If you’ve ever strolled around outside after a storm, you’ve probably noticed a… distinctive smell. This aroma, known as petrichor, is often associated with spring, just like we frequently associate the smell of fresh-cut grass with summer. So what’s going on here? I mean, water itself doesn’t have this smell, so why does rain? As it turns out, there’s more than one kind of ‘rain smell’, and a number of factors that cause it.
One smell comes before the rain even hits the ground. It’s slightly sweet, sharp and (according to some people) reminiscent of chlorine. That’s ozone. During a storm, lightning can split oxygen and nitrogen molecules. These can recombine into nitric oxide, and that oxide can react with other atmospheric chemicals to form ozone. The wind from an approaching storm can carry this ozone scent from the clouds straight to your nose. But that’s not the only aroma in the mix.
One of the more pleasant rain smells is actually caused by bacteria. Yep. A type of bacteria called actinomycetes grows in soil when conditions are damp and wet. When the soil dries, this bacteria produces spores. The wetness and force of rain kicks these tiny spores into the air. The moisture after rainfall acts kind of like an aerosol air freshener, because this moist air can easily carry the spores, we breathe them in and boom – we sense that distinctive, ‘earthy’ scent so often associated with everything from a drizzle to a storm. This bacteria is extremely common, and people across the world will recognize the aroma. Because the spores release when soil dries out, this scent is strongest after rain that breaks a dry smell.
These are some of the most common after-rain smells, but there are all sorts of other scents that could come into play. This is why when you and a friend talk about the smell of rain, you might be describing a few completely different things.