Why is it that when salt water evaporates from the ocean, it comes down as fresh water in rain?

Dear Tom,
Why is it that when salt water evaporates from the ocean, it comes down as fresh water in rain?

Chuck Mieko,
Round Lake Beach

Dear Chuck,
The Earth’s water is constantly on the move, evaporating into the atmosphere and then returning as rain or snow. It’s an ongoing process known as the hydrologic cycle. Salt in seawater is merely dissolved in the water, not chemically bonded to it. When water evaporates (one molecule at a time) only pure water returns to the atmosphere. Salt (and other impurities) is left behind. Seaside residents know that air blowing in from the ocean contains salt particles — their rusting automobiles attest to it — and, surprisingly, it’s for the same reason that the oceans are salty. When airborne droplets of salty ocean spray evaporate, their minute loads of salt are left floating in the air. So, the answer to your question is simple: only pure water evaporates.