Introduction to Cloud Classification
Clouds are visible masses of water droplets or ice crystals suspended in the atmosphere. Understanding cloud types helps in weather prediction, aviation, and appreciating the natural world. This cheatsheet covers the primary cloud classifications, their characteristics, and what weather they typically indicate. The classification system we use today was developed by Luke Howard in 1803 and has been expanded into the International Cloud Atlas maintained by the World Meteorological Organization.
Core Classification System
Three Primary Altitude Levels
| Level | Altitude Range | Prefix/Types |
|---|---|---|
| High Clouds | 16,500-45,000 ft (5-14 km) | Cirro- (Cirrus, Cirrostratus, Cirrocumulus) |
| Middle Clouds | 6,500-23,000 ft (2-7 km) | Alto- (Altocumulus, Altostratus) |
| Low Clouds | Surface-6,500 ft (0-2 km) | Stratus, Stratocumulus, Nimbostratus |
Four Basic Forms
| Form | Description | Examples |
|---|---|---|
| Cirrus | Thin, wispy, feather-like | Cirrus, Cirrostratus |
| Cumulus | Puffy, cotton-like, with flat bases | Cumulus, Cumulonimbus |
| Stratus | Layered, sheet-like, covering large areas | Stratus, Altostratus |
| Nimbus | Rain-bearing | Nimbostratus, Cumulonimbus |
High-Level Clouds (Cirro-form)
Cirrus (Ci)
Characteristics:
- Thin, wispy, delicate white filaments
- Often described as “mare’s tails” or “horse tails”
- Composed entirely of ice crystals
- Appear fibrous or hair-like
- Semi-transparent
Weather Indication:
- Fair weather
- When increasing and thickening from the west: approaching warm front (weather change within 24 hours)
- Often precede other cloud types in a sequence of approaching frontal systems
Appearance Variations:
- Cirrus uncinus: Hook-shaped ends
- Cirrus fibratus: Straight or slightly curved filaments
- Cirrus spissatus: Dense patches with a gray appearance
Cirrocumulus (Cc)
Characteristics:
- Small white patches or puffs in groups or lines
- Rippled appearance, sometimes called “mackerel sky”
- Very small elements with no shading
- Composed of ice crystals
- Rarely cover the entire sky
Weather Indication:
- Generally fair but cold weather
- May signal atmospheric instability at high levels
- Sometimes precede stormy weather in winter
Appearance Variations:
- Cirrocumulus undulatus: Wave-like pattern
- Cirrocumulus lenticularis: Lens-shaped
- Cirrocumulus floccus: Small tufted appearance
Cirrostratus (Cs)
Characteristics:
- Transparent, whitish veil covering part or all of sky
- Often produces halo phenomena around sun or moon
- Sheet-like, without distinct features
- Composed of ice crystals
Weather Indication:
- Often indicates approaching warm front
- May precede precipitation by 12-24 hours
- Usually thickens as weather system approaches
Appearance Variations:
- Cirrostratus fibratus: Fibrous veil
- Cirrostratus nebulosus: Uniform nebulous veil
- Cirrostratus undulatus: Shows undulations
Mid-Level Clouds (Alto-form)
Altocumulus (Ac)
Characteristics:
- White or gray patches, sheets, or layers
- Usually composed of water droplets
- Regular pattern of rounded masses or rolls
- Elements larger than cirrocumulus
- Shows shadowing from self-shading
- Often displays a “cauliflower” appearance
Weather Indication:
- Generally stable weather, but may indicate:
- Potential for thunderstorms later in the day (especially if developing in morning)
- If “towers” are present (castellanus form), indicates instability at mid-levels
Appearance Variations:
- Altocumulus castellanus: Tower-like extensions from the top
- Altocumulus lenticularis: Lens-shaped, stationary (often forms over mountains)
- Altocumulus stratiformis: Layer-like
- Altocumulus floccus: Small tufts with ragged bases
Altostratus (As)
Characteristics:
- Grayish or bluish sheet covering part or all of sky
- Sun/moon visible as if through frosted glass
- No halo phenomena (unlike cirrostratus)
- Uniform appearance without distinct features
- Composed of water droplets and some ice crystals
Weather Indication:
- Usually associated with approaching warm front
- Often precedes precipitation
- Indicates steady precipitation likely within 12-24 hours
- Gradual lowering suggests increasingly likely precipitation
Appearance Variations:
- Altostratus translucidus: Thin enough to reveal sun/moon
- Altostratus opacus: Thick enough to completely hide sun/moon
- Altostratus undulatus: Wave-like patterns visible
Low-Level Clouds
Stratocumulus (Sc)
Characteristics:
- Low, lumpy layer or patches of clouds
- Gray or whitish patches, rolls, or waves
- Larger elements than altocumulus
- Often arranged in lines, waves, or patches
- Usually has dark shaded parts
Weather Indication:
- Generally indicates fair weather
- May follow rain as weather improves
- Usually does not produce precipitation (light drizzle at most)
- When widespread in morning, often disperses by afternoon
