Cloud Structures and Features

Cloud Classification: Species, Varieties, and Features Introduction Clouds are classified into different categories based on their appearance, structure, and origin. Beyond the basic cloud types (genera) like cumulus and stratus, meteorologists use additional descriptive systems to precisely communicate cloud characteristics. This classification system includes species (structural sub-types), varieties (descriptive modifiers), supplementary features (attached structures), and accessory clouds (detached formations). Understanding these distinctions is essential for reading weather observations, interpreting satellite imagery, and understanding cloud physics. Cloud Species: Structural Categories Cloud species describe the internal structure and overall appearance of a cloud. They provide more specific information than the cloud genus alone. Stable or Mostly Stable Species Cirrostratus nebulosus appears as a diffuse, featureless layer without distinct structure. It forms a thin, uniform veil across the sky. Cirrostratus fibratus consists of semi-merged filaments that gradually transition toward cirrus characteristics. You'll see these as delicate, thread-like patterns. Cirrus species fall into several structural categories: Cirrus fibratus displays straight, wavy, or twisted filaments Cirrus uncinus (also called "hooked cirrus") features filaments with characteristic hooked or curved ends Cirrus spissatus appears as opaque patches rather than delicate filaments—these are thicker and denser than other cirrus varieties Stratocumulus and altocumulus share two important species: Stratiformis describes extensive, continuous sheets of clouds Lenticularis describes lens-shaped clouds that form downwind of mountain ranges due to standing waves in the atmosphere Ragged Species The term "ragged" indicates fragmented, broken cloud appearance, often associated with instability or storm activity. Stratus fractus consists of ragged, broken sheets—partly stable layers with irregular edges. These often appear as broken remnants of larger cloud formations. Cumulus fractus refers to small, ragged cumulus remnants. These can accompany heavy precipitation and are sometimes called pannus when they form as accessory clouds beneath larger precipitating systems. Partly Unstable Species Castellanus appears as small turret-like protrusions rising from an otherwise stable, stratiform or cirriform cloud layer. The name comes from their resemblance to castle turrets, and they indicate weak instability within an otherwise stable layer. Floccus consists of small, tufted, detached clouds that appear cotton-like or woolly. These can occur with cirrus, cirrocumulus, altocumulus, or stratocumulus clouds. Volutus is a dramatic roll cloud with a cylindrical, rotating appearance. It can develop ahead of a cumulonimbus or in association with geographic features. The "Morning Glory" clouds that occur over northern Australia are famous examples of volutus clouds. Cloud Varieties: Descriptive Modifiers Varieties are additional descriptive terms that modify how we describe a cloud's appearance. While species describe structural characteristics, varieties describe optical properties and patterns. A single cloud can have both a species and a variety designation. Opacity-Based Varieties These varieties describe how much light passes through the cloud: Translucidus indicates a thin, translucent cloud through which you can clearly see the sun or moon. This is typical of thinner altocumulus or stratocumulus layers. Perlucidus describes a thick, mostly opaque cloud with small translucent breaks or gaps. The cloud blocks most light, but thin sections allow some light penetration. Opacus refers to a completely opaque, thick cloud that blocks all direct light. The sun or moon is completely obscured. Pattern-Based Varieties These varieties describe visual patterns and arrangements within clouds: Intortus describes filaments twisted into irregular, contorted shapes. This is commonly seen in cirrus fibratus clouds. Vertebratus refers to a fishbone-like arrangement of cloud filaments, also observed in cirrus fibratus. The parallel lines resemble the skeleton of a fish. Radiatus indicates rows or lines of clouds that appear to converge toward the horizon—similar to railroad tracks perspective. This optical effect results from clouds being arranged in parallel lines as seen from below. Radiatus can occur with cirrus species, altocumulus, stratocumulus, certain cumulus types, and altostratus. Duplicatus describes closely spaced, parallel layers of the same cloud type stacked vertically. These layers appear to be doubled or layered versions of each other. Supplementary Features: Structures Attached to Clouds Supplementary features are additional structures that attach to or extend from the main cloud body. They provide important information about cloud dynamics and precipitation characteristics. Precipitation-Based Features Virga is precipitation that falls from a cloud but completely evaporates before reaching the ground. You'll see it as visible streaks extending downward from the cloud base that don't reach the surface. Virga is common in dry lower atmospheric layers and