High-speed video shows us why rain smells

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.

The fact that even whole bacteria can catch a ride on the wind in this way could explain why rainfall has long been associated with the spread of certain diseases — though it’s assumed that rain was physically throwing infectious agents into the air, this is the first direct evidence in favor of such a mechanism. It will be interesting to see if scientists now identify any species of microorganism that is tailored specifically to the rain-distribution strategy, perhaps settling into cracks and increasing surface area so they can be dragged along as readily as possible.
There’s a point of diminishing returns on the violence of an impact with the ground, confirming the long-known principle that a light rain produces a stronger smell than a heavy one; lighter impacts tend to allow more aerosolizing. Additionally, a heavy rain probably has stronger winds and a generally less favorable environment for letting tiny micro-bubbles of water hang around. A light rain, on the other hand, leaves the scent of Earth hanging in the air and a quiet feeling that fits with the weather.
This could be conditioning on my part, or some sort of inborn simian association with the smell. Rain has certainly been associated with life ever since the invention of agriculture, and it makes sense that it’s pleasant to have a stronger connection to the world around you; getting so many more smellable molecules into the air basically amps up the sense

Earlier work had identified a particular oil excreted by plants during dry seasons as the source of the petrichor smell — and it may be largely responsible. The distinctive, Earthy smell is likely due mostly to aromatic compounds released by plants, this one and others, and it does seem to be strongest early in the rainy season. Waterlogged ground would produce fewer air bubbles and thus less bubbling-up to create particles in the air, tending to create splashes of large water globs that jump a small height and quickly fall back to the ground.

source: geek