Cloud
A cloud is a visible aggregate of minute particles of water droplets or ice crystals suspended in the atmosphere, formed by the condensation of water vapor.[1] Clouds arise when rising air cools to its dew point, enabling water vapor to condense onto aerosol particles serving as nuclei, with the process governed by atmospheric temperature, humidity, and dynamics.[2][3] They are ubiquitous features of Earth's atmosphere, influencing local weather through precipitation and global patterns via radiative interactions.[4] Clouds are systematically classified by the World Meteorological Organization into genera based on altitude, morphology, and internal structure, yielding ten primary types: high-level cirrus, cirrostratus, and cirrocumulus composed mainly of ice crystals; mid-level altocumulus and altostratus mixing water droplets and ice; and low- to vertically developing cumulus, stratus, stratocumulus, nimbostratus, and cumulonimbus often producing rain or storms.[5][6] This nomenclature, rooted in 19th-century observations by Luke Howard, facilitates forecasting and climatological analysis.[4] In the climate system, clouds exert dual radiative forcings by scattering and reflecting shortwave solar radiation to cool the surface—predominantly via albedo enhancement—while absorbing and re-emitting longwave terrestrial radiation to warm it, with empirical assessments indicating a net cooling influence that modulates Earth's energy balance.[7][8] Their feedback responses to warming, including shifts in coverage and altitude, represent a major uncertainty in climate projections, as unresolved microphysical processes affect precipitation efficiency and optical properties.[9][10] Beyond radiation, clouds drive convective transport of heat, moisture, and momentum, shaping circulation patterns and extreme weather events like thunderstorms and cyclones.[8]Etymology
Linguistic Origins
The English word cloud originates from Old English clud (also spelled clūd), attested around the 9th century, initially denoting "a mass of rock" or "hill," akin to a lump or clod of earth.[11] This sense reflects a metaphorical extension to atmospheric formations resembling earthy clumps, with the modern meaning of "visible mass of condensed water vapor" emerging by the late 13th to early 14th century, as recorded in texts like the Ancrene Wisse.[11][12] The term traces to Proto-Germanic *klūtaz or *kludaz, meaning "boulder, rock-mass, or clod," which carried connotations of solidity and aggregation before application to transient sky phenomena.[11] This root links to Proto-Indo-European *gel- or *gʷel-, implying "to ball up, clench, or form into a lump," evident in cognates like Old Norse klute ("mass") and suggesting an ancient perceptual analogy between terrestrial clods and billowing vapors.[13][11] In broader Indo-European linguistics, words for "cloud" often derive instead from *nébʰos, a reconstructed term for "cloud" or "mist" from the root *nebʰ- ("to become damp or cloudy"), yielding Latin nūbēs (cloud), Greek néphos (cloud-mass), and Sanskrit nábhas- (sky or cloud).[14] Germanic languages, however, favored the *klūtaz lineage for English and related tongues, diverging from the dampness-focused PIE term, possibly due to regional environmental emphases on visible lumpiness over moisture.[12] Old English retained *weolcen (from Proto-Germanic *welkanaz, meaning "cloud" or "sky") as an alternative until the 14th century, when clud supplanted it, influencing modern usage.[11]Historical Terminology Shifts
Prior to the early 19th century, cloud descriptions relied on vernacular, ad hoc terms varying by culture and observer, often poetic or morphological without systematic categorization. Ancient texts, such as Aristotle's Meteorologica (circa 340 BCE), employed Greek terms like nephē for general clouds alongside qualifiers such as "wool-like" or "rain-bearing," but these lacked consistent typology.[15] Medieval and Renaissance observers continued descriptive approaches, referencing shapes like "mackerel sky" in English or nubes subtypes in Latin, influenced by artistic depictions rather than scientific uniformity.[16] A transitional effort came from Jean-Baptiste Lamarck, who in 1802 published the first formal cloud classification using French descriptors, including nuage filamenteux (filamentous, akin to cirrus) and nuage lenticulaire (lenticular), aiming to link forms to weather phenomena but limited by national language.[17] This preceded Luke Howard's seminal 1803 essay "On the Modifications of Clouds," which marked a decisive shift to Latin-derived, internationally accessible nomenclature modeled on Linnaean taxonomy. Howard defined primary genera—cirrus (from Latin for "curl of hair," denoting high, fibrous forms), cumulus ("heap," for puffy, low-level accumulations), and stratus ("spread out," for layered sheets)—plus compounds like cirrostratus and nimbus for rain clouds, reducing redundancy and enabling global standardization.