can occur with many cloud types including cirrocumulus and cumulonimbus. Praecipitatio is precipitation that reaches the ground without completely evaporating. This is the "real" precipitation you can actually measure. It's typical of thick, water-rich clouds like altostratus opacus and vertically developed clouds. Cloud-Based Features Incus is the characteristic anvil-shaped top of a cumulonimbus capillatus. When a thunderstorm cloud reaches the tropopause (the upper boundary of the troposphere), it can no longer rise vertically. Instead, it spreads horizontally, creating the distinctive flat-topped anvil shape. This feature indicates a mature thunderstorm. Mammatus are pouch-like or breast-like protrusions hanging from the underside of clouds. While most commonly seen on cumulonimbus clouds, they also appear on cirrus and stratocumulus. Mammatus indicate atmospheric instability and sinking air motion. Tuba is a cloud column or lowering hanging from a cumulus or cumulonimbus cloud. It can develop into a funnel cloud or tornado. Not all tubas become tornadoes, but this feature indicates rotating motion in the cloud. Arcus is a roll cloud attached to the forward edge of a cumulus congestus or cumulonimbus. It often forms along the leading edge of squall-line outflows as cold, dense air from the downdraft spreads outward along the surface. Fluctus forms when strong wind shear (wind changing speed or direction with height) creates regularly spaced wave-like crests in a cloud layer. Also called a Kelvin–Helmholtz cloud, fluctus indicates instability from wind shear. Murus is a rotating wall cloud that appears beneath a cumulonimbus. This feature is particularly important for forecasters because it can precede tornado formation. The rotating motion in the wall cloud is connected to rotation in the storm itself. Cauda is a horizontal tail cloud extending from a murus cloud. It indicates strong inflow of warm, humid air toward the storm system and suggests the storm is intensifying. Accessory Clouds: Detached from the Main Cloud Accessory clouds form separately from the main cloud but are closely associated with it. They differ from supplementary features because they are not physically attached to the main cloud. Pannus are ragged, low clouds formed beneath heavy precipitating clouds such as nimbostratus, cumulonimbus, or towering cumulus. These form in the very humid air beneath the main cloud where precipitation is falling. Pileus is a smooth cap cloud that forms over a rapidly rising cumulus or cumulonimbus. As the main cloud pushes upward into stable air, it forces air up and over the cloud top, and moisture condenses on the windward side, creating a cap-like appearance. Velum is a thin, sheet-like cloud that may wrap around the middle or front of a parent cloud. It forms when air is forced to rise around an obstruction (like a growing cumulus) and moisture condenses. Flumen (also called "beaver's tail") forms from the warm, humid inflow of a supercell thunderstorm. It has a distinctive tail-like appearance and can sometimes be mistaken for a tornado, but it does not rotate and does not have the same danger. Cloud Origin: Mother, Genitus, and Mutatus Classifications Clouds don't exist in isolation—they change as atmospheric conditions evolve. Meteorologists use special terminology to describe how clouds transform. A mother cloud is the original cloud that changes its genus (type) as atmospheric conditions evolve. When a mother cloud changes but retains much of its original appearance while adopting a new genus, it's called a genitus cloud. For example, stratocumulus cumulogenitus forms when spreading cumulus clouds flatten and merge into a stratocumulus layer. The parent cumulus structure is still somewhat visible, even though the cloud is now classified as stratocumulus. A mutatus cloud fully changes its genus, losing most of its original form. This represents a more complete transformation than a genitus cloud. Special Origin Clouds Flammagenitus clouds (cumulus or cumulonimbus) are generated by large fires, volcanic eruptions, or nuclear blasts. The intense heat from these sources forces air upward, creating convective clouds. Contrail homogenitus clouds form when persistent contrails (aircraft condensation trails) spread and transform into cirrus-like formations. As contrails spread and thicken, they eventually resemble natural cirrus clouds. <extrainfo> Large-Scale Cloud Patterns: Stratocumulus Fields When viewed from above (such as from satellites), stratocumulus clouds arrange themselves into distinctive patterns that reveal information about the atmospheric dynamics below. Closed-cell fields have a cloudy center with clear edges, resembling a solid honeycomb pattern. The clouds form organized cells with cloud in the center and clear air at the boundaries. Open-cell fields display clear (cloud-free) interiors surrounded by cloud edges, resembling an empty or inverted honeycomb pattern. These fields form through different atmospheric processes than closed-cell patterns. Understanding these large-scale patterns helps meteorologists recognize atmospheric conditions, particularly over oceans where these formations are common. </extrainfo>