The Sweet Smell of Rain

In this week's Science out of the Box, Debbie Elliott gets answers to the question: Why does the air smell so good after it rains? It has to do with ozone and soil bacteria.

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DEBBIE ELLIOTT, host:

This is ALL THINGS CONSIDERED from NPR News. I'm Debbie Elliot.

(Soundbite of thunder and rain)

ELLIOTT: Few things smell better than the air after a good, strong rain. And this week in Science Out of the Box, we've decided to find out why.

(Soundbite of music)

ELLIOTT: We called up Dr. Charles Wysocki at Philadelphia's Monell Chemical Senses Center and asked him what is so delicious about the smell of rain.

Dr. CHARLES WYSOCKI (Member, Monell Chemical Senses Center): Oh, there are many reasons for that. After a rainstorm, there's plenty of disruption in the environment - all of the rain hitting the ground, the leaves, the environment in general. Also, there are usually some strong winds that are present during storms that just mix up the environment and generally bring a lot of smells to the nose of the appreciating person. For example, if it was a thunderstorm, there's plenty of ozone that's generated. Ozone has a pleasant smell for many people.

ELLIOTT: What does ozone actually smell like?

Dr. WYSOCKI: Oh, that's a difficult one because everyone's nose is so personal. To me, the fresh smell of a storm reminds me very much of my childhood days growing up in the countryside in Upstate New York. And my mother bringing in the fresh laundry as it dried on the line and I would just thrust my face into the laundry basket. And to me, the smell of those freshly dried clothes is so much like the smell of a thunderstorm just passing through.

ELLIOTT: Now, there are a lot of companies that have tried to bottle that smell and capture that scents that you have, making laundry detergents, hair sprays, all kinds of things that try to smell like rain, but none of them can quite capture the scent. Why is that so much harder to do, say, than reproducing the smell of lemon or apple?

Dr. WYSOCKI: It's very difficult to mimic exactly all of the minute ingredients that are present in a lot of the naturally occurring odors. Some of them are so low that they go undetected by the instrumentation that a lot of the perfumers and chemists used. And for most the part, we know what some of the ingredients are in some of the smells that are generated during storms, especially the ones that might come from the soils.

ELLIOTT: One of those ingredients turns out to be bacteria with a pretty unpronounceable name - actinomycetes. These bacteria flourish in damp, warm soils.

Mr. BILL YPSILANTIS (Soil Condition and Health Specialist, Bureau of Land Management): And they actually play a very important role in decomposition of organic materials and that's how soils get a lot of their nutrients.

ELLIOTT: That's Bill Ypsilantis, a soil condition and health specialist at the Bureau of Land Management.

Mr. YPSILANTIS: So when the soil now dries out, these actinomycetes produce force. And then when it rains, the force of rain kicks up these tiny spores in the air, and it sends some really - a very pleasant aroma. It's sort of a musky smell. You'll also smell that when you are in your garden and you're turning over your soil. And that's a good indication because that tells you, you have good organic material in the soil. The soil is not too wet. It's not too dry.

ELLIOTT: Actinomycetes bacteria mean healthy soil and healthy people. Researchers have isolated several antibiotics from ordinary garden dirt. Something to think about the next time you take a stroll after a rainstorm.

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