十万个为什么·科学启蒙30篇（1）
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Why Do Some Clouds Produce Rain While Others Don’t?
为什么有些云会下雨，而另一些不会？
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Rain forms only when cloud droplets grow heavy enough to fall past rising air currents.
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Tiny water droplets usually stay suspended unless they collide and merge repeatedly.
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In cold clouds, ice crystals form and attract vapor, growing faster than droplets.
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Wind patterns, humidity, and temperature layers all affect whether droplets grow big enough.
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Cumulonimbus clouds are tall and turbulent, giving droplets time and space to grow.
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Stratus clouds are thin and stable, so droplets rarely reach raindrop size.
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Dust, salt, or pollen particles act as nuclei around which water condenses.
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Without enough nuclei, clouds may hold moisture but never release rain.
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Meteorologists use radar to track droplet size and predict rainfall chances.
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That’s why fog looks like a cloud on the ground—but rarely rains.