Good Question: Why Is Everything Dirtier After It Rains?

MINNEAPOLIS (WCCO) — You might think you don’t need a car wash after all of the rain we’ve been getting this past week.

Turns out, at least on Tuesday, that’s not the case. That was when WCCO viewers started sending us photos of the dirty cars and patio furniture.

They wanted to know: Why is everything dirtier after it rains?

“I was out and about with my 3-year-old and I came out to the truck after it poured and I’m like, ‘What is all over my truck?'” said Tracy Penn. “Can there be such a thing as dirty rain?”

According to the Minnesota Climatology Office, just before it rains, the wind kicks up any dust that’s on or near the ground. That dust then mixes with the rain.

“It falls with the rain and lands on wherever the water hits, whether it’s a tree, your coat or your car,” said Minnesota State Climatologist Luigi Romolo.

The water then evaporates and leaves behind a grimy mess. Romolo says the particles left behind are pollen, soil, salt, car or airplane pollutants and seedlings.

How much dust is left behind depends on how often and how hard it rains.

“For a longer, harder rain, much of the pollution would get washed out early on, so that the subsequent rainfall would be cleaner,” said Dylan Millet, a professor of atmospheric chemistry at the University of Minnesota.

As for why a car wash will get a car cleaner than a good soaker, Romolo says the water in the car crash is cleaner than rain, and the soap gives the dust something to stick to.

More from Heather Brown
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