Understanding clouds goes beyond basic identification. Each cloud type reveals information about atmospheric stability, moisture, and potential hazards. This knowledge is essential for visual meteorology and correlating what you see outside with what the instruments and forecasts are telling you.
Cloud Formation Basics
Clouds form when air rises and cools to its dew point, causing water vapor to condense. The type of cloud that forms depends on:
Stability
- Unstable air: Vertical (cumuliform) clouds
- Stable air: Horizontal (stratiform) layers
Lifting Mechanism
- Convection: Produces cumulus
- Frontal lift: Layered clouds ahead of fronts
- Orographic: Mountain-induced clouds
Low Clouds (Surface to 6,500 ft)
Stratus (ST)
METAR: Not specifically coded - reported as layer
Flat, gray, featureless layers covering the sky. Often forms from fog lifting or moist air cooling over a surface.
- • Low ceilings, reduced visibility
- • Smooth flight conditions (stable air)
- • Possible drizzle or light precipitation
- • Icing risk if temps below freezing
Stratocumulus (SC)
METAR: Not specifically coded
Low, lumpy layers with some vertical development. Larger elements than altocumulus, often covering large portions of the sky.
- • Light turbulence possible
- • Generally light precipitation only
- • Often breaks into VFR conditions
- • Moderate icing potential
Cumulus (CU)
METAR: Not specifically coded
Puffy, white clouds with flat bases and cauliflower tops. The classic "fair weather" cloud indicating thermal activity.
- • Bumpy conditions below/between clouds
- • Usually benign unless growing rapidly
- • Indicates unstable air
- • Watch for vertical development
Cumulonimbus (CB)
METAR: BKN040CB = Broken cumulonimbus at 4,000 ft
The thunderstorm cloud. Massive vertical development from low levels to the tropopause, with characteristic anvil top.
- • Severe turbulence, wind shear
- • Hail, heavy precipitation
- • Lightning, tornadoes possible
- • AVOID by at least 20 NM
Middle Clouds (6,500 - 20,000 ft)
Altostratus (AS)
Gray or blue-gray sheets covering the sky. Sun may appear as through frosted glass. Often precedes warm fronts by 12-24 hours.
- • Generally smooth flight
- • Continuous light precipitation possible
- • Warns of approaching frontal weather
- • Moderate icing in freezing temps
Altocumulus (AC)
White or gray patches of rounded masses, often in rows. Can indicate instability at middle levels. "Mackerel sky" pattern.
- • Light to moderate turbulence possible
- • May indicate instability developing
- • "Altocumulus castellanus" warns of afternoon thunderstorms
High Clouds (Above 20,000 ft)
Cirrus (CI)
Thin, wispy, white clouds made of ice crystals. Often indicate jet stream location or approaching frontal systems.
- • Generally smooth conditions
- • No precipitation reaches surface
- • May indicate jet stream nearby (clear air turbulence)
Cirrostratus (CS)
Thin, whitish veil covering the sky. Creates halos around the sun or moon. Often precedes warm fronts by 24-48 hours.
- • Excellent visibility through layer
- • Weather system approaching
- • Smooth conditions
Special Cloud Types
Towering Cumulus (TCU)
Rapidly growing cumulus with significant vertical development. Pre-thunderstorm stage.
METAR: BKN050TCU
Warning: May produce moderate turbulence, icing, and develop into CB within minutes
Lenticular (ACSL)
Lens or disc-shaped clouds forming in mountain wave conditions. Appear stationary but air flows through continuously.
Warning: Severe turbulence and strong vertical currents nearby
Rotor Clouds
Ragged, rotating clouds below lenticular clouds, indicating violent rotational turbulence.
Warning: Extreme turbulence - avoid at all costs
Wall Cloud
Lowering beneath a cumulonimbus, often rotating. Tornado formation zone.
Warning: Indicates possible tornado - maximum avoidance
METAR Cloud Reporting
Understanding how clouds are reported in METARs helps correlate ground observations with what you see from the cockpit.
Coverage Codes
| Code | Coverage | Oktas |
|---|---|---|
| SKC/CLR | Clear | 0 |
| FEW | Few (1-2/8) | 1-2 |
| SCT | Scattered (3-4/8) | 3-4 |
| BKN | Broken (5-7/8) | 5-7 |
| OVC | Overcast (8/8) | 8 |
Example Decoding
FEW015 SCT025 BKN040CB OVC100
- FEW015: Few clouds at 1,500 ft AGL
- SCT025: Scattered clouds at 2,500 ft
- BKN040CB: Broken cumulonimbus at 4,000 ft
- OVC100: Overcast at 10,000 ft
Frequently Asked Questions
What is the difference between cumulus and cumulonimbus?
Cumulus are fair-weather clouds with limited vertical development. Cumulonimbus are thunderstorms that have grown into the upper atmosphere, producing severe hazards including lightning, hail, and extreme turbulence.
What do stratus clouds indicate?
Stratus clouds indicate stable air with low ceilings and reduced visibility. Flight through stratus is typically smooth but may involve icing conditions if temperatures are near or below freezing.
What is a lenticular cloud?
Lenticular clouds are lens-shaped clouds forming over mountains in wave conditions. They indicate strong winds aloft and mountain wave turbulence.
What clouds indicate icing?
Any cloud between +2°C and -20°C may produce icing. Cumuliform clouds are most dangerous due to high liquid water content in updraft regions.