[18][19] Refinements accelerated in the mid-19th century, with Hervé-Maurice Renou introducing mid-level genera such as altocumulus and altostratus in works from 1855 to 1877, incorporating altitude-based prefixes (alto- from Latin "high") to address gaps in Howard's framework.[18] The 20th century saw international bodies like the International Meteorological Committee (1926–1930) and the Committee for Clouds (1949–1953) standardize species and varieties, replacing imprecise terms: for instance, Clayton's 1896 cirrus filosus shifted to fibratus in 1951 for its superior connotation of fibrous texture, while castellatus evolved to castellanus to align etymologically with "castle-like" protuberances.[18] These changes emphasized morphological precision over historical precedent, informed by photographic evidence and aerial observations. Modern updates by the World Meteorological Organization, including 2017 additions like asperitas (for wave-like formations) and cauda (for flammagenitus clouds), reflect technological advances in imaging and remote sensing, extending nomenclature to supplementary features while preserving Howard's core genera.[18] Such evolutions prioritize empirical consistency, with the WMO's International Cloud Atlas serving as the authoritative reference since 1896, ensuring terms evolve with observational data rather than rigid tradition.[17]Physical Fundamentals
Definition and Basic Properties
A cloud is defined as a hydrometeor consisting of minute particles of liquid water or ice crystals, or a mixture of both, suspended in the atmosphere above Earth's surface and typically not touching the ground.[20] This definition, established by the World Meteorological Organization, emphasizes visibility arising from the aggregation of these particles, which scatter incoming solar radiation.[20] Clouds occur in Earth's troposphere and stratosphere, as well as on other planetary bodies, but terrestrial clouds predominantly form through adiabatic cooling of moist air leading to supersaturation and condensation or deposition.[21] The basic composition of clouds involves water in liquid or solid form, with liquid droplets predominant in warmer conditions and ice crystals in colder altitudes or mixed-phase clouds.[7] Cloud droplets are spherical or near-spherical, with diameters ranging from approximately 1 to 100 micrometers, though most fall between 5 and 20 micrometers in effective radius, enabling prolonged suspension due to low terminal fall velocities of 1-10 cm/s.[22][23] Ice crystals exhibit diverse habits, including plates, columns, and dendrites, with sizes from 10 micrometers to several millimeters, influenced by temperature and supersaturation; these shapes affect sedimentation rates and radiative interactions.[24][25] Particles form on condensation nuclei such as aerosols, dust, or sea salt, with concentrations typically 10-1000 cm⁻³ in liquid clouds, determining optical depth and precipitation efficiency.[26] Clouds remain aloft because their microphysical properties—small size and low density (around 0.5 g/m³ for liquid water content)—counteract gravitational settling, sustained by atmospheric turbulence or weak updrafts exceeding fall speeds.[21] Visibility requires optical thickness sufficient for Mie scattering, rendering clouds opaque or translucent based on particle density and size distribution; optically thin cirrus clouds, for instance, transmit more light due to separated ice crystals.[7] These properties underpin clouds' role in Earth's energy balance, reflecting shortwave radiation while absorbing and re-emitting longwave infrared.[7]Composition and Microstructure
Clouds consist primarily of suspended microscopic particles of liquid water or ice, with their composition determined by temperature and humidity conditions. In clouds where temperatures exceed 0°C, the dominant phase is supercooled liquid water droplets, while below -40°C, ice crystals predominate; mixed-phase clouds feature both phases interacting via processes like the Wegener-Bergeron-Findeisen mechanism.[27] These hydrometeors form on cloud condensation nuclei (CCN), which are hygroscopic aerosol particles—such as sulfates, sea salt, and mineral dust—with diameters typically exceeding 0.05–0.2 μm, enabling activation at supersaturations of 0.1–1%.[28] [29] CCN composition influences droplet activation efficiency, with soluble components enhancing water uptake and insoluble ones like dust providing heterogeneous nucleation sites.[30] The microstructure of clouds is characterized by the size distributions, shapes, phases, and concentrations of these particles, which govern microphysical processes like coalescence and sedimentation. Liquid droplet diameters generally range from 1 to 100 μm, with effective radii averaging 5–15 μm in continental and marine stratiform clouds, respectively, and number concentrations varying from 10 to 1000 cm⁻³ based on updraft strength and CCN availability.