Slightly salty rain

Q: When the sun draws water from the ocean how come we don't get salt-water rain? Elliot, Swansea, South Carolina

A: The Sun draws water from the ocean only in the sense of heating the water enough to evaporate it. The water molecules near the sea’s surface bop around. As the Sun heats the water; the water molecules jiggle more. That’s what heating water means — increasing the average speed of the molecules.

The Sun evaporates seawater and leaves the salt behind.  Photo courtesy of John Bortniak, NOAA

Seawater also contains dissolved salt (sodium chloride), which is a collection of electrically charged particles called ions. Sodium ions have a positive and chloride ions a negative charge. Water molecules, coincidentally, also have slight end charges — plus on one end and minus on the other. (The overall charges of a molecule balance but the positive-charge and negative-charge centers don’t coincide. This permanent mismatch charges the ends.)

Salt ions drift around in the water and attract the oppositely-charged ends of water molecules. So, electrical bonds lightly hold salt ions to water molecules.

The Sun heats the water molecules, they jiggle faster and kick nearby water molecules. The extra kick energy breaks the electrical bonds binding the water molecules with salt ions, and careens the water molecules into the air. Almost all the salt ions stay behind.

Salt also evaporates but at a much slower rate — negligible at sea temperatures. So essentially no salt accompanies the water as it evaporates into the air.

Salt, however, does enter the atmosphere, just not through evaporation. Instead, it comes in as tiny salty water droplets — for example, via "bubbles formed by breaking waves," says Craig Bohren, author of Clouds in a Glass of Beer. "These droplets can evaporate, leaving behind small salt grains."

So, when it rains, it rains salty water. Not very salty — not enough to taste, but still salty. In fact, "cloud droplets form by condensation of water vapor on small soluble particles, often salt (sodium chloride)," says Bohren. "These droplets can coalesce to form much larger rain drops.

Of course, much more junk makes its way into our atmosphere and rains out — "disintegrating micrometeorites, industrial pollution, dust, and an occasional small frog or fish that gets picked up by a particularly energetic storm," says Bob Harbort of Southern Polytechnic State University

Further Reading:

Craig Bohren. Clouds in a glass of beer. New York: John Wiley & Sons, Inc, 1987.

ESPERE, Environmental Science Published for Everyone Round the Earth: Clouds and particles by Justine Gourdeau

Plymouth State College: How rain forms by Derek W. Brown

(Answered May 20, 2005; updated Sep. 9, 2007)

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