Rain cleans the sky

Why does the sky appear clear after it rains?  Shylaja, Bangalore, India

Rain cloud over Swifts Creek, Victoria Australia. Photo courtesy of Peter (aka Fir0002) and Wikipedia.

Rain cleans the sky of pollutants.  That's why the sky appears clear after a rain.

Each cloud droplet forms on a particle, ridding air of one particle.  Droplets within a cloud collect more particles.

"It takes about a million cloud droplets to form a raindrop," emails physicist and meteorologist Craig Bohren, distinguished professor emeritus at the Pennsylvania State University.

So each raindrop cleans more than a million particles from the air.  Furthermore, raindrops are big enough to reach the ground, before they completely evaporate, and "hence, they transport particles downward," says Bohren.  On the way down, the drops collect and remove even more refuse from the sky.  Rain washes the sky clean.

Further Reading

Clouds in a glass of beer by Craig Bohren

The unclean sky by Louis J. Battan

Readers' Comments

• I have a question for you. As a child we used to make a concoction called Snow Cream - snow cream sugar. Folks wouldn't let us bring in snow till at least 3" had fallen and then only top inch as said that was how much it took to clean out the sky. Know that they were on to something as understand about particles and precipitation. Question: is there a point when snow becomes pollution free?
  
  ShenValleyFlyFish, Greenwood, Virginia
   
• If the rain isn't particularly heavy, the evidence of the cleansing process will show up all over the deck of a boat. From time to time, when the wildlife refuges or rice fields are being burned, or when the pollution from Mexico heads NE on a strong wind, a drizzle or fine rain will precipitate out the "solids", and leave a fine, soot-like residue over everything. It's amazing how much glop does get washed out of the air!
  
  ShoreAcres, League City, Texas
   
• The same idea of water particles to precipitate out pollutants is used in industrial settings for acidic and caustic fumes. The fumes are sucked through a "scrubber" full of pH balanced water droplets that neutralize the fumes and take some of the nasties out before exhausting to atmosphere.
  
  GardenGrrl, Lewisville, Texas
   
• Reply:  Raindrops clean the sky of particles by washing them from the sky. But that's incidental. As far as the raindrop is concerned, particles are seeds that foster growth by electrical attraction.
  
  Water molecules are partly electrically polarized. There's a slightly negative and a slightly positive end. The particles (aerosols, microscopic dust, salt or mineral specks) also have a slight charge, which attracts and holds water molecules.
  
  As the water vapor in a cloud cools, the water molecules slow down. As a water molecule slows, a particle 'seed' grabs it. Voila! A droplet is born.
  
  That's a start. Then more and more water molecules link (through their hydrogen bonds) to the growing droplet. The average size of a cloud droplet is about 100 times that of its seed. Many layers of water molecules pile on and bond to form a cloud droplet.
   

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