[22] [31] Size distributions often approximate a gamma function, with shape parameters indicating narrower spectra (variance ~3–6 μm) in cleaner environments and broader ones under polluted conditions, reflecting local aerosol loading.[32] [33] Ice crystals exhibit diverse habits—such as hexagonal plates, columns, bullets, and dendrites—shaped by vapor deposition growth regimes, with sizes from 10 μm to millimeters and concentrations typically 0.1–100 L⁻¹ in cirrus clouds.[34] [25] These morphologies arise from temperature-dependent diffusion, with dendritic growth favored near -15°C and columnar forms at lower temperatures, impacting radiative transfer and precipitation efficiency.[35] In mixed-phase regions, riming of ice by supercooled droplets alters crystal density and fall speeds, while secondary ice production via mechanisms like rime splintering amplifies crystal numbers beyond primary ice nuclei.[36] Overall, cloud microstructure varies spatially and temporally, with small-scale heterogeneities (e.g., cm-scale narrow distributions) challenging bulk parameterizations in models.[37][38]Formation Mechanisms
Cooling Processes
Cooling processes in the atmosphere drive cloud formation by reducing air temperature to the dew point, where water vapor condenses into droplets or ice crystals. The predominant mechanism is adiabatic cooling, occurring when a parcel of moist air ascends and expands due to decreasing atmospheric pressure, performing work that lowers its internal energy and temperature without heat exchange with surroundings.[2] This process follows the dry adiabatic lapse rate of approximately 9.8 °C per kilometer until saturation, after which latent heat release from condensation moderates cooling to the moist adiabatic lapse rate of about 6 °C per kilometer.[39] Adiabatic cooling is triggered by various lifting mechanisms, including convection from surface heating, where solar warming destabilizes air near the ground, prompting buoyant ascent; orographic lift over terrain, forcing air upward; and frontal lifting at weather fronts, where warmer air overrides cooler air masses.[2] In convective scenarios, unsaturated air cools at the dry rate until reaching the lifting condensation level, typically 1-2 km above the surface in humid tropics, beyond which cloud growth accelerates.[40] Empirical observations confirm this dominance, as most tropospheric clouds form via uplift-induced cooling rather than horizontal mixing or surface contact.[41] Secondary processes include radiative cooling, where air loses heat through infrared emission to space, particularly at night or in clear skies over polar regions, lowering temperatures isobarically to induce condensation without significant vertical motion.[42] This contributes to fog and stratiform clouds, with cooling rates up to 5-10 °C per hour in thin boundary layers, though it is less efficient for thick cloud development compared to adiabatic processes.[43] Mixing with subsaturated cooler air can also promote cooling, as in entrainment at cloud edges, but this often dissipates clouds unless offset by sustained uplift.[41] These mechanisms collectively ensure that cooling to saturation—typically requiring a 10-20 °C drop depending on humidity—precedes nucleation on aerosols like sea salt or sulfates.[39]Moisture Condensation Dynamics
![Cumulus humilis clouds in Ukraine.jpg][float-right] Moisture condensation dynamics encompass the physical processes governing the phase transition of atmospheric water vapor into liquid droplets within clouds. Condensation initiates when air parcels, cooled primarily through adiabatic expansion during ascent, reach saturation at the lifting condensation level, after which relative humidity exceeds 100%, creating supersaturation. This supersaturation drives the deposition of water molecules onto aerosol particles, forming the initial cloud droplets.[2][44] The nucleation phase relies predominantly on heterogeneous nucleation, where water vapor condenses on cloud condensation nuclei (CCN)—hygroscopic aerosols such as sulfates, sea salt, and organic compounds, typically 0.05 to 1 micrometer in diameter. These particles reduce the critical supersaturation required for droplet formation to 0.1-1%, enabling activation at realistic atmospheric conditions; homogeneous nucleation, necessitating supersaturations over 400%, occurs rarely in the troposphere due to its high energy barrier. CCN concentrations dictate droplet numbers, ranging from 10-100 per cubic centimeter in clean maritime air to 300-1000 or more in polluted continental environments, influencing cloud albedo and precipitation efficiency.[45][46][47] Following nucleation, droplets grow via diffusional condensation, as water vapor diffuses toward the droplet surface driven by a vapor pressure deficit caused by droplet curvature (Kelvin effect) and solute presence (Raoult's law). The growth rate, described by \frac{dr}{dt} = \frac{G (S - 1)}{r} where r is radius, S is supersaturation, and G incorporates diffusion and thermodynamic factors, results in initial rapid expansion from sub-micrometer sizes to 10-20 micrometers within minutes. This phase sustains cloud opacity until larger droplets transition to collision-coalescence for precipitation development.[48][49]Tropospheric Clouds
