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Cumulus Clouds:Low, Puffy Heaps

Cumulus clouds are detached, low-based clouds with relatively flat bases and sharply defined, rounded tops. Small forms often accompany fair weather, while growing cumulus can produce showers and eventually develop into cumulonimbus.

Cumulus (Cu)
Cumulus (Cu)

How to identify cumulus clouds

Low-altitude, fluffy heaps of clouds with cotton-like appearance

Cumulus clouds are the clouds that we all drew as kids. They’re cotton ball clouds, popcorn clouds, and the clouds in the opening scene of The Simpsons. Though they come in different shapes and sizes, they’re generally the easiest type of cloud to pick out of the ten different cloud types.

When the average person is asked to visualize a cloud, cumulus clouds are generally the first to come to mind. The different sizes of cumulus clouds are described by their four associated species. If the cumulus cloud you’re looking at is wider than it is tall, then it’s a cumulus humilis.

If it’s as wide as it is tall, then it’s a cumulus mediocris. But when a cumulus cloud is taller than it is wide, known as cumulus congestus, things can get… interesting. If cumulus congestus clouds continue their vertical growth, they’re capable of producing rain, and can eventually morph into a cumulonimbus cloud, or thunderstorm.

If its vertical growth is impressive enough, you might find a cap cloud on top of a cumulus cloud, otherwise known as a pileus cloud. While cumulus clouds are known as low level clouds, cumulus congestus clouds with more extensive vertical growth can rise into the middle level. Finally, if you see a cumulus cloud that’s ragged and broken up, consider it of the species cumulus fractus.

You might witness these clouds in the evening as cumulus clouds begin to dissipate, or perhaps on a windy day.

Cumulus cloud examples

Cumulus fractus (Cu fra)
Cumulus congestus virga (Cu con vir)
Cumulus mediocris (Cu med)
Cumulus humilis (Cu hum)
Cumulus (Cu)
Cumulus congestus (Cu con)

Is it cumulus? Check these four clues

Use the clues together. One puffy edge is not enough to identify a cloud when its height, base, and overall coverage point elsewhere.

  1. Detached shape

    Look for a separate mound, dome, or tower rather than one continuous cloud blanket.

  2. Relatively flat base

    The lower edge often shares a level, darker base where rising air first condenses.

  3. Crisp, rounded top

    Actively growing edges look sharply defined and cauliflower-like, not fibrous or wispy.

  4. Visible vertical growth

    Even a small cumulus rises above its base; deeper forms build upward into larger towers.

Read the cloud's growth, not just its name

Cumulus can remain a small fair-weather cloud or grow into a shower-producing tower. The changing vertical structure is the useful weather signal.

  1. 1Little vertical growthHumilisOften fair weatherSmall, flattened heaps show that convection is present but still shallow.Open the field note →
  2. 2Moderate growthMediocrisBuilding convectionThe cloud becomes about as tall as it is wide while keeping crisp outlines.Open the field note →
  3. 3Strong vertical growthCongestusShowers possibleA tall cauliflower tower signals vigorous rising air and can produce precipitation.Open the field note →
  4. 4Deep, glaciated growthCumulonimbusThunderstormWhen the upper cloud loses its crisp outline and becomes smooth or fibrous, it is no longer cumulus.Open the field note →

Classifying cumulus clouds beyond the cloud type

Begin with cumulus as the cloud type, then add species, varieties, supplementary features, accessory clouds, or origin terms only when each detail is visible. Every illustration below links to its complete field note.

Cumulus cloud species

4 cloud species are associated with cumulus clouds. Open any field note to compare the illustration with photographs and identification guidance.

Illustration of cumulus congestus cloudsCumulus congestus (Cu con)Vertical, taller than it is wideExplore this subtype →Illustration of cumulus fractus cloudsCumulus fractus (Cu fra)Ragged, broken upExplore this subtype →Illustration of cumulus humilis cloudsCumulus humilis (Cu hum)Flattened, wider than it is tallExplore this subtype →Illustration of cumulus mediocris cloudsCumulus mediocris (Cu med)Medium height, tall as it is wideExplore this subtype →

Cumulus cloud varieties

1 cloud variety is associated with cumulus clouds. Open any field note to compare the illustration with photographs and identification guidance.

