Why Rain Isn't Salty
Dear Tom,
A lot of Chicago's moisture comes from the saltwater Gulf of Mexico, so why isn't our
rain salty?
Bob Johnson, Oak Park
Dear Bob,
Since the beginning of time, the saltwater oceans have been the reservoir of about 97
percent of this planet's water supply. The water is constantly being recycled,
evaporating into the atmosphere and returning to Earth as rain or snow in an ongoing
hydrologic cycle. Sea water is indeed salty, with about 35,000 parts of salt per million
parts of water. However, the salt in sea water is only dissolved in it and not chemically
bonded, so it is left behind when the water evaporates, like when a pot of saltwater is
boiled dry on a stove. That is the reason sea air is salty when the ocean's spray
evaporates, and also why precipitation falls as fresh water. When salty ocean water
surges inland as in a hurricane storm surge, it can kill vegetation.