Primary Classification Schemes
The primary classification scheme for tropospheric clouds, as codified by the World Meteorological Organization (WMO) in its International Cloud Atlas, divides clouds into categories based on altitude of their base above the surface and morphological form, using Latin-derived terms to denote height, shape, and precipitation potential.[50] This system, originating from Luke Howard's 1803 nomenclature and refined through international consensus, employs 10 principal genera that reflect observable textures such as fibrous (cirrus-like), layered (stratus-like), or heaped (cumulus-like), enabling consistent identification for meteorological forecasting and research.[51] Altitude divisions are approximate and latitude-dependent: high-level clouds form above 5,000 meters (16,500 feet) in tropical regions but lower in polar areas; middle-level clouds occupy 2,000–7,000 meters (6,500–23,000 feet); and low-level clouds lie below 2,000 meters (6,500 feet).[52] These groupings prioritize empirical visual and structural traits over microphysical processes, though vertical development in convective clouds like cumulonimbus can span multiple levels.[4] The 10 genera are assigned to altitude levels as follows, with prefixes like cirro- indicating high altitude, alto- for middle, nimbo- for rain-bearing, cumulo- for piled or convective forms, and strato- for horizontal layers:| Altitude Level | Genera |
|---|---|
| High (>5,000 m) | Cirrus (detached wispy filaments), Cirrocumulus (small white patches), Cirrostratus (thin sheet-like veil)[53] |
| Middle (2,000–7,000 m) | Altocumulus (patchy layers with rounded elements), Altostratus (uniform grayish sheet, often precipitating)[6] |
| Low (<2,000 m) | Stratocumulus (lumpy layers), Stratus (uniform low fog-like layer), Nimbostratus (thick rain-bearing layer)[53] |
| Variable/Vertical | Cumulus (detached heaped towers with flat bases), Cumulonimbus (towering anvil-topped storm clouds)[4] |
Genera by Altitude and Form
High-level clouds, occurring above 5 km in temperate latitudes (with variations by region: 3–8 km in polar areas and 6–18 km in tropical regions), consist primarily of ice crystals and exhibit fibrous, silky, or uniform sheet-like forms. These include cirrus (Ci), detached clouds appearing as delicate, white, ice-crystal filaments, patches, or narrow bands, often without shadows; cirrocumulus (Cc), small, white flakes or globules arranged in groups, lines, or ripples, sometimes showing optical phenomena; and cirrostratus (Cs), a transparent veil of thin, whitish clouds covering the sky in a uniform layer, producing halos around the sun or moon.[56][57][58] Mid-level clouds, typically between 2–7 km in temperate zones (2–4 km polar, 2–8 km tropical), form from water droplets, ice crystals, or a mix, and display wavy, lens-shaped, or patchy layered structures. Key genera are altocumulus (Ac), white or gray layers or patches composed of rounded masses, rolls, or ripples, often with shaded elements; altostratus (As), a fibrous or smooth grayish veil or layer, thick enough to obscure the sun's disk but not producing precipitation at the surface; and nimbostratus (Ns), a thick, amorphous, dark gray layer from which continuous rain or snow falls, often with low ragged fragments called praecipitatio.[57][58][5] Low-level clouds, below 2 km regardless of latitude, are predominantly water-droplet based and feature rounded masses, rolls, or continuous horizontal bases. Genera encompass stratocumulus (Sc), large dark and rounded masses or rolls in groups or bands, usually with gaps revealing blue sky; stratus (St), a uniform grayish layer with a relatively uniform base, resembling fog lifted off the ground and producing drizzle or mist; and low forms of cumulus (Cu), detached clouds with sharp outlines, flat bases, and dome-shaped upper parts showing vertical growth from surface heating, but without significant vertical development.[57][58][4] Clouds with notable vertical extent transcend altitude levels, spanning from near the surface to the tropopause. Cumulonimbus (Cb) represents the most intense, with strong vertical development forming towering masses, anvil-shaped tops (incus), and heavy precipitation, hail, or thunderstorms, while moderate cumulus exhibits limited but distinct upward growth. These genera reflect fundamental atmospheric stability: high and mid-level forms indicate upper-air dynamics, low-level suggest boundary-layer mixing, and vertical types signal convective instability.[58][5][59]| Genus Abbreviation | Primary Altitude (Temperate) | Characteristic Form | Composition |
