Storm Scents: It's True, You Can Smell Oncoming Summer Rain

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http://www.scientificamerican.com/article.cfm?id=storm-scents-smell-rain

When people say they can smell a storm coming, they're right. Weather patterns produce distinctive odors that sensitive noses sniff out. This year's peculiar weather patterns—such as drought in the Midwest and a "super derecho" of thunderstorms earlier this summer—are no exception. In fact, as the rains return after a dryspell, many of these odors are stronger than ever. So, what are the scents of a summer storm? Here's a breakdown of three common odors:

—Oh, ozone
Before the rain begins, one of the first odors you may notice as winds pick up and clouds roll in is a sweet, pungent zing in your nostrils. That's the sharp, fresh aroma of ozone—a form of oxygen whose name comes from the Greek word ozein (to smell). Tropospheric chemist Louisa Emmons at the National Center for Atmospheric Research explains that ozone emanates from fertilizers and pollutants as well as natural sources. An electrical charge—from lightning or a man-made source such as an electrical generator—splits atmospheric nitrogen and oxygen molecules into separate atoms. Some of these recombine into nitric oxide, and this in turn reacts with other atmospheric chemicals, occasionally producing a molecule made up of three oxygen atoms—ozone, or O3. (Most atmospheric oxygen is made up of two atoms—O2.) The scent of ozone heralds stormy weather because a thunderstorm's downdrafts carry O3 from higher altitudes to nose level.

—Petrichor potpourri
Once the rains arrive, other odors come with them. Falling water disturbs and displaces odoriferous molecules on surfaces, particularly on dry ones, and carry them into the air. If you happen to be near vegetation, these molecules may come from plants and trees. For city dwellers, these odorants rise up from concrete and asphalt. Some are fragrant, others foul.