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July 26, 2026Sources cited
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Clouds

Clouds are visible collections of tiny water droplets, ice crystals, or both suspended in Earth’s atmosphere.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 26, 2026|2 min read| Fact-checked

Clouds are visible collections of tiny water droplets, ice crystals, or both suspended in Earth’s atmosphere. They form when moist air cools to saturation and water vapour condenses or deposits onto particles in the air. Clouds are a key part of the water cycle and strongly influence weather and climate.

Formation

Clouds form when moist air rises, expands, and cools until it reaches saturation. Water vapour then condenses onto cloud condensation nuclei or freezes into ice crystals. Rising air cools as pressure decreases, which often leads to cloud formation.

Types and classification

Clouds are classified by height and appearance. Common groupings include high, middle, and low clouds, along with forms such as cirrus, cumulus, stratus, and cumulonimbus. Clouds are commonly grouped by height and shape, including cirrus, stratus, and cumulus.

Weather effects

Clouds can produce rain, snow, drizzle, lightning, and thunder. They also influence temperature by shading the surface and affecting how much heat is retained in the atmosphere. Some clouds produce precipitation, lightning, and thunder. Clouds can cool Earth by reflecting sunlight and warm it by trapping outgoing heat.

Role in the water cycle

Clouds are a visible stage of the water cycle, linking evaporation, condensation, and precipitation. Their formation and movement help redistribute water around the planet. Clouds become visible when water vapour condenses into droplets or ice crystals.

Key facts

  • Clouds become visible when water vapour condenses into droplets or ice crystals.
  • Rising air cools as pressure decreases, which often leads to cloud formation.
  • Most clouds occur in the troposphere, with rare exceptions higher up.
  • Clouds are commonly grouped by height and shape, including cirrus, stratus, and cumulus.
  • Clouds can cool Earth by reflecting sunlight and warm it by trapping outgoing heat.
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Canadian context

Clouds are a central feature of Canada’s weather, affecting precipitation, storms, aviation, and seasonal conditions across the country.

Frequently asked questions

What are clouds made of?
Clouds are made of tiny water droplets, ice crystals, or a mixture of both suspended in the atmosphere.
How do clouds form?
They form when moist air cools to saturation, causing water vapour to condense onto particles or freeze into ice crystals.
What are the main types of clouds?
Common cloud groupings include cirrus, stratus, and cumulus, with many subtypes based on height and shape.
Why do clouds matter?
Clouds affect precipitation, storms, and Earth’s energy balance by reflecting sunlight and trapping heat.
Where do clouds usually occur?
Most clouds are found in the troposphere, the lowest layer of Earth’s atmosphere.

References

  1. Encyclopaedia Britannica — https://www.britannica.com/science/cloud-meteorology
    Supports: Cloud formation by rising, cooling air; adiabatic cooling; condensation onto nuclei.
  2. National Weather Service — https://www.weather.gov/lmk/cloud_classification
    Supports: Cloud classification by height and examples of high and low cloud types.
  3. Royal Meteorological Society / MetLink — https://www.metlink.org/fieldwork-resource/clouds/
    Supports: Cloud formation mechanisms and composition of droplets or ice crystals.
  4. NASA — https://www.nasa.gov/learning-resources/for-kids-and-students/what-are-clouds-grades-5-8/
    Supports: Cloud definition, formation, and climate/weather effects.
  5. Met Office — https://www.metoffice.gov.uk/binaries/content/assets/metofficegovuk/pdf/research/library-and-archive/library/publications/factsheets/factsheet_1-clouds_2023.pdf
    Supports: Cloud definition and typical location in the troposphere.
  6. National Weather Service — https://www.weather.gov/source/zhu/ZHU_Training_Page/clouds/cloud_development/clouds.htm
    Supports: Cloud formation from water vapour condensing into visible droplets or ice crystals.
  7. NOAA — https://www.noaa.gov/jetstream/clouds/four-core-types-of-clouds
    Supports: Core cloud categories based on Luke Howard's classification.