CogKnoSys

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Physical

Cloud Formation

Clouds form when air becomes saturated with water vapor, usually by cooling to its dew point, so tiny droplets or ice crystals appear.

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The idea

A cloud is a visible mass of tiny liquid droplets, ice crystals or other particles floating in the atmosphere. On Earth, the droplets and crystals are made mainly of water. Clouds form when air becomes saturated, meaning it holds as much water vapor as it can. That happens when the air cools to its dew point, or when it gains enough moisture from a nearby source. Air often cools as it rises, and the extra vapor then condenses into droplets.

The modern international system sorts clouds into five physical forms: stratiform sheets, cumuliform heaps, stratocumuliform rolls or ripples, cumulonimbiform towers and cirriform wisps. Split by altitude, these give ten basic genera. Mid-level types get the prefix alto- and high-level types the prefix cirro-. Very low stratiform clouds that reach the ground are called fog and mist.

Clouds matter for climate. Clouds in the troposphere, the lowest layer of the atmosphere, can reflect sunlight back to space, which cools the area below, or trap heat rising from Earth's surface, which warms it. Their altitude, form and thickness decide how much they heat or cool. Clouds are the main uncertainty in how sensitive Earth's climate is to change. Clouds also form on other planets and moons, where they may contain methane, ammonia or sulfuric acid.

Check yourself

Question 1 of 2

What happens to air when it cools to its dew point?

Pick one. The first pick counts.

Composition

what it's made of

  • Water droplets — Tiny liquid droplets, made mainly of water, that make up many clouds.
  • Ice crystals — Frozen particles that make up high, cold clouds.
  • Cooling to the dew point — Air that cools to its dew point becomes saturated, so vapor condenses.
  • Added moisture — Air can also become saturated by gaining water vapor from a nearby source.
  • Cloud forms — Sheets, heaps, rolls or ripples, towers and wisps, the five physical forms.

Connection

how it relates

Under Water Cycle

Cause and effect

  1. Climate Patternsstrength 70 of 100

    Clouds can cool the Earth by reflecting sunlight or warm it by trapping heat, so they affect climate.

Related

  1. Convection Currentsstrength 60 of 100

    Rising warm air cools as it climbs, and its water vapor condenses into clouds.

  2. Thermal Energystrength 40 of 100

    When water vapor condenses into cloud droplets, it releases heat into the surrounding air.

Change

how and why it evolves

Past
In 1802 Luke Howard formally proposed Latin names for cloud types, and his system became the basis of the modern international classification.
Now
The World Meteorological Organization names clouds with both Latin and common names, and researchers study how different clouds warm or cool the Earth.
Next
As satellites and computer models improve, scientists will likely get better at predicting how clouds respond as the planet warms.

Knowledge you can walk. Read the idea, then step up, across or down.

Cloud Formation

Clouds form when air becomes saturated with water vapor, usually by cooling to its dew point, so tiny droplets or ice crystals appear.

Learn, discuss, create, buy or invest

The idea

A cloud is a visible mass of tiny liquid droplets, ice crystals or other particles floating in the atmosphere. On Earth, the droplets and crystals are made mainly of water. Clouds form when air becomes saturated, meaning it holds as much water vapor as it can. That happens when the air cools to its dew point, or when it gains enough moisture from a nearby source. Air often cools as it rises, and the extra vapor then condenses into droplets.

The modern international system sorts clouds into five physical forms: stratiform sheets, cumuliform heaps, stratocumuliform rolls or ripples, cumulonimbiform towers and cirriform wisps. Split by altitude, these give ten basic genera. Mid-level types get the prefix alto- and high-level types the prefix cirro-. Very low stratiform clouds that reach the ground are called fog and mist.

Clouds matter for climate. Clouds in the troposphere, the lowest layer of the atmosphere, can reflect sunlight back to space, which cools the area below, or trap heat rising from Earth's surface, which warms it. Their altitude, form and thickness decide how much they heat or cool. Clouds are the main uncertainty in how sensitive Earth's climate is to change. Clouds also form on other planets and moons, where they may contain methane, ammonia or sulfuric acid.

Check yourself

2 quick questions, instant feedback.