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8.3: Clouds

Difficulty Level: Basic Created by: CK-12
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Where should you go if you don't want to see the sun?

The Pacific Northwest has the most days of heavy cloud cover. Sixty-two percent of days have more than three-quarters of the sky covered in cloud in Seattle. Conditions are nearly the same in Portland, Oregon. If you want to see the most sunshine, the least cloudy cities are in Arizona.


Clouds form when air in the atmosphere reaches the dew point. Clouds may form anywhere in the troposphere. Clouds that form on the ground are called fog.

How Clouds Form

Clouds form when water vapor condenses around particles in the air. The particles are specks of matter, such as dust or smoke. Billions of these tiny water droplets come together to make up a cloud. If the air is very cold, ice crystals form instead of liquid water.

Classification of Clouds

Clouds are classified on the basis of where and how they form. Three main types of clouds are cirrus, stratus, and cumulus. Figure below shows these and other types of clouds.

  • Cirrus clouds form high in the troposphere. Because it is so cold they are made of ice crystals. They are thin and wispy. Cirrus clouds don’t usually produce precipitation, but they may be a sign that wet weather is coming.
  • Stratus clouds occur low in the troposphere. They form in layers that spread horizontally and may cover the entire sky like a thick blanket. Stratus clouds that produce precipitation are called nimbostratus. The prefix nimbo- means “rain.”
  • Cumulus clouds are white and puffy. Convection currents make them grow upward and they may grow very tall. When they produce rain, they are called cumulonimbus.

Find the cirrus, cirrostratus, and cirrocumulus clouds in the figure. What do they have in common? They all form high in the troposphere. Clouds that form in the mid troposphere have the prefix “alto-”, as in altocumulus. Where do stratocumulus clouds form?

Clouds and Temperature

Clouds can affect the temperature on Earth’s surface. During the day, thick clouds block some of the sun’s rays. This keeps the surface from heating up as much as it would on a clear day. At night, thick clouds prevent heat from radiating out into space. This keeps the surface warmer than it would be on a clear night.


  • cloud: Tiny water or ice particles that are grouped together in the atmosphere.
  • fog: Air condensed below its dew point that is near the ground like a cloud.


  • Water vapor condenses on particles in the air to form clouds.
  • Clouds block sunlight in the day. Clouds trap heat in the atmosphere at night.
  • Cloud types include cirrus, stratus and cumulus.


Use this resource to answer the questions that follow.


  1. What is a cloud?
  2. What causes clouds to form?
  3. What is the Foehn effect?
  4. What causes a front to form?
  5. Explain what causes fog.


  1. What happens to turn water vapor into a cloud?
  2. What effects to clouds have on temperature?
  3. Compare and contrast cirrus, stratus and cumulus clouds.

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cirrus cloud Cold, high clouds made of ice crystals.
cloud Tiny water or ice particles that are grouped together in the atmosphere.
cumulus cloud Tall, puffy, white clouds, some types of which may rain.
fog Air condensed below its dew point that is near the ground like a cloud.
stratus cloud Low thick layers of clouds.

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6 , 7
Date Created:
Jan 04, 2013
Last Modified:
Dec 24, 2016
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