11.3: Clouds

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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. 62% of days have more than three-quarters of the sky covered in clouds 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

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; there are other types of clouds as well (Figure below).

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.

Picture showing classification of clouds

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.

Science Friday: Growing Snowflakes in a Bottle

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Summary

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.

Review

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