十万个为什么·科学阅读30篇（6）
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Why Do Some Clouds Look Like Fluffy Cotton Balls?
为什么有些云看起来像蓬松的棉花球？
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These puffy clouds are called cumulus clouds and form on sunny days.这些蓬松的云称为积云，通常在晴天形成。
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Warm air rises and cools as it goes higher in the sky.暖空气上升时，随着高度增加而逐渐冷却。
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When cooled air reaches its dew point, water vapor turns into tiny droplets.当冷却的空气达到露点时，水蒸气便凝结成微小水滴。
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Droplets gather around dust or salt particles to build visible clouds.水滴围绕尘埃或盐粒聚集，形成肉眼可见的云。
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Strong updrafts keep them tall and fluffy in shape.强烈的上升气流使其保持高耸蓬松的形态。
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They usually mean fair weather unless they grow very large.它们通常预示好天气，除非变得异常庞大。
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No rain falls from small cumulus clouds most of the time.小型积云大多数时候不会降雨。
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Watching cloud shapes helps us understand air movement and moisture.观察云的形状有助于我们了解空气运动和湿度状况。