Cloud Types for Pilots
Clouds are one of the most important weather indicators for pilots. Identifying cloud types allows you to anticipate weather conditions, icing risk, turbulence, and visibility changes before they affect your flight.
High Clouds
High clouds are composed entirely of ice crystals. They rarely produce precipitation reaching the ground but can signal deteriorating weather over the coming hours.
Cirrus (Ci)
Thin, wispy streaks or curls of white cloud. Composed entirely of ice crystals. Often called "mare's tails." Fair-weather cirrus appears in isolated patches with no particular pattern; cirrus ahead of a warm front forms in organised sheets thickening toward the horizon.
Pilot significance: No turbulence or icing in the cloud itself, but organised cirrus increasing and lowering may indicate deteriorating weather within 12–24 hours. Be alert for the approach of a warm front system.
Cirrostratus (Cs)
A thin, transparent sheet of ice crystal cloud covering all or part of the sky. Produces a characteristic halo around the sun or moon — a reliable indicator of this cloud type. The sun remains visible but may appear milky.
Pilot significance: Light to moderate icing possible for aircraft flying within the layer. Strong warm front indicator — thickening and lowering cirrostratus typically precedes altostratus by several hours.
Cirrocumulus (Cc)
Small white puffs or ripples arranged in rows or patches, often referred to as a "mackerel sky." Relatively rare. Individual elements appear small (less than 1° finger-width at arm's length). No shading visible on the puffs — distinguishes from altocumulus.
Pilot significance: Slight turbulence possible. May indicate approaching frontal weather or upper-level instability. Generally benign for flight.
Middle Clouds
Middle clouds are composed of water droplets, ice crystals, or a mixture. They are associated with frontal systems and can produce significant icing conditions.
Altostratus (As)
A grey or blue-grey sheet of cloud covering large areas of sky. The sun appears as though viewed through frosted glass — a key identifying feature. Associated with warm fronts and extensive precipitation areas. Can be many thousands of feet thick.
Pilot significance: Moderate icing likely, especially near 0°C isotherm. Light to moderate turbulence possible. Precipitation falling through (rain, snow, or freezing rain below). Poor visibility in cloud. High likelihood of IFR conditions below cloud base.
Altocumulus (Ac)
Grey or white patches, sheets, or waves of cloud arranged in groups or rolls. Elements appear larger than cirrocumulus — roughly 1–5° finger-width. Shading visible on the darker sides of elements. Can occur in multiple layers.
Pilot significance: Moderate icing possible. Watch for Altocumulus Castellanus (ACcas) — towering altocumulus with vertical development, visible in the morning, is a strong indicator that afternoon convective thunderstorm activity is likely.
Altocumulus Castellanus (ACcas)
Altocumulus with turret-like vertical extensions rising from a common base. The turrets appear as small cumuliform towers growing upward from the altocumulus layer. Most significant when observed in the morning hours on a day with expected surface heating.
Pilot significance: A serious warning sign. ACcas in the morning strongly indicates that the atmosphere has sufficient moisture and instability for significant afternoon convective development, including thunderstorms. Revise flight plans accordingly.
Low Clouds
Low clouds are predominantly water droplet clouds. They directly impact takeoff and landing conditions, often producing low ceilings, reduced visibility, and icing near the freezing level.
Stratus (St)
A low, grey, uniform layer of cloud with a flat, featureless base resembling fog that has lifted off the surface. Often covers large areas. May produce drizzle. Ceiling can be extremely low — sometimes below 100 ft.
Pilot significance: Light icing possible. Poor to very poor visibility in cloud. Ceiling and visibility often below VFR and IFR approach minimums. Typically forms in stable air; can persist for extended periods, especially in coastal and valley areas.
Stratocumulus (Sc)
Low, lumpy grey or whitish cloud layer arranged in patches, rolls, or waves. The most common cloud type globally. Has more structure than stratus. Breaks may appear and the sun occasionally visible through gaps. Produces drizzle or light rain.
Pilot significance: Light icing possible. Drizzle. Breaks in cover may allow VFR flight, but conditions can change. Base typically higher than stratus but still operationally significant. Horizon scanning important to identify gaps for departure/arrival.
Nimbostratus (Ns)
Dark, thick, grey cloud producing continuous, prolonged rain or snow. No well-defined cloud base — precipitation and virga often obscure the lower boundary. Can extend from low levels to middle cloud levels (several thousand feet thick). Associated with warm fronts and occluded fronts.
