Glossary · 40 terms

Glossary

Forty terms, from the ten genera to the supplementary features and the named species and varieties. Each entry gives the definition and one field note about how to use the term outdoors.

Genus · 10

Altocumulus Genus Altocumulus · Ac A genus of middle-level cloud, usually a layer of patches, rolls or rounded masses, sometimes with a rippled appearance. Elements are thumbnail-sized at arm's length and often show slight shading on their undersides.Altostratus Genus Altostratus · As A genus of middle-level cloud forming a greyish or bluish-grey sheet, thin enough to show the sun as a bright patch but thick enough to hide its disc completely. No halo is produced.Cirrocumulus Genus Cirrocumulus · Cc A genus of high cloud forming a thin, sometimes rippled sheet of very small white grains, mostly ice with some supercooled water. Elements are fingertip-sized at arm's length and show no shading.Cirrostratus Genus Cirrostratus · Cs A genus of high cloud forming a transparent, whitish veil of ice crystals, usually covering much or all of the sky. The sun's disc remains visible through it, though it may be fuzzy, and a 22° halo is common.Cirrus Genus Cirrus · Ci A genus of high cloud, composed almost entirely of ice crystals, appearing as detached white filaments, tufts or narrow bands. Cirrus has no shading and never produces precipitation that reaches the ground.Cumulonimbus Genus Cumulonimbus · Cb A genus of heavy, dense cloud of considerable vertical extent, in the form of a mountain or huge tower, whose upper part is at least partly glaciated, often with a smooth, fibrous or striated anvil shape on top.Cumulus Genus Cumulus · Cu A genus of detached, generally dense cloud with sharp outlines, developing vertically in the form of rising mounds, domes or towers, with a flat, often dark base and a brilliant white top when lit by the sun.Nimbostratus Genus Nimbostratus · Ns A genus of thick, dark, featureless cloud layer producing continuous, moderate precipitation. It has no clearly defined base, and the sun is never visible through it.Stratocumulus Genus Stratocumulus · Sc A genus of low cloud forming a layer of joined patches, rolls or cells, usually with visible shading and darker undersides, and often with blue sky showing between elements.Stratus Genus Stratus · St A genus of low cloud forming a generally grey, uniform layer with a fairly even base, not showing any distinct elements. It may produce drizzle or small snow grains, and it can reach the ground, at which point it is fog.

Species · 13

Castellanus Species castellanus A cloud species in which small turrets rise from a common base, giving the cloud a crenellated, castle-like appearance. It occurs in cirrocumulus, altocumulus and stratocumulus.Congestus Species congestus A species of cumulus that is strongly developed, with sprouting or cauliflower-like tops and a vertical extent clearly greater than its width. It is the largest cumulus form and the immediate precursor of cumulonimbus.Fibratus Species fibratus A cloud species applied to cirrus and cirrostratus, describing straight or slightly curved filaments that do not end in hooks or tufts. The cloud looks like a set of fine hairs stretched across the sky.Floccus Species floccus A cloud species consisting of small, ragged, roughly equal-sized tufts, each with a more or less ragged base, often with virga trails hanging below. It occurs in cirrus, cirrocumulus and altocumulus.Fractus Species fractus A cloud species made up of ragged, broken shreds with no well-defined base, usually seen in stratus and cumulus. The shreds are the remains of a cloud that is being torn apart by wind or that is evaporating.Humilis Species humilis A species of cumulus with a flattened shape, greater in width than in height, whose tops are relatively flat or only slightly domed. It is the smallest and lowest form of the cumulus genus.Lenticularis Species lenticularis A cloud species in the shape of a lens, almond or saucer, often with smooth, sharply defined edges unlike any other cloud. It occurs in cirrocumulus, altocumulus and stratocumulus and is a signature of standing waves.Mediocris Species mediocris A species of cumulus of moderate vertical extent, with somewhat bulging tops, roughly as tall as it is wide. It lies between cumulus humilis and cumulus congestus in the convective sequence.Nebulosus nebulosus A cloud species describing a cloud that is a featureless mist or veil with no distinguishable detail. It applies to cirrostratus and stratus, precisely the two genera that have no elements to describe.Spissatus Species spissatus A species of cirrus that is dense enough to appear greyish rather than white when viewed against the sun. It is the thickest form of cirrus and often occurs as the remains of a cumulonimbus anvil.Stratiformis Species stratiformis A cloud species forming an extensive horizontal layer or sheet, with the individual elements flattened so that the layer dominates over the elements. It is the most common species of altocumulus and stratocumulus.Uncinus Species uncinus A species of cirrus whose filaments end in a hook or comma shape rather than a tuft. The hook points in the direction of the wind shear and marks where the ice crystals are falling most slowly.Volutus Species volutus A cloud species consisting of a long, horizontal, tube-shaped roll cloud that appears to rotate slowly about a horizontal axis. It was added to the International Cloud Atlas in 2017 and applies to altocumulus and stratocumulus.

