Ask Mr. WeatherBlogger: Why isn't the rain salty?
Dorothy Crumb writes from upstate New York to ask:
Can you tell me where the rain comes from in a hurricane? If it picks it up from the ocean, why isn't it salty? Thanks, Dorothy W. Crumb, Pompey, New York (Near Syracuse)
Dear Ms. Crumb,
The answer is that the rain does indeed come from the ocean. But as the seawater evaporates under the hot tropical sun, and moves up into the atmosphere as water vapor, it leaves its salts behind.
It's just like distilling water by boiling it, capturing the steam and condensing it again as a liquid. The process leaves most everything that isn't water behind. And the water you condense and capture is fresh.
That's how people have obtained salt for centuries - by evaporating seawater and scraping up all the salt that's left behind when the water is gone.
So, the tropical sun heats the ocean, turning some of the seawater into water vapor and separating it from its salts. The water vapor rises inside the hurricane's thunderstorms, cools and condenses, and falls again as rain - freshwater rain.
Thanks for asking.
For more than you ever wanted to know about evaporation's role in the water cycle, click here.