Cloud formation & identification

Which process makes vapour become visible?

Identify the cooling or moisture change before naming the lifting mechanism.

Several routes to a cloud

A cloud is a visible suspension of liquid droplets, ice particles or both above the surface. Its particles form when the vapour and temperature conditions favour condensation or ice formation. Adiabatic cooling during ascent is a major route: a parcel expands as pressure decreases and reaches saturation if lifted far enough. This does not require the environmental temperature to follow the parcel's own cooling rate.

Cooling can also begin at a cold surface through conduction and turbulent exchange, eventually producing fog or a low cloud layer. Advection brings air over a colder surface or carries a moist layer into a region where cooling occurs. Radiation can cool an air or cloud layer, including infrared cooling near a cloud top. Horizontal transport itself is not necessarily a local cooling process, so specify what changes as the air arrives.

Turbulent mixing can carry moist air to its condensation level in a shallow layer or mix parcels of different temperature and humidity into a saturated mixture. Because saturation vapour pressure depends non-linearly on temperature, two nearly saturated air streams at similar pressure can form droplets when mixed. Moisture addition by evaporation can also bring a cool layer to saturation. The visible cloud marks the resulting particle field, not every place where the air has moved vertically.

Why does one lift make a sheet and another a tower?

Combine the lifting geometry with the environment's stability.

Forced, free and mixed-layer lift

At a front, broad forced ascent of moist stable air often forms extensive stratiform layers. Lifting over a mountain can make a cap cloud on the windward slope or lenticular cloud in a stable wave. A shallow layer stirred by turbulence can reach saturation intermittently near its top, producing broken stratocumulus or irregular fragments. These examples can generate cloud without freely buoyant deep convection.

Free convection begins when a parcel becomes buoyant relative to the environment. Discrete thermals tend to form cumuliform clouds with separate rising towers, often with fairly flat bases if they start from a mixed layer with similar moisture. An unstable environment can allow much deeper growth, including towering cumulus and cumulonimbus, but only if moisture, lift and the absence or removal of a cap permit it.

Stable air favours limited vertical extent and horizontal spreading; unstable air favours vertical development. Neither rule guarantees cloud: a dry stable air mass can be clear, and an unstable dry layer may contain invisible thermals rather than a visible tower. Embedded convection can also exist inside a broad stratiform cloud shield, so a layered appearance does not exclude a concealed convective hazard.

What do cirriform, stratiform and cumuliform describe?

Use broad forms as clues, then refine the identification with level and detail.

The form and the classification hierarchy

Cirriform describes fibrous, filament-like forms, usually associated with ice at high levels. Stratiform describes a horizontally extensive sheet or layer. Cumuliform describes separate heaps, rounded tops or towers associated with vertical development. These are broad visual forms; a cloud can combine a layered arrangement with rounded elements, as stratocumulus does.

WMO classifies ordinary clouds into ten genera. A genus gives the main form, with species and varieties refining the structure; supplementary features describe additional aspects such as precipitation trails or an anvil. Do not identify a cloud only because it is white or grey: lighting, viewing direction and optical depth alter colour. Compare edge texture, element size, layering, vertical growth, precipitation and the height range.

Cumulus often has crisp cauliflower contours and a common base, while cirrus has a fibrous outline. A uniform grey low sheet is more consistent with stratus than with an isolated cumulus heap. A sheet of rounded low rolls suggests stratocumulus. Use official Cloud Atlas photographs to practise real identification alongside the schematic diagrams: atmospheric examples are more varied than any single icon.

Why is a high cloud not at the same height in every latitude?

Use overlapping height ranges and recognise the genera that span several levels.

Levels are guides, not hard ceilings

WMO divides the troposphere into low, middle and high cloud levels, historically called étages. Their approximate height ranges vary with latitude, and the middle and high ranges overlap. Low-level genera occur most often from the surface to about 2 km in all regions. Middle and high cloud levels extend much farther upwards in the tropics than near the poles, following the deeper tropical troposphere.

Cirrus, cirrocumulus and cirrostratus are the usual high genera. Altocumulus, altostratus and nimbostratus are associated with the middle level, while stratus, stratocumulus, cumulus and cumulonimbus usually have low-level bases. Altostratus can extend higher, nimbostratus commonly spans several levels, and Cu or Cb can rise from low bases into the middle and high troposphere. Do not classify a CB as harmless high cloud merely because its anvil is high.

Which combination of form and level fits the cloud?

Compare the defining appearance, then compare an original photograph with a schematic and practise using the exercise context.

High and middle genera

Cirrus is a high fibrous or streaky ice cloud. Cirrocumulus has very small white grains or ripples, usually without visible shading in the small elements. Cirrostratus is a thin widespread veil, often associated with a halo around the Sun or Moon. Altocumulus has larger patches, rounded elements or rolls that can show shading. Altostratus is a widespread grey or blue-grey layer; thin parts can show the Sun as a diffuse disc rather than producing the typical cirrostratus halo.

Nimbostratus is a thick widespread precipitation layer, usually dark enough to hide the Sun, with a diffuse base and often ragged low fragments beneath it. Its sustained rain or snow and broad vertical extent help distinguish it from a thin altostratus sheet. Viewing angle and apparent element size can mislead, so compare several visual clues and the height information rather than relying on one small patch of sky.