Variety · 7

Duplicatus Variety duplicatus A cloud variety in which two or more layers of cloud are superposed at slightly different levels, sometimes partially merged. It occurs in cirrus, cirrostratus, altocumulus, altostratus and stratocumulus.Lacunosus Variety lacunosus A cloud variety in which the layer has regular round holes, often with fringed edges, giving a honeycomb or net-like appearance. It occurs in cirrocumulus and altocumulus, and more rarely in stratocumulus.Opacus Variety opacus A cloud variety describing a layer thick and opaque enough that the sun or moon cannot be seen through it at all. It applies to altocumulus, altostratus, stratocumulus and stratus.Perlucidus Variety perlucidus A cloud variety describing a layer of broad elements with distinct but small gaps between them, through which the sky or a higher cloud layer can be seen. It applies to altocumulus and stratocumulus.Radiatus Variety radiatus A cloud variety in which bands or elements are arranged in parallel lines that, because of perspective, appear to converge towards a point on the horizon. The bands are physically parallel; the convergence is an optical effect.Translucidus Variety translucidus A cloud variety describing a layer thin enough that the position of the sun or moon is clearly visible through it. It applies to altocumulus, altostratus, stratocumulus and stratus.Undulatus Variety undulatus A cloud variety in which the layer or elements are arranged in parallel bands or waves with a consistent wavelength, giving a regular, ridged appearance. It occurs in cirrocumulus, altocumulus, stratocumulus and cirrostratus.

Supplementary feature · 7

Incus Supplementary feature incus A supplementary cloud feature of cumulonimbus consisting of the flattened, broad top of the cloud, shaped like an anvil and made largely of ice crystals. Its presence marks a cumulonimbus in or past its mature stage.Mamma (mammatus) Supplementary feature mamma A supplementary cloud feature consisting of hanging, pouch-like protuberances on the underside of a cloud, most often a cumulonimbus anvil or the underside of an altocumulus or stratocumulus layer.Pannus Supplementary feature pannus A supplementary cloud feature consisting of ragged shreds of cloud in a more or less continuous layer, occurring below another cloud and separated from it. It is the same thing observers often call scud.Pileus Supplementary feature pileus A supplementary cloud feature in the form of a cap or hood of cloud sitting directly above the top of a cumulus or cumulonimbus, looking like a small hat. It is thin, short-lived, and often only lasts a few minutes.Praecipitatio praecipitatio A supplementary cloud feature indicating that precipitation is reaching the ground from the cloud, whether or not it is visible as a shaft. It can be applied to stratus, stratocumulus, cumulus, cumulonimbus, altostratus and nimbostratus.Tuba Supplementary feature tuba A supplementary cloud feature consisting of a column of cloud hanging from the base of a cumulonimbus or cumulus congestus, looking like a funnel or a cone with the narrow end downward. A tuba that reaches the surface becomes a tornado or a waterspout.Virga Supplementary feature virga A supplementary cloud feature consisting of precipitation that falls from a cloud but evaporates or sublimates before reaching the ground, appearing as vertical or slanted streaks that end in mid-air.

Special origin · 3

Cataractagenitus Special origin cataractagenitus A cloud formed as a result of spray from a waterfall or from the turbulent water at the base of one. The spray adds moisture and the turbulence lifts it, producing cloud that is localised to the immediate vicinity of the falls.Flammagenitus Special origin flammagenitus A cloud whose formation is directly caused by fire, either a wildfire or an industrial fire. The heat of the fire drives strong convection, and the smoke particles provide abundant nuclei for condensation. It was added to the atlas in 2017.Homogenitus Special origin homogenitus A cloud whose formation is directly caused by human activity, most commonly an aircraft contrail. The suffix was introduced in the 2017 revision of the atlas and is usually attached to the parent genus, as in cirrus homogenitus.