Appearance Variations:
- Stratocumulus stratiformis: Extended layers
- Stratocumulus lenticularis: Lens-shaped
- Stratocumulus castellanus: With turret-like extensions
- Stratocumulus undulatus: Wave-like appearance
Stratus (St)
Characteristics:
- Uniform grayish cloud layer
- Fog-like but not touching the ground
- Featureless, blanket-like appearance
- Low altitude, often below 2,000 ft
- May produce drizzle, ice prisms, or snow grains
Weather Indication:
- Overcast, dreary conditions
- Associated with light precipitation or drizzle
- Often forms overnight and may burn off during the day
- Can persist for days in winter or coastal areas
Appearance Variations:
- Stratus nebulosus: Uniform fog-like layer
- Stratus fractus: Ragged patches (often under precipitation)
- Stratus undulatus: Slight wave-like pattern
Nimbostratus (Ns)
Characteristics:
- Dark gray, wet-looking layer clouds
- Diffuse appearance, sun/moon completely obscured
- Continuous precipitation (rain or snow)
- Thick, with a base usually below 2,000 ft
- Often accompanied by stratus fractus below main cloud base
Weather Indication:
- Steady, continuous precipitation
- Associated with warm fronts and occluded fronts
- Persistent precipitation lasting hours or days
- Reduced visibility due to precipitation
Appearance Variations:
- Generally uniform in appearance
- Often has precipitation that reaches the ground
- May have virga (precipitation that evaporates before reaching ground)
Vertical Development Clouds
Cumulus (Cu)
Characteristics:
- Distinct, puffy, cotton-like appearance
- Flat bases with rounded, cauliflower-like tops
- Bright white tops with darker bases
- Distinct outlines and edges
- Vertical development
Weather Indication:
- Fair weather type (cumulus humilis) – indicates good weather
- Moderate development (cumulus mediocris) – generally stable
- Towering cumulus (cumulus congestus) – potential for showers
- Diurnal cycle: often form mid-morning, peak afternoon, dissipate evening
Appearance Variations:
- Cumulus humilis: Little vertical development, “fair weather cumulus”
- Cumulus mediocris: Moderate vertical development
- Cumulus congestus: Strong vertical development, towering
- Cumulus fractus: Ragged, broken fragments
Cumulonimbus (Cb)
Characteristics:
- Massive, dense, tall with anvil-shaped top (incus)
- Great vertical development (up to 60,000 ft)
- Dark, often with mammatus formations under anvil
- Produces lightning, thunder, heavy precipitation
- May have wall cloud, funnel cloud, or tornado
- Upper portion spread out in anvil or plume shape
Weather Indication:
- Thunderstorms, heavy rain, possible hail
- Strong, gusty winds
- Potential for severe weather (tornadoes, downbursts)
- Localized but intense precipitation
- Rapid development and movement possible
Appearance Variations:
- Cumulonimbus calvus: Rounded top without anvil
- Cumulonimbus capillatus: With fibrous or striated upper portion
- Cumulonimbus incus: Classic anvil-shaped top
- Cumulonimbus with arcus (shelf) cloud: Roll cloud at leading edge
Special Cloud Formations
Lenticular Clouds (Lenticularis)
Characteristics:
- Lens or almond-shaped
- Stationary, despite strong winds
- Smooth, polished appearance
- Form at high altitudes over mountains
- Can stack in multiple layers
Formation:
- Created by stable, moist air flowing over mountains
- Forms in standing wave pattern downwind of mountains
- Constant formation and dissipation (appears stationary)
Weather Indication:
- Strong winds at cloud level
- Stable atmospheric conditions with vertical waves
- Turbulence for aircraft
Mammatus Clouds
Characteristics:
- Pouch-like protrusions hanging from base of cloud
- Most commonly observed on underside of cumulonimbus anvil
- Can also appear on cirrus, cirrocumulus, altostratus, stratocumulus
- Dramatic, bulbous appearance
Formation:
- Cold air pockets descending into warmer air
- Often associated with severe weather
- Results from reverse convection
Weather Indication:
- Appears after thunderstorm peak
- Often indicates storm is weakening
- Visually dramatic but generally not dangerous itself
Kelvin-Helmholtz Clouds
Characteristics:
- Distinctive breaking-wave or curling pattern
- Short-lived (typically only lasts a few minutes)
- Forms at boundary of air masses with different speeds
Formation:
- Wind shear between two air layers of different densities
- Similar to ocean waves breaking on shore
- Result of velocity difference creating instability
Weather Indication:
- Strong wind shear
- Atmospheric instability between layers
- Possible clear air turbulence for aircraft
Noctilucent Clouds
Characteristics:
- Extremely high (76,000-85,000 ft / 50-53 miles)
- Visible only when sun is below horizon (twilight hours)
- Electric blue or silver color
- Wavy, filamentary structure
- Visible in summer months at high latitudes
Formation:
