Ten genera cover every sky. The first three live in the high etage, the next three in the medium (with one important escape downward), the rest keep their bases low. Three of the ten refuse to stay in any single band: nimbostratus thickens downward through the low etage, and cumulus and cumulonimbus are clouds of vertical development, low-based but growing through the levels above.
Cirrus (CI) — high; delicate white filaments and hooks of ice crystals.
Cirrocumulus (CC) — high; tiny white grains and ripples without shading, in patches or sheets.
Cirrostratus (CS) — high; a transparent, milky veil that still lets the sun or moon show a halo, its signature.
Altocumulus (AC) — medium; white or grey rolls, patches and rounded masses with light shading.
Altostratus (AS) — medium; a grey, featureless sheet through which the sun appears as through ground glass, with no halo: the discriminator against cirrostratus.
Nimbostratus (NS) — medium, thickening and lowering into the low etage; a dark, ragged layer deep enough to hide the sun entirely, with continuous rain or snow.
Stratocumulus (SC) — low; grey and white rolls and clumps merged into a sheet, the most common cloud over Europe.
Stratus (ST) — low; a uniform grey layer close to the ground, at most drizzle: effectively a lifted fog.
Cumulus (CU) — low base, tops often into the medium etage; detached heaps with flat bases and cauliflower tops.
Cumulonimbus (CB) — low base, top toward the tropopause; the towering shower and thunderstorm cloud, often crowned with an anvil.
Species and supplementary features
Within the genera, the syllabus names five species and one supplementary feature worth recognising from the flight deck, because each is a message about the air:
Castellanus — turrets rising from a common cloud base, like battlements; most often on altocumulus. It announces instability at that level: a morning warning of afternoon thunderstorms.
Lenticularis — smooth, almond- or lens-shaped clouds standing still over ridges: the visible crests of mountain waves, with the turbulence of that system implied.
Congestus — cumulus grown tall, sharp-edged and cauliflower-topped, deeper than it is wide: one step short of a cumulonimbus.
Calvus — a cumulonimbus whose upper outlines have begun to soften and lose their sharp cauliflower detail: the top is starting to glaciate, freezing into ice.
Capillatus — a cumulonimbus whose top has gone fully fibrous and cirrus-like, typically with the anvil spread: a mature storm.
Virga — trails of precipitation hanging beneath a cloud and evaporating before reaching the ground; a supplementary feature, and a sign of dry air below the base.
Ice clouds, mixed clouds and water clouds
What a cloud is made of follows the temperature bands of the humidity lesson:
Warmer than 0 °C — a cloud is pure water.
Between 0 °C and about −20 °C — supercooled droplets remain common, so clouds there are water or mixed: droplets and ice crystals side by side.
From −20 °C down toward −40 °C — the mixture shifts increasingly to ice.
Colder than −40 °C — every droplet freezes spontaneously; cloud is pure ice.
Mapped onto the etages: the high clouds (cirrus, cirrocumulus, cirrostratus) are ice clouds; medium clouds are typically mixed; low clouds are mostly water. A cumulonimbus, spanning the whole ladder, is water at its base, mixed through its middle and ice in its anvil.
Inversions and cloud
An inversion (temperature rising with height, from the atmosphere module) is the strongest lid the sky can carry, and it shapes cloud four ways. First, it shuts down vertical motion: a parcel rising into air warmer than itself becomes colder and heavier than its surroundings and stops, so convection, and the mixing of anything (heat, moisture, pollution) ends at the inversion base. Second, that same lid builds stratus: moist air stirred beneath an inversion cannot break through it, so the lifting spreads sideways and condenses into a flat sheet (stratus or stratocumulus with a strikingly level top) just below the inversion base. Third, a ground inversion on a clear, calm night traps the coldest, moistest air in the lowest metres, and if the cooling reaches the dew point the result is fog: the radiation fog of the next lesson. Fourth, at the top of the troposphere the tropopause inversion stops even the deepest convection: a cumulonimbus top that reaches it flattens and spreads sideways into the anvil of the capillatus stage, the shape of a storm that has hit the ceiling.
Icing and turbulence, genus by genus
The exam asks for an assessment of all ten genera against the two airframe hazards. Two rules organise the list before it is memorised: icing is worst in deep clouds of large supercooled droplets (the 0 °C to −20 °C band again) and turbulence is worst where vertical motion is strongest. Genus by genus:
Cirrus — ice crystals only: icing negligible, flight smooth.
Cirrocumulus — icing negligible; at most a slight ripple.
Cirrostratus — icing negligible, flight smooth.
Altocumulus — light icing possible in its supercooled droplets; light turbulence, and the castellanus species warns of more to come.
Altostratus — light to moderate icing, worst in its lower part; little turbulence.
Nimbostratus — moderate to severe icing in a deep supercooled layer (the most serious icing cloud after the cumulonimbus), with moderate turbulence in and beneath it.
Stratocumulus — light to moderate icing near its tops; light to moderate low-level turbulence.
Stratus — light icing at most; smooth: its hazard is a base low enough to hide the ground, not what it does to the wings.
Cumulus — moderate icing in the upper half; moderate turbulence throughout.
Cumulonimbus — severe icing and severe turbulence: the worst entry on both lists, and the reason the flight-hazards module gives thunderstorms lessons of their own.