What really causes that amazing "after the rain" smell?S

After a rainstorm, especially a rain storm that breaks a long dry spell, the world smells different. What causes that strange, clean, earthy smell? A few things that aren't so clean.

One of the benefits of living in California is the extraordinary smell you get after the first rain of the fall. Generally it's in October or November, in a climate where warm summer rainstorms are unheard of and nothing has seen moisture in half a year. Right after the rain there's an intense smell that you have to huff while you can. It's incredibly strong. But we're not the only place that gets such a smell. People everywhere notice it, and scientists have a few theories as to what causes it.

Some of the smell, especially the parts that people identify as "clean," probably come from ozone. Oxygen atoms usually travels in pairs. Ozone consists of three oxygen atoms linked together. It's naturally created in the upper atmosphere, where the energy from ultraviolet light splits oxygen pairs in two. Each of the newly-single oxygen atoms is then taken up by oxygen pairs to form an ozone threesome. The same process can happen around waterfalls and after lightning strikes.

Ozone is sometimes used to kill off odors, especially smoky and moldy odors. The clean scent after a rainfall is partially caused by ozone cleaning away some of the scents we take for granted. Ozone also has a scent all its own. Some say it's like geraniums, but others compare it to a light bleach scent. Few sniff it for more than a moment because it does terrible damage to the lungs, splitting open cells and causing them to leak enzymes. A little ozone after the rain, however, does little damage for all the pleasure it brings.

The other main scent of a rainstorm is a sort of rich earthy smell, like freshly turned-over dirt. Actually, it's exactly like freshly turned-over dirt.

In pretty much any soil on Earth you'll find actinomycetes. These are bacteria that tend to grow in rich, wet dirt. They're beneficial both to the dirt and to us. Scientists have isolated antibiotics from dirt, and found that they're produced by these actinomycetes. When the soil dries out, the bacteria put out spores that can survive in the dry dirt. And then when it rains, the rain dampens the soil and kicks up the spores, so the scent is the of the bacteria in the air and dirt, going to work again.

Image: Leon Brooks

[Via NPR, The Weather Guys, NOAA]