# Cloud formation and description

> Learn how clouds are formed, cloud types, and cloud classification for ATPL aviation meteorology.

- **Subject:** 050 — Meteorology
- **Section:** 050-04 — Clouds and Fog
- **Topic code:** 050-04-01
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/meteorology/clouds-and-fog/cloud-formation-and-description/

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Clouds form when air rises and cools adiabatically until it reaches its dew point (condensation level), at which point water vapor condenses into visible water droplets or ice crystals.

## Cloud Classification (WMO)

![Cloud Types](https://upload.wikimedia.org/wikipedia/commons/5/57/Cloud_types_en.svg)

The World Meteorological Organization classifies clouds into three levels or "etages" according to the height of their base.

| Level | Base Height | Prefix | Main Composition | Types |
| :--- | :--- | :--- | :--- | :--- |
| **High Clouds** | 16,500 - 45,000 ft | Cirro- | Ice Crystals | Cirrus (Ci), Cirrocumulus (Cc), Cirrostratus (Cs) |
| **Medium Clouds** | 6,500 - 23,000 ft | Alto- | Water Droplets / Crystals | Altocumulus (Ac), Altostratus (As), Nimbostratus (Ns)* |
| **Low Clouds** | 0 - 6,500 ft | - | Water Droplets | Stratus (St), Stratocumulus (Sc), Cumulus (Cu), Cumulonimbus (Cb), Nimbostratus (Ns)* |

> **Note:** Nimbostratus (Ns), Cumulus (Cu), and Cumulonimbus (Cb) have great vertical development and can extend through multiple levels.

## Cloud Types and Characteristics

### High Clouds
Composed almost entirely of ice crystals. Generally do not produce significant icing or turbulence.

![Cirrus](https://upload.wikimedia.org/wikipedia/commons/f/fc/Quill-shaped_cirrus_cloud.jpg)

*   **Cirrus (Ci):** Thin clouds, looking like "hair" or strands, silky. Indicate fair weather or the approach of a warm front aloft.

![Cirrocumulus](https://upload.wikimedia.org/wikipedia/commons/e/ea/2023-07-22_-_Cirrocumulus_lacunosus_-_solitary_cloud_-_zenith_-_DSG1929-11_-_crop_mid_right.jpg)

*   **Cirrocumulus (Cc):** Small rounded patches or "mackerel sky". The only high clouds with cumuliform characteristics.

![Cirrostratus](https://upload.wikimedia.org/wikipedia/commons/c/c5/2024-03-01_-_Cirrostratus_-_halo_%2822_deg%29_centered_on_sun_-_DSN2151-1.jpg)

*   **Cirrostratus (Cs):** Transparent whitish veil that can produce halos around the sun or moon.

### Medium Clouds

![Altocumulus](https://upload.wikimedia.org/wikipedia/commons/8/85/Altocumulus_cloud.jpg)

*   **Altocumulus (Ac):** Patches or layers of rounded masses.
    *   **Ac Castellanus:** Look like castle turrets. Indicate instability at medium levels and are precursors to thunderstorms.
    *   **Ac Lenticularis:** Lens-shaped clouds. Indicate the presence of **mountain waves** and severe turbulence, even if they appear static and smooth.

    ![Lenticular cloud](https://upload.wikimedia.org/wikipedia/commons/3/37/Altocumulus_lenticularis_radiatus_undulatus_bei_Greensburg_II.jpg)

![Altostratus](https://upload.wikimedia.org/wikipedia/commons/1/12/Sun_through_Altostratus.JPG)

*   **Altostratus (As):** Grey or bluish sheet covering the sky. Can give a "ground glass" effect to the sun. Light to moderate icing.

### Low Clouds

![Stratus](https://upload.wikimedia.org/wikipedia/commons/5/55/Stratus_Clouds_-_Knoxville%2C_Tennessee_-_March_19%2C_2014.jpg)

*   **Stratus (St):** Uniform grey layer. Can produce drizzle. Dissipates by solar heating.

![Stratocumulus](https://upload.wikimedia.org/wikipedia/commons/7/73/Stratocumulus_cumulogenitus_-_20150804_16h39_%2810909%29.jpg)

*   **Stratocumulus (Sc):** Patches or layers of low clouds, grey or whitish. Rarely produce precipitation.

![Nimbostratus](https://upload.wikimedia.org/wikipedia/commons/f/f8/Nimbostratus_pannus.jpg)

*   **Nimbostratus (Ns):** Dense, dark grey layer. Continuous rain or snow cloud. Can completely hide the sun. Risk of icing and freezing rain.

### Clouds of Vertical Development

![Cumulus](https://upload.wikimedia.org/wikipedia/commons/2/2b/Cumulus_cloud_above_Lechtaler_Alps_at_tannheim%2C_Austria.jpg)

*   **Cumulus (Cu):** Detached, dense clouds with sharp outlines (cauliflower shape). Dark, flat base.
    *   *Cu Humilis:* Fair weather, wider than they are tall.
    *   *Cu Mediocris:* Moderate vertical development.
    *   *Cu Congestus (TCU):* Great vertical development (Towering Cumulus). Showers, turbulence, and icing.

![Cumulonimbus](https://upload.wikimedia.org/wikipedia/commons/e/e3/Cloud_cumulonimbus_at_baltic_sea%281%29.jpg)

*   **Cumulonimbus (Cb):** The thunderstorm cloud. Huge tower that can reach the tropopause and form an anvil (*incus*).
    *   *Cb Calvus:* Rounded top, not yet fibrous.
    *   *Cb Capillatus:* Fibrous top (cirriform) or with anvil.
    *   **Hazards:** Severe turbulence, severe icing, hail, lightning, windshear.

## Formation Processes

1.  **Convection:** Heating of the earth's surface warms the adjacent air, causing it to rise (instability). Forms cumuliform clouds.
2.  **Orographic Lift:** Air is forced to rise by a mountain. Forms lenticular clouds and stratus on the windward side.
3.  **Convergence:** In low-pressure systems, air converges at the surface and rises.
4.  **Turbulence/Mixing:** Wind mixes a layer of moist air near the surface, forming Stratus or Stratocumulus.
5.  **Frontal Lift:** Warm air rising over cold air (fronts).

## Stability vs Instability

The type of cloud depends on the stability of the air mass:
*   **Stable Air:** Tends to resist vertical movement. Forms **Stratiform** clouds (layers, large horizontal extent, little vertical development). Continuous precipitation (if any).
*   **Unstable Air:** Favors vertical movement. Forms **Cumuliform** clouds (vertical development, updrafts/downdrafts). Showers.

**Inversions:** A temperature inversion (temperature increase with height) acts as a "lid", stopping the vertical development of clouds and trapping cloud layers (like Stratus) or fog below it.

## Practical Calculations

**Cloud Base (Cumulus):**
A general rule for estimating the height of the cumulus base is:
$$ \text{Base (ft)} = (\text{Temperature} - \text{Dew Point}) \times 400 $$

*   Larger difference (spread) between T and Td $\rightarrow$ Higher base.
*   High Relative Humidity (T and Td close) $\rightarrow$ Lower base.

**Cloud Tops:**
Vertical growth stops when the temperature of the rising air parcel (cooling at the SALR inside the cloud) equals the temperature of the surrounding air (ELR).

## Cloud Dissipation

Clouds dissipate mainly by:
1.  **Heating:** Increases the air's capacity to hold water vapor (evaporation of droplets).
2.  **Subsidence:** Sinking air that compresses and warms adiabatically.
3.  **Mixing:** With drier air from the surroundings.

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