When you step outside after a big rainstorm and take a deep whiff of that fresh, earthy smell, you're mostly smelling a chemical called geosmin.
It's a byproduct of bacteria and fungi. And something about rain lofts the chemical — and sometimes the organisms themselves — into the air, a process that not only helps release that earthy smell but may, in very rare conditions, spread diseases.
Somehow raindrops launch tiny living things off the ground.
Wind can pick bacteria up from the ground and bring into the sky, sometimes settling in clouds thousands of feet above us. Sea spray can also send bacteria flying. The mystery has been, how does rain do it?
Now, mechanical engineers writing Wednesday in the journal Nature Communications say they've figured it out. It has to do with tiny bubbles.
Using high-speed cameras and fluorescent dye, the researchers filmed drops of water as they fell on different types of soil infused with bacteria. They watched as the drops delicately catapulted the microbes into the air.
The white lines are the trajectories of tiny water droplets ejected when a raindrop hits the ground. Airflow in the room pushes the droplets off to the side.
When a raindrop hits the ground at the right speed, it traps air bubbles beneath it, each one no wider than a human hair.
Like an air bubble stuck under a container at the bottom of a pool, those bubbles then rise, bursting when they reach the surface.
As the bubbles burst, they send a spray of tiny water jets into the air, sometimes carrying bacteria with them.
"It's very similar to what happens if you pour a glass of champagne or any carbonated beverage and you see that fizz on top," says Cullen Buie, a mechanical engineer at the Massachusetts Institute of Technology and an author on the paper. In that case, the popping bubbles are full of carbon dioxide; in this case, they're full of air.
In a few microseconds, Buie and his colleagues found, a single raindrop can create hundreds of tiny airborne droplets, each one carrying as many as several thousand live bacteria.
It's as if the ground has gently sneezed.
The more bubbles there are, the more chances there are for bacteria to get launched into the air.
The soaring microbes can survive inside a tiny water droplet for at least an hour.
"The next step is studying how far they can potentially travel," says Cullen, who estimates that bacteria could travel for miles, if the winds are right and they don't get dragged down by other raindrops.
The three kinds of soil bacteria involved in this study — Corynebacterium glutamicum, Pseudomonas syringae, Bacillus subtilis — are harmless. But pathogens can also get a lift from water droplets.
Cullen and his colleagues got started on this study after a British scientist contacted them after noticing an uptick in a rare disease called melioidosis during the rainy season in parts of Southeast Asia and northern Australia. The lung infection is caused by the soil-dwelling bacterium Burkholderia pseudomallei.
Cullen's results suggest rain could be a reason for that uptick. He and his colleagues varied the speed of the falling raindrops and the type and temperature of the soil. They found that the Goldilocks conditions for producing spray loaded with bacteria is exactly what you'd find in certain tropical places — sandy clay soils, a soil temperature of 86 degrees Fahrenheit and droplets falling at about 3 miles an hour.
"I don't want people thinking they shouldn't go outside after it rains because they might get some bacterial infection. I definitely don't want people thinking that," says Cullen.
Melioidosis, he says, is very rare. And, if rain really is the culprit behind a small rise in cases, that would only be the case in the rare locations with exactly the right conditions — and with the disease-causing bacterium living in the soil.
"Bacteria, unfortunately, get a bad rap," says Buie. Most, he says, are harmless or even beneficial. And sometimes they even smell good.