十万个为什么·科学阅读30篇（1）

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为什么有些云会降雨，而另一些却始终干燥？

Rain forms only when cloud droplets grow heavy enough to fall before evaporating.
Tiny water droplets usually stay suspended unless they collide and merge into larger drops.
Clouds over oceans often contain more salt particles, which act as better condensation nuclei.
In dry air below clouds, raindrops may evaporate completely—a phenomenon called virga.
Cumulonimbus clouds rise high and cool rapidly, allowing ice crystals to form and trigger snow-rain cycles.
Stratus clouds are shallow and stable, so droplets rarely reach falling size without strong updrafts.
Wind shear and humidity layers determine whether droplets survive the descent to ground.
Scientists use radar to track droplet size distribution and predict rainfall likelihood accurately.
Pollution can create too many tiny nuclei, leading to many small drops that never coalesce.
That’s why some polluted cities see frequent drizzle but rarely heavy downpours.