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.只有当云中水滴变得足够重，能穿过上升气流下落时，才会形成降雨。
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Tiny water droplets usually stay suspended unless they collide and merge repeatedly.微小水滴通常悬浮在空中，除非反复碰撞并合并。
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In cold clouds, ice crystals form and attract vapor, growing faster than droplets.在冷云中，冰晶形成并吸附水汽，比水滴增长得更快。
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Wind patterns, humidity, and temperature layers all affect whether droplets grow big enough.风向、湿度和温度层共同决定水滴能否长大到足以降雨。
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Cumulonimbus clouds are tall and turbulent, giving droplets time and space to grow.积雨云高大且湍急，为水滴生长提供了充足的时间和空间。
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Stratus clouds are thin and stable, so droplets rarely reach raindrop size.层云薄而稳定，水滴极少能长到雨滴大小。
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Dust, salt, or pollen particles act as nuclei around which water condenses.尘埃、盐粒或花粉等微粒充当水汽凝结的凝结核。
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Without enough nuclei, clouds may hold moisture but never release rain.若凝结核不足，云中虽含水分，却难以形成降雨。
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Meteorologists use radar to track droplet size and predict rainfall chances.气象学家利用雷达监测水滴大小，预测降雨概率。
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That’s why fog looks like a cloud on the ground—but rarely rains.因此雾就像贴地的云——但极少降雨。