11.clouds-and-cloud-types. Clouds and Cloud Types
Clouds are visible aggregates of minute water droplets, ice crystals, or both, suspended in the atmosphere. They form when water vapor condenses or sublimates onto microscopic particles called condensation nuclei (dust, salt, smoke, pollen). Three ingredients must be present for cloud formation: sufficient water vapor, condensation nuclei, and a method of cooling the air to its dew point—most commonly by lifting. As an unsaturated parcel of air rises, it cools at the dry adiabatic lapse rate of approximately 3 °C per 1,000 feet. Once saturated, continued lifting cools the parcel at the moist (saturated) adiabatic lapse rate of roughly 1.1 to 2.8 °C per 1,000 feet, depending on moisture content. The altitude at which condensation begins is the lifted condensation level (LCL), which approximates the height of cloud bases.
Clouds are classified by altitude of base and by form. The four altitude families are:
- Low clouds — surface to 6,500 feet AGL: stratus, stratocumulus, nimbostratus.
- Middle clouds — 6,500 to 20,000 feet AGL: altostratus, altocumulus. Prefix alto-.
- High clouds — above 20,000 feet AGL: cirrus, cirrostratus, cirrocumulus. Prefix cirro- or cirrus.
- Clouds with extensive vertical development — bases as low as 1,000 feet AGL with tops to 60,000 feet or higher: towering cumulus and cumulonimbus.
By form, clouds are either stratiform (layered, sheet-like, indicating stable air) or cumuliform (heaped, puffy, indicating unstable air). The Latin roots are useful shorthand: cirrus = curl of hair, cumulus = heap, stratus = layer, nimbus = rain-bearing. Combining roots gives the cloud's character—nimbostratus is a layered rain cloud; cumulonimbus is a heaped rain cloud (the thunderstorm).
Low clouds are composed primarily of water droplets, including supercooled droplets when temperatures are below freezing—a serious structural icing hazard. Stratus produces low ceilings, restricted visibility, and sometimes drizzle. Stratocumulus appears as a lumpy gray layer. Nimbostratus is the classic continuous-rain cloud and frequently extends through the middle level.
Middle clouds consist of water, ice, or supercooled droplets. Altostratus is a gray or bluish sheet through which the sun appears as if seen through ground glass; it often precedes a warm front. Altocumulus appears as patches or rolls; altocumulus castellanus—turreted altocumulus—indicates mid-level instability and is a reliable precursor to afternoon thunderstorms.
High clouds are composed almost entirely of ice crystals and rarely affect VFR operations directly, though they can indicate approaching weather. Cirrus appears as delicate white filaments. Cirrostratus forms a thin veil that produces halos around the sun or moon and often signals an approaching warm front 24–36 hours out. Cirrocumulus appears as small white patches in ripples (a 'mackerel sky').
Clouds with vertical development are the most operationally significant. Cumulus clouds with little vertical extent—'fair weather cumulus'—indicate shallow convection. Towering cumulus (TCU) signal strong updrafts and the potential for thunderstorm development within about 30 minutes. Cumulonimbus (CB) is the mature thunderstorm: a single CB can contain severe turbulence, lightning, hail, severe icing, microbursts, wind shear, and tornadoes. Anvil tops, often pushed downwind by upper-level winds, indicate the storm has reached the tropopause. Pilots must avoid CBs by at least 20 NM when severe or topping 35,000 feet.
Special cloud types worth knowing:
- Lenticular (altocumulus standing lenticular, ACSL) — smooth, lens-shaped clouds that form on the lee side of mountains in mountain wave conditions. They mark severe turbulence even though the cloud itself appears stationary and benign.
- Rotor clouds — turbulent cloud rolls beneath the lee waves; extreme low-level turbulence.
- Cap clouds — sit atop mountain peaks; indicate strong downslope winds on the lee side.
- Funnel cloud / tornado / waterspout — pendant from a CB, indicating extreme rotation.
Cloud reporting in METARs uses coverage in eighths (oktas): SKC/CLR (clear), FEW (1–2/8), SCT (3–4/8), BKN (5–7/8), OVC (8/8). A ceiling is the lowest BKN or OVC layer (or vertical visibility into an obscuration). For example, BKN035 means a broken layer with base at 3,500 feet AGL.
For flight planning, recognizing cloud type provides immediate insight into the air mass: stratiform = stable, smooth, possible icing in cold temperatures, widespread IFR; cumuliform = unstable, turbulent, showery precipitation, good visibility between cells, but localized severe weather. The pilot who reads the sky has already begun the weather briefing.