Illustration of cumulus radiatus cloudsCumulus radiatus (Cu ra)Parallel bands and stripsExplore this subtype →

Cumulus supplementary features

5 supplementary features are associated with cumulus clouds. Open any field note to compare the illustration with photographs and identification guidance.

Illustration of cumulus arcus cloudsCumulus arcus (Cu arc)Shelf cloud, gust collarExplore this subtype →Illustration of cumulus fluctus cloudsCumulus fluctus (Cu flu)Kelvin-Helmholtz waves, curlsExplore this subtype →Illustration of cumulus praecipitatio cloudsCumulus praecipitatio (Cu pra)Precipitation reaching the surfaceExplore this subtype →Illustration of cumulus tuba cloudsCumulus tuba (Cu tub)Funnel-shaped vortex cloudExplore this subtype →Illustration of cumulus virga cloudsCumulus virga (Cu vir)Precipitation trails that do not reach the groundExplore this subtype →

Cumulus accessory clouds

3 accessory clouds are associated with cumulus clouds. Open any field note to compare the illustration with photographs and identification guidance.

Illustration of cumulus pannus cloudsCumulus pannus (Cu pan)Ragged frazzles, scudExplore this subtype →Illustration of cumulus pileus cloudsCumulus pileus (Cu pil)Cap or hood over a cumuliformExplore this subtype →Illustration of cumulus velum cloudsCumulus velum (Cu vel)Horizontal cloud veilExplore this subtype →

Cumulus other clouds

3 other clouds are associated with cumulus clouds. Open any field note to compare the illustration with photographs and identification guidance.

Illustration of cumulus cataractagenitus cloudsCumulus cataractagenitus (Cu cagen)Waterfall condensation and sprayExplore this subtype →Illustration of cumulus flammagenitus cloudsCumulus flammagenitus (Cu flgen)Generated by fire or volcanic heatExplore this subtype →Illustration of cumulus homogenitus cloudsCumulus homogenitus (Cu hogen)Caused by human activityExplore this subtype →

Compare cumulus with similar cloud types

Use cloud-base level, texture, structure, and sky coverage together before choosing the closest match.

Cumulus vs. altocumulus

Cumulus grows from a low, usually well-defined base into detached mounds or towers. Altocumulus sits higher and more often appears as a field of smaller rounded elements or bands without one shared low base.

Best clueLook for one low, flat base supporting crisp vertical growth. A broad mid-level field of smaller elements points toward altocumulus.

Illustration of cumulus clouds
Cumulus
Illustration of altocumulus clouds
Altocumulus

Cumulus vs. stratocumulus

Cumulus is usually separated into individual heaps with noticeable vertical growth. Stratocumulus spreads into a low patch, sheet, or layer whose large rounded elements connect or organize into rolls.

Best clueCheck the sky coverage: detached heaps suggest cumulus; a joined layer or broad field of rolls suggests stratocumulus.

Illustration of cumulus clouds
Cumulus
Illustration of stratocumulus clouds
Stratocumulus

Cumulus vs. cumulonimbus

Cumulus keeps a sharply outlined, cauliflower-like top while it is made mostly of liquid droplets. Cumulonimbus has much deeper growth and an upper portion that begins to look smooth, fibrous, striated, or anvil-shaped as ice forms.

Best clueA fibrous or glaciated upper edge, or thunder and lightning, means the cloud has crossed into cumulonimbus.

Illustration of cumulus clouds
Cumulus
Illustration of cumulonimbus clouds
Cumulonimbus

See cumulus cloud classifications in action

Follow real photographs through the visible clues that lead to each complete cloud classification.

Photograph of a cumulus congestus cloudCumulus congestus (Cu con)See how height, shape, coverage, and other clues build this full classification.Read the step-by-step lesson →Photograph of cumulus humilis cloudsCumulus humilis (Cu hum)See how height, shape, coverage, and other clues build this full classification.Read the step-by-step lesson →
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