples (Photo 16) | Cc, Cs, Ac, As, Sc, St |
| vertebratus | ve | Fan out like ribs or fish skeleton (Photo 17) | Ci |
Photo 14. Cirrostratus duplicatus with sheets arranged at multiple different levels.
Robin Cannon
Photo 15. Altocumulus radiatus with broad parallel bands converging towards a point based on perspective.
Jennifer Leese
Photo 16. Altostratus undulatus with sheets of parallel undulating waves or ripples.
Jennifer Leese
Photo 17. Cirrus vertebratus where clouds fan out like ribs or a fish skeleton.
Jennifer Leese
Supplementary features and accessory clouds
Clouds may also have one or more supplementary features or accessory clouds associated with them. Supplementary clouds are formations that are either attached or partly merged with a larger cloud. They may occur at any level of the cloud, or above or below it (International Cloud Atlas, 2017). There are 11 types of supplementary clouds (Table 3).
| Supplementary cloud | Abbreviation | Description | Associated genera |
|---|---|---|---|
| arcus | arc | Dense, horizontal roll often associated with the leading edge of a thunderstorm | Ci, Cs, Ac, As, Sc |
| asperitas | asp | Chaotic, wave-like clouds resembling rough sea waves | Ci |
| cauda | cau | Horizontal tail (not a funnel) extending away from the main cloud towards the wall cloud | Cc, Ac, Sc |
| cavum | cav | Well-defined, often circular hole in thin layer of supercooled water droplet cloud | Ac, As, Sc, St |
| fluctus | flu | Short-lived, breaking wave formation typically on top surface of the cloud (Photo 18) | Ac, Sc |
| incus | inc | Anvil-shaped upper portion of cumulonimbus cloud (Photo 19) | Ci, Ac, As, Sc, Cu |
| mamma | mam | Hanging protuberances on underside of the cloud often resembling the udders of a cow (Photo 20) | Ac, As, Sc, St |
| murus | mur | Localized, persistent, abrupt lowering of cloud from base of cumulonimbus. Often associated with supercell storms and tornado formation (aka wall cloud) | Cc, Cs, Ac, As, Sc, St |
| praecipitatio | pra | Precipitation that reaches the ground | Ci |
| tuba | tub | Column extending down from base indicating intense vortex (aka funnel cloud or tornado) | — |
| virga | vir | Precipitation that evaporates before reaching the ground | — |
Clouds may also have associated smaller clouds, known as accessory clouds which are separate from the main cloud but are partly merged with it. The four accessory clouds are:
- Flumen (flm): Bands of low clouds associated with supercell thunderstorms (Cb) that are arranged parallel to the low-level winds that often resemble a beaver's tail.
- Pannus (pan): Ragged shreds of clouds situated below or sometimes attached to another cloud (Photo 21).
- Pileus (pil): A smooth cap-cloud that forms above a cumulus or cumulonimbus cloud.
- Velum (vel): A broad horizontal cloud veil attached or just above the upper part of one or several cumulus clouds that often pierce it.
A complete cloud classification can include the cloud genus, species, variety, and any supplementary features or accessory clouds that are present. These descriptors are added in a specific order, beginning with the genus and followed by the species and variety (Tables 1 and 2). For example, a cloud identified as altocumulus stratiformis perlucidus belongs to the genus altocumulus, has the species stratiformis because it forms an extensive horizontal layer, and the variety perlucidus because gaps between the cloud elements allow sunlight or blue sky to be seen through the cloud deck. Another example is cirrus fibratus radiatus, where fibratus describes the cloud's long, fibrous appearance and radiatus indicates cloud bands appear to converge toward the horizon.
Supplementary features and accessory clouds can be added to further describe unique characteristics (Table 3). Supplementary features are attached to the main cloud name and identify structures associated with the cloud itself. For example, altocumulus lenticularis virga identifies a lens-shaped cloud producing precipitation that evaporates before reaching the ground. Accessory clouds are separate clouds physically connected to or generated by the parent cloud. Examples include pileus, a smooth cap cloud that forms above a rapidly growing cumulus or cumulonimbus cloud, and pannus, ragged fragments that often develop beneath precipitating clouds.
Not every cloud receives all classification levels. Some clouds can only be assigned a genus, while others may include a species, variety, and one or more supplementary features. The naming system functions much like scientific classification in biology, with each additional descriptor providing more information about the cloud's appearance, structure, and atmospheric processes.
