Welcome to You Ask Andy

 Etheleen Duckett, age 12, of Winston Salem, North Carolina, for her question:

Why does every raindrop contain a speck of dust?

Most raindrops contain several specks of dust, plus assorted particles of air pollutants. However, even if a raindrop fell through a clean, unpolluted atmosphere, most likely toe would find one particle of dust in its pure water. Meteorologists suspect that a solid particle of some sort is necessary to create a drop of rain. It seems that the misty moisture of a cloud cannot merge into sizeable drops unless tiny rainmaking nuclei are present.

Near the ground, the air's moisture can gather on all sorts of cool surfaces. In the atmosphere there are no such solid surfaces    but the air may contain fine fragments of dust, smoke, perhaps salt and other particles. Cloud moisture is infinitely small, widely separated droplets of water. Apparently they amass around dusty nuclei to form raindrops heavy enough to fall. Chances are, the dusty nuclei stays in a raindrop until it hits the ground.

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