Why does some rain smell?

19 May 2015

Question

As it has been so sunny recently, we will inevitably have rain to ruin this lovely sunshine. What I have been wondering is why does some rain have a certain smell and some rain doesn't, and what is it that is giving the rain its smell?

Answer

We put this question to Chris Smith, and he had this to say:

Chris - It's got this fancy name petrichor because petros in Greek means rocky and chor of course means smells. The bottom line here is that no one knows exactly what it is, but a lady in the 1970s, her name is Nancy Gerber, she actually did some chemical experiments and made some measurements. She found there were several chemicals. Some of which coincide with a soil bug called actinomycetes. These filamentous bugs live in the soil and grow throughout the soil. They break down various detritus and things that's in the soil and a product of their metabolism, a bit like sort microbial pooh if you like, are these chemicals. When the raindrops come down, they splat into the ground and they elevate particles of these microorganisms and their by-products and their metabolic waste into the air and it happens to have this kind of smell. One name that's given to it is geosmin for one of the chemicals that's up there. it's nice, isn't it? But interestingly, there was another paper that came out from MIT earlier this year. Caulin Buwy and his colleagues actually said, "Well, it's all very well, raindrops splatter on the ground and they release these particles from the ground. But how do they do it? That was their question. They did some beautiful photography where they used a very fast camera of raindrops splatting down against the ground. If you look carefully, very, very detailed picture of these raindrops landing, what you see are almost like bubbles of champagne coming up through a champagne glass, coming up in the raindrop. What they speculate is that as the drop hits the ground, it compresses and captures a little bleb of particulate matter and air - some bubbles - as it lands. And that stuff then has to come bubbling up through the liquid. As it does so, it carries with it some of these particles which are in the soil and they get aerosolised or distributed out into the air and then you can smell them.

Kat - But this doesn't happen all the time. so presumably, it's when the conditions are just at the right weight of rain and the right kind of soil.

Chris - If you've had relentless rain day after, after day, after day, then it's damped down all the particles probably anyway. They've all been washed away or everything that's been thrown up into the air is there and you're already used to the smell because there's also this whole question if you adapt to the presence of a smell. You must've noticed you go around someone's house, you think, "God! It's a bit whiffy in here" especially around your house. After about 5 minutes, you start noticing the smell and that's because you have adapted. Your nose as learned that that smell is always there. so, when you notice this most is when it hasn't been present. So if you had a long profound pronounced dry spell, lots of the stuff is in dry soil, easy to release into the air, comes down, splat up into the air, you breath it and then you really notice. And that's what we think geosmin is.

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