- Ice crystals forming on meteoric dust
- Located in mesosphere (highest cloud type)
- Requires extremely cold temperatures (-190°F/-123°C)
Observation:
- Best seen in summer months
- 50°-65° latitude north or south
- Just after sunset or before sunrise
Cloud Formations and Weather Prediction
Frontal Systems and Associated Clouds
| Front Type | Cloud Sequence | Weather Implication |
|---|---|---|
| Warm Front | Cirrus → Cirrostratus → Altostratus → Nimbostratus | Gradual deterioration, steady precipitation |
| Cold Front | Cumulus → Towering Cumulus → Cumulonimbus | Rapid deterioration, intense but brief precipitation |
| Occluded Front | Mixed cloud types, often nimbostratus | Complex weather patterns, extended precipitation |
| Stationary Front | Variable clouds depending on air masses | Persistent conditions with little change |
24-Hour Forecast Based on Clouds
| Cloud Observation | Likely Weather Within 24 Hours |
|---|---|
| Morning cumulus growing into afternoon cumulonimbus | Afternoon thunderstorms, clearing by evening |
| Thickening and lowering cirrus/cirrostratus | Approaching warm front, precipitation within 24 hours |
| Altocumulus castellanus in morning | Atmospheric instability, possible thunderstorms later |
| Stratocumulus or stratus persisting all day | Continued overcast conditions, possible light precipitation |
| Clearing skies with scattered cumulus | Improving conditions, fair weather likely |
Cloud Coverage Terminology
| Term | Coverage (oktas) | Description |
|---|---|---|
| Clear (SKC or CLR) | 0/8 | No clouds |
| Few (FEW) | 1/8 – 2/8 | Less than a quarter of sky covered |
| Scattered (SCT) | 3/8 – 4/8 | Between a quarter and half of sky covered |
| Broken (BKN) | 5/8 – 7/8 | More than half but not complete coverage |
| Overcast (OVC) | 8/8 | Complete cloud coverage |
Clouds and Aviation
Aviation Significance
| Cloud Type | Aviation Hazards | Pilot Actions |
|---|---|---|
| Cumulonimbus | Severe turbulence, lightning, hail, icing, microbursts | Avoid by at least 20 nm horizontally |
| Embedded thunderstorms | Hidden turbulence and icing | Use weather radar, request ATC assistance |
| Stratiform clouds | Icing (especially if temperature -10°C to 0°C) | Request altitude change if icing occurs |
| Mountain wave clouds | Severe turbulence, rapid altitude excursions | Avoid or cross at right angles to wave |
| Fog/Low stratus | Low visibility, obscured terrain | Delay operation or use instrument procedures |
METAR Cloud Reporting
METAR KJFK 121851Z 18004KT 10SM FEW035 BKN120 OVC250 23/18 A3001
Cloud layer coding:
FEW035: Few clouds at 3,500 feetBKN120: Broken cloud layer at 12,000 feetOVC250: Overcast cloud layer at 25,000 feet
Cloud Photography Tips
Best Practices for Cloud Photography
| Cloud Type | Best Time | Camera Settings |
|---|---|---|
| Cumulus | Mid-day with sunlight | Fast shutter (1/500+), polarizing filter |
| Cumulonimbus | Late afternoon, sunset | Wide-angle lens, HDR technique |
| Cirrus | Early morning, sunset | Polarizing filter, slight underexposure |
| Mammatus | After storm, sunset | Medium telephoto, enhance contrast |
| Foggy stratus | Dawn/early morning | Long exposure, tripod, careful exposure |
Post-Processing Recommendations
- Contrast adjustment: Enhances cloud texture
- Graduated filters: Balance bright sky with darker foreground
- Clarity/Texture: Brings out detail in cloud formations
- White balance: Can dramatically affect mood (cooler for dramatic storms, warmer for sunset clouds)
Common Challenges in Cloud Identification
| Challenge | Solution |
|---|---|
| Mixed cloud types | Focus on predominant type and altitude |
| Transitional forms | Observe development over time |
| Distance perspective | Use known landmarks for altitude estimation |
| Poor visibility | Use partial views and edges for identification |
| Rapidly changing conditions | Make multiple observations over time |
Resources for Further Learning
Apps and Websites
- Cloud Spotter app by Cloud Appreciation Society
- GLOBE Observer app (NASA citizen science project)
- National Weather Service Cloud Chart
- World Meteorological Organization Cloud Atlas (online)
Field Guides
- “A Field Guide to the Atmosphere” by Vincent Schaefer and John Day
- “The Cloudspotter’s Guide” by Gavin Pretor-Pinney
- “Peterson First Guide to Clouds and Weather” by John A. Day
Scientific Resources
- American Meteorological Society resources
- University Corporation for Atmospheric Research (UCAR) education resources
- NASA Earth Observatory cloud resources
Citizen Science Projects
- GLOBE Program cloud observations
- Cloud Appreciation Society
- S’COOL (Students’ Cloud Observations On-Line)
By understanding cloud types and their implications, you can “read” the sky to predict weather changes, appreciate atmospheric processes, and develop a deeper connection to the natural world above us.