Low bases and vertical growth

Stratus is a low, fairly uniform sheet, sometimes producing drizzle and obscuring higher terrain. Stratocumulus is a low layer of larger rounded or rolled elements, often with darker parts and openings. Cumulus consists of separate heaps with distinct edges, a relatively flat base and rounded sunlit tops; vertical extent varies greatly. Cumulonimbus is a deep convective cloud whose upper parts lose the crisp cauliflower outline and can become fibrous and spread into an anvil.

A cumulonimbus need not show a textbook broad anvil from every viewing angle, and an embedded CB may not be visible as a separate tower from below. Radar, lightning information, forecasts and reports help assess concealed convection. The Cloud Atlas defines the genus by its cloud structure; a pilot should not wait for audible thunder to recognise a potentially hazardous deep convective cloud.

Which extra feature changes the interpretation?

Use the named species and supplementary features as visible evidence of a process.

Recognise the required features

Castellanus has turret-like protrusions rising from a common cloud base, resembling battlements. Altocumulus castellanus is a useful sign of instability at its level and can support a wider assessment of convective potential; it is not a timed guarantee of a thunderstorm at the observing point. Lenticularis has a smooth lens-like outline, commonly associated with a standing wave; an apparently fixed cloud can contain strong through-flow.

Cumulus congestus has large vertical development and distinct cauliflower towers. Cumulonimbus calvus has upper parts beginning to lose that sharp cumuliform appearance but without a fully developed fibrous top. Cumulonimbus capillatus has visibly fibrous, hair-like upper structure, often with spreading at the top. An anvil is the supplementary feature incus, not a separate eleventh genus.

Virga consists of precipitation trails below cloud that evaporate or sublimate before reaching the surface. It can reveal dry air beneath a cloud and evaporative cooling that supports descending currents. Do not call virga surface rain just because a falling streak is visible. These appearance clues add to, rather than replace, the wind, temperature and humidity profile.

Why can stratus spread while a cumulus top flattens?

Connect the inversion's restoring force to cloud depth at the surface, boundary-layer top and tropopause.

Stable caps alter the geometry

A parcel lifted into an inversion cools while the surrounding air becomes warmer, so it becomes negatively buoyant and vertical growth is suppressed. If a moist boundary layer reaches saturation beneath such a lid, cloud can spread horizontally as stratus or stratocumulus. The inversion does not create water by itself: sufficient moisture and a cooling or lifting process are still required.

A ground inversion formed by nocturnal cooling can bring a moist shallow layer to saturation and support radiation fog. Weak mixing can spread the cooling through enough depth for fog to become substantial, while excessive dry-air mixing may inhibit it. Cloud beneath an elevated inversion and fog within a ground-based inversion have different positions relative to the stable layer; both depend on the moisture and cooling history.

Near the tropopause, the lapse-rate change or inversion strongly limits ordinary convective growth. A cumulonimbus top often spreads into an anvil as rising air encounters the stable layer. A powerful updraught can overshoot before returning, so the tropopause does not guarantee that every storm top remains neatly below it. Anvil spread and an overshooting top are signs to interpret with the storm's full structure.

What do the genera suggest about icing and turbulence?

Combine the visual category with temperature, liquid water and the generating motion.

Water phase is a separate classification

Pure-water clouds contain liquid droplets, which can be supercooled below 0 °C. Ice clouds contain ice particles, while mixed-phase clouds contain both liquid and ice. Temperature is an important clue but not a complete phase measurement. Liquid and mixed-phase layers can occur at sub-zero temperatures well above the freezing level, while high fibrous cloud is usually glaciated. The icing chapter examines these microphysical conditions in more detail.

Assess all ten genera without making a guarantee

Cirrus, cirrostratus and cirrocumulus are predominantly high ice clouds and usually have little conventional supercooled-liquid airframe-icing potential; shear can still produce turbulence, and convective outflow can contain ice-crystal engine hazards. Altostratus and nimbostratus can include extensive sub-zero liquid or mixed layers and embedded convection, so both icing and turbulence require assessment. Altocumulus can contain supercooled droplets, while castellanus suggests instability and lenticularis suggests waves.

Stratus and stratocumulus commonly have liquid droplets and can cause icing when supercooled, even with limited vertical motion. Cumulus has convective updraughts and downdraughts; deeper congestus increases concern for significant turbulence and icing in its supercooled region. Cumulonimbus combines potentially severe turbulence and icing with hail, lightning and strong outflow. None of these broad comparisons specifies a safe penetration route or proves that a given cloud will have a particular intensity.

What must change for the droplets or ice to vanish?

Follow the energy and moisture budget rather than assuming sunshine always clears every cloud.

Remove saturation or remove the particles

Cloud droplets evaporate when warming, drying or mixing makes the surrounding air sufficiently unsaturated. Descent warms a parcel by compression and often dissipates cloud, as on the lee side of terrain. Entrainment of drier air can erode edges, while a reduction of moisture supply or lifting can stop replacement of evaporating particles. Ice can sublimate in an environment unsaturated with respect to ice.

Precipitation removes cloud water or ice, but a cloud can persist if condensation and moisture transport replenish it. Solar heating can evaporate a shallow fog or stratus layer, yet it can also strengthen convection and build cumulus elsewhere. Night-time cloud-top radiative cooling can maintain or thicken some stratiform layers even after surface convection weakens.

A useful forecast explanation therefore names the process changing the layer: rising temperature, dry-air entrainment, altered advection, subsidence, weakening ascent or particle loss. It also tests whether that change is strong enough through the full cloud depth. A clear patch at one point or one level does not establish that an extensive layer has dissipated along the entire route.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.