Welcome to You Ask Andy

Danny Boher, age 13, of Charlotte, NC., for his question:

Why isn't rain water salty?

This is a very logical question. Clouds are formed from moisture that evaporates from the earth, and most of the surface of our planet is covered by the salty seas. At some stage in the weathery water cyc1e the salt is sifted out and the rain that falls is the earth's freshest water.

The water cyc1e is powered by the beaming sun. As the earth rotates, its warm rays evaporate moisture from every watery surface. The moisture becomes gaseous vapor that condenses into misty cloud droplets. The cloudy moisture condenses and precipitates rain and snow, hail and sleet upon the land and sea. Life on earth depends on this precipitation, and it is adapted to this supply of fresh, salt free water. If the clouds suddenly started dousing salty water, most of the earth's plants and the animals that depend upon them would perish.

Every year some 350 billion tons of rain water rain down around the world, and some 80,000 cubic miles of moisture are evaporated from the sea to contribute to this precipitation. A cubic mile of sea water would fill a tank a mile high and a mile wide. Some parts of the sea are saltier than others, but an average cubic mile of ocean water contains 166 million tons of dissolved salts.

You would expect 166 million tons of salty chemicals to fall with every cubic mile of water evaporated from the sea. This does not happen because of the nature of heat evaporation and the water molecule. This frisky particle of hydrogen and oxygen is so small, that we need 18 billion of them to measure one inch. It gains the frisky energy to change from liquid water to gaseous vapor from the heat of the sun.

Water molecules attract each other and in liquid water this attraction links them together in a sort of follow the leader game. The sun's heat gives them energy to move faster. Countless molecules on the surface of the sea get up enough speed energy to break the attraction that holds them together. They becoane separate particles and break away to mingle with other gaseous particles of the air, but the sun does not provide enough heat to change the dissolved chemicals into gases. The salts stay behind in the ocean, and the evaporated sea water falls as fresh rain.

The chemicals in the sea are dissolved from the land by the rivers and dumped into the ocean, trace by trace. Since the evaporating water leaves its chemicals behind, the sea becomes saltier every day, and its waters contain dissolved metals and other chemicals besides salts. A cubic mile of the ocean contains about 38 tons of gold. If this evaporated with the water, we could expect golden showers to fall upon us from the clouds.


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