I happen to come from the Pacific Northwest, a region of the world that combines a long and elaborate coastline with countless rivers, lakes, and of course the infamous rain. We fish at every depth of water there is, make our power with hydroelectric dams, export drinking water to less glacial parts of the world — we know wet. And I’m telling you, there’s is something about the smell of the first rain after a dry spell; it’s a musty scent that bears no resemblance to the smell of a long-term torrential downpour. Is this smell carried down to us by the clouds? Kicked up from the ground? A new study using high-speed cameras to capture the impacts of raindrops on the ground shows that it is almost the latter.
The technical term for this smell is “petrichor,” and it turns out that it is caused when raindrops actually aerosolize as they bounce off a porous surface — the micro-scale pits actually end up with air dropped inside them as the drop comes down from above. As the tiny pockets of air become compressed, they eventually burst upward due to the pressure and bubble up through the puddled raindrop, carrying small amounts of the raindrop with them. This aerosol effect can create droplet-carrying particles of soil or rock, bacteria, or even viruses from the ground and carry them up on bubbles of air, like champagne molecules entering our nose on bubbles of rapidly expanding CO2.
