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.有些云朵看起来像棉球一样蓬松洁白。
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These are called cumulus clouds.这类云被称为积云。
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They form when warm air rises quickly from the ground.它们由地面暖空气快速上升形成。
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Water vapor cools and turns into tiny water droplets.水蒸气遇冷凝结成微小水滴。
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Sunlight reflects off all these droplets evenly.阳光均匀地反射在这些水滴上。
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That makes the clouds appear bright and puffy.使云朵显得明亮而蓬松。
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Cumulus clouds usually mean fair weather.积云通常预示着晴好天气。
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But big ones can grow into storm clouds.但较大的积云可能发展成雷雨云。