Photo 18. Stratus fluctus with a short-lived, breaking wave formation on the top surface of the cloud.
Andrew Brischke
Photo 19. Cumulonimbus incus with a distinct anvil shape on the upper portion of the cumulonimbus cloud.
Michael Crimmins
Photo 20. Stratocumulus mamma with hanging protuberances on the underside of the cloud often resembling the udders of a cow.
Roberta Brischke
Photo 21. Cumulonimbus with pileus, a smooth cap cloud that forms above a rapidly growing cumulus or cumulonimbus cloud.
Michael Crimmins
Special clouds and upper atmospheric clouds
In addition to cloud genus, species, and varieties, there are two other classifications of clouds. Upper atmospheric clouds (nacreous and noctilucent clouds) and special clouds, which tend to be rarely observed, or in some cases only in certain parts of the world. Noctilucent clouds (NLCs) are the highest clouds in the Earth's atmosphere, forming in the mesosphere about 50 miles above Earth's surface. NLCs are composed of tiny ice crystals and are known for their eerie, electric-blue and silvery appearance during the summer twilight.
Special clouds may form or grow as a consequence of specific, often localized factors. These clouds may be either natural or the result of human activity (International Cloud Atlas, 2017). One example of a natural special cloud is commonly known as a pyrocumulonimbus cloud. This cloud develops from the convection of air by the intense heat from wildfires or volcanic eruption activity.
An example of a human-caused cloud is commonly known as contrails (short for condensation trails). These form in the wake of hot, humid exhaust from an aircraft engine, mixing with the freezing and moist air, turning water vapor into a trail of ice crystals. Contrails are often confused with chemtrails, which are an erroneous, unproven conspiracy theory where some believe aircraft are spraying chemical or biological agents into the atmosphere.
Summary
Whether you are taking a moment to admire the clouds in the sky, imagining a story of rather familiar shapes and outlines, or are concerned with what the weather will bring in the next hours or days, clouds can inform us about our constantly changing atmospheric conditions.
The formation of clouds requires water vapor, CCNs, and the right temperature conditions to form. When the air temperature cools to the dew point, water vapor condenses on CCNs and forms clouds. How and where in the atmosphere the temperature cools to below the dew point to form clouds can provide clues as to what is happening to the air around us, as it is constantly in motion. With the constant motion of air moving both horizontally and vertically, clouds are simultaneously forming and dissipating, appear and disappear, grow and shrink, and change their shape so we can imagine new narratives, forecast incoming weather, or rapidly challenge our cloud identification skills.
Much like the Linnean system for identifying living organisms (Kingdom, Phylum, Class, etc.), clouds have a hierarchical method to identify clouds. There are four core types of clouds based on form and appearance, and ten basic cloud genera based partly on altitude that include features of two or more categories. From the ten basic clouds (genera), they can be further described by cloud species (Table 1), variety (Table 2), and accessory clouds (Table 3). Being able to read the constantly changing clouds can tell us how the atmosphere is behaving and what we may expect for weather in the next hours, days, or seasonally.
So while you are out enjoying the outdoors, take some time to admire the clouds, imagine a story, and try to identify the clouds you are admiring and what they are telling you!
Photo 22. Cumulus congestus on its way to forming a cumulonimbus calvus flammagentitus, or a pyrocumulonimbus cloud, observed from across the Grand Canyon looking north towards the Dragon Bravo Fire, 2024.
Iric Burden
References
Cloud Appreciation Society. (2022). Luke Howard: The namer of clouds. https://cloudappreciationsociety.org/luke-howard-the-namer-of-clouds/
Cloud Appreciation Society. (2025). Cloud library. https://cloudappreciationsociety.org/cloud-library/
Howard, L. (1803). Essay on the modifications of clouds. John Churchill & Sons.
National Oceanic and Atmospheric Administration. (2023a). How clouds form. JetStream School for Weather. https://www.noaa.gov/jetstream/clouds/how-clouds-form
National Oceanic and Atmospheric Administration. (2023b). The four core types of clouds. JetStream School for Weather. https://www.noaa.gov/jetstream/clouds/four-core-types-of-clouds
National Oceanic and Atmospheric Administration. (2023c). Ten basic clouds. JetStream School for Weather. https://www.noaa.gov/jetstream/clouds/ten-basic-clouds
World Meteorological Organization. (2017). International cloud atlas. https://cloudatlas.wmo.int/en/home.html