十万个为什么·科学阅读30篇（1）
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What Makes Some Clouds Rain While Others Stay Dry?
为什么有些云会降雨，而另一些却始终干燥？
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Rain forms only when cloud droplets grow heavy enough to fall before evaporating.
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Tiny water droplets usually stay suspended unless they collide and merge into larger drops.
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Clouds over oceans often contain more salt particles, which act as better condensation nuclei.
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In dry air below clouds, raindrops may evaporate completely—a phenomenon called virga.
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Cumulonimbus clouds rise high and cool rapidly, allowing ice crystals to form and trigger snow-rain cycles.
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Stratus clouds are shallow and stable, so droplets rarely reach falling size without strong updrafts.
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Wind shear and humidity layers determine whether droplets survive the descent to ground.
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Scientists use radar to track droplet size distribution and predict rainfall likelihood accurately.
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Pollution can create too many tiny nuclei, leading to many small drops that never coalesce.
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That’s why some polluted cities see frequent drizzle but rarely heavy downpours.