|---|---|---|---|
| Cirrus (Ci) | High (5–13 km) | Filaments, patches | Ice crystals |
| Cirrocumulus (Cc) | High (5–13 km) | Globules, flakes | Ice crystals |
| Cirrostratus (Cs) | High (5–13 km) | Uniform sheet | Ice crystals |
| Altocumulus (Ac) | Middle (2–7 km) | Rounded masses, rolls | Water/ice mix |
| Altostratus (As) | Middle (2–7 km) | Fibrous layer | Water/ice mix |
| Nimbostratus (Ns) | Middle to low (2 km downward) | Amorphous veil | Water droplets |
| Stratocumulus (Sc) | Low (0–2 km) | Rolls, patches | Water droplets |
| Stratus (St) | Low (0–2 km) | Uniform layer | Water droplets |
| Cumulus (Cu) | Low (0–2 km base) | Detached heaps | Water droplets |
| Cumulonimbus (Cb) | Low to high (0–13+ km) | Towering with anvil | Water/ice mix |
Species, Varieties, and Supplementary Features
Cloud species delineate specific morphological forms and internal structures within each genus, providing finer distinctions based on observable shapes such as thread-like, hooked, or turreted arrangements. For instance, in the genus Cirrus, species include fibratus (fine, uncinus-like threads or plates appearing as detached filaments), uncinus (hooked filaments with tufted heads), spissatus (dense and opaque patches), castellanus (small turrets resembling cumuliform protuberances), and floccus (small, tufted clouds with ragged lower edges). These species reflect variations in ice crystal aggregation and wind shear effects at high altitudes. Similarly, for Cumulus, species encompass humilis (small, flat-based heaps with little vertical development), mediocris (moderate vertical extent without reaching free convection levels), and congestus (well-developed heaps showing continued growth). The World Meteorological Organization (WMO) recognizes 14 primary species across high, middle, and low genera, with updates including the addition of volutus (roll-shaped, low horizontal tubes) in 2017 as a new species for undulatus-like forms.[61] Varieties further qualify species by attributes like transparency, internal structure uniformity, or spatial arrangement, aiding in assessing cloud opacity and layering. Common varieties include intortus (twisted or tangled, often in cirrus), radiatus (arranged in parallel bands converging to a point), verticatus (verticose or whirled), undulatus (wavy undulations), opacus (opaque, obscuring the sun or moon), translucidus (translucent, allowing partial solar disk visibility), perlucidus (perforated with clear holes), and duplex or triplex (multi-layered). These descriptors, standardized by the WMO, derive from empirical observations of droplet or crystal density and atmospheric stability, with castellanus and floccus also serving dual roles as both species and varieties in some genera to denote instability indicators. Varieties are not exhaustive but emphasize optical and textural differences verifiable through ground or satellite imagery.[53] Supplementary features comprise distinct appendages or modifications attached to or embedded within the main cloud body, often signaling dynamic processes like precipitation trails or shear-induced shapes. Key examples include virga (pendulous precipitation streaks evaporating before reaching the ground, common in altocumulus or stratocumulus), praecipitatio (actual precipitation reaching the surface), incus (anvil-shaped dome or plume on cumulonimbus caps from overshooting convection), mamma or mammatus (pouch-like protrusions from cloud undersides due to sinking air pockets, typically under cumulonimbus or altocumulus), incus (wait, duplicate? No: also tuba for funnel clouds), flumen (beaver's tail inflow bands under thunderstorms), and asperitas (wavy, undulating undersurfaces, added in 2017). The WMO updated its atlas in 2017 to include five new supplementary features—asperitas, cauda (roll-like tails), flumen, homomutatus (lens-shaped altocumulus variants), and tuba refinements—based on photographic evidence and community submissions, totaling around 11 recognized types. These features are distinguished from accessory clouds (separate but associated formations like pannus or pileus) by their integral attachment to the parent cloud.[62][63][64]| Category | Examples | Key Characteristics |
|---|---|---|
| Species | Fibratus, uncinus, castellanus, humilis, congestus, volutus | Shape-specific forms (e.g., threads, turrets, rolls); indicate crystal/droplet organization and instability. |
| Varieties | Opacus, translucidus, radiatus, undulatus | Opacity, patterning, or layering qualifiers; reflect density and viewing geometry. |
| Supplementary Features | Virga, incus, mammatus, asperitas, flumen | Attached elements (e.g., trails, pouches, waves); denote precipitation, shear, or subsidence dynamics.[62] |