Pilot significance: Extensive icing throughout the cloud layer — potentially severe near the freezing level. Very poor visibility in cloud and precipitation. No thunderstorms (no embedded convection), but conditions are sustained and widespread. Often requires IFR alternates far removed from the route.
Convective Clouds
Convective clouds develop vertically through atmospheric instability. They range from benign fair-weather cumulus to the most hazardous cloud type in aviation — the cumulonimbus.
Cumulus (Cu) — Fair Weather
Heaped, puffy clouds with flat bases and rounded tops. Fair-weather cumulus has limited vertical extent — the top is relatively flat or only slightly domed. Base altitude corresponds to the lifting condensation level. Thermal activity below and within the cloud.
Pilot significance: Turbulence below the cloud base from thermal activity. Light icing if above the freezing level. Good visibility in surrounding areas. Flat-topped cumulus is benign; monitor for vertical development into towering cumulus.
Towering Cumulus (TCu)
Cumulus with significant vertical development — height is greater than base width. Cauliflower-like tops still visible (not yet glaciated). Rapidly developing in unstable conditions. Precursor to cumulonimbus. May be embedded in other cloud layers, making visual detection difficult in IMC.
Pilot significance: Significant turbulence inside and around the cloud. Moderate to severe icing above the freezing level. May develop into cumulonimbus within minutes. Treat with the same caution as Cb. Give wide horizontal clearance.
Cumulonimbus (Cb)
The most hazardous cloud type in aviation. A deep convective cloud extending from low levels to the tropopause (and sometimes above in overshooting tops). Tops glaciate and spread into the characteristic anvil shape. Contains all aviation hazards simultaneously.
AVOID — All hazards present: Extreme turbulence (including outside the cloud), severe to extreme structural icing, large hail, lightning, very heavy precipitation, severe downdraughts and updraughts, microbursts, wind shear, possible tornado. Never attempt to fly through a Cb.
VMC minimum clearances: 5 nm horizontal separation, 1,000 ft above cloud top, avoid flying below the anvil. In IMC, divert around; do not attempt to navigate through embedded Cb by radar alone.
Pilot Significance Quick Reference
Summary of icing risk, turbulence, in-cloud visibility, and operational category for each cloud type. Use this table for rapid pre-flight briefing reference.
| Cloud Type | Abbr. | Icing Risk | Turbulence | In-Cloud Visibility | Operational Impact |
|---|---|---|---|---|---|
| Cirrus | Ci | Low | None | N/A (above most traffic) | Monitor for frontal approach |
| Cirrostratus | Cs | Light–Moderate | Light | Moderate | Warm front precursor |
| Cirrocumulus | Cc | Trace | Slight | Good | Low concern |
| Altostratus | As | Moderate | Light–Moderate | Poor | High — IFR likely below |
| Altocumulus | Ac | Moderate | Light–Moderate | Moderate | Medium |
| ACcas | ACcas | Moderate | Moderate | Moderate | High — CB development warning |
| Stratus | St | Light | Light | Very Poor | High — low ceiling |
| Stratocumulus | Sc | Light | Light | Poor | Medium — breaks possible |
| Nimbostratus | Ns | Moderate–Severe | Light | Very Poor | High — sustained IFR |
| Cumulus | Cu | Low | Moderate below | Good | Low (monitor development) |
| Towering Cumulus | TCu | Moderate | Severe | Poor | High — give wide clearance |
| Cumulonimbus | Cb | Severe–Extreme | Extreme | Very Poor | AVOID |
The Okta Scale — Cloud Cover Measurement
Cloud cover is reported in oktas — eighths of the sky covered by cloud. METARs use standardised cover descriptors derived from the okta count. Understanding oktas is essential for reading cloud groups in METARs and TAFs.
| Oktas | METAR Code | Description | Sky Coverage |
|---|---|---|---|
| 0 | SKC / CLR | Sky Clear | No cloud detected |
| 1–2 | FEW | Few | 1/8 to 2/8 of sky covered |
| 3–4 | SCT | Scattered | 3/8 to 4/8 of sky covered |
| 5–7 | BKN | Broken | 5/8 to 7/8 of sky covered — defines ceiling |
| 8 | OVC | Overcast | 8/8 sky completely covered — defines ceiling |
| 9 | VV/// | Sky Obscured | Vertical visibility reported; sky not discernible |
Ceiling definition: The ceiling is the height of the lowest layer reported as BKN or OVC. FEW and SCT layers are not ceilings. For example, FEW010 BKN025 OVC060 gives a ceiling of 2,500 ft (the BKN layer). This directly determines VFR/IFR flight categories.