十万个为什么·科学阅读30篇（3）
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Why Do Some Clouds Look Like Cotton Balls?
为什么有些云像棉花糖？
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Some clouds look fluffy and white like cotton balls.
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These are called cumulus clouds.
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They form when warm air rises quickly from the ground.
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Water vapor cools and turns into tiny water droplets.
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Sunlight reflects off all these droplets evenly.
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That makes the clouds appear bright and puffy.
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Cumulus clouds usually mean fair weather.
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But big ones can grow into storm clouds.