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It rained. In May. Just goes to show how intense climate change has become. These unexpected showers in Mumbai were caused by unusually strong western disturbances.
But that is not what we are here to talk about.
Yesterday, after the rain and wild winds had passed, my family and I went out for a long drive. The weather was unusually pleasant, so I asked if we could roll the windows down. The moment we did, a strong earthy scent of herbs and plants rushed in. It felt like my olfactory nerves had just been treated to a five star experience.
“I do not remember it smelling this strong after the later rains,” I said aloud.
And that got me thinking. What is the science behind this?
This phenomenon is called petrichor.
So what causes it? Let’s break it down.
1) Plants and soil prepare ahead of time
(Maybe take notes from them. Just saying.)
During dry spells, plants secrete oils into the soil. These oils remain there, waiting for rain to break them apart. Not a study strategy you should adopt, though. Go study, please.
At the same time, a group of soil dwelling bacteria called actinomycetes (very hardworking, by the way) produces geosmin. This compound is responsible for the earthy smell we associate with rain. It might even be the inspiration behind those fancy petrichor themed dishes. Yes, I am still thinking about food.
2) Cue the rainfall
When the first rain hits the ground, it breaks apart those oils and geosmin molecules, sending them into the air. That is when the earthy scent hits your nose.
The ozone layer’s role
Sometimes, especially during thunderstorms, the smell of rain has a slightly metallic or sharp tang. This happens because ozone is carried down from the atmosphere by wind before or during the rain. Yes, this is the same gas that protects us from the sun. And no, this is not an excuse to skip sunscreen.
Why only the first rain?
You mostly notice petrichor during the first rain because that is when the soil is full of dry weather oils and geosmin. Later rains do not have the same dramatic effect. It is like showing up as the third wheel to your own friend group.
Third wheeling or not, all rains are equally important. And now you know the science behind that amazing earthy smell that fills the air after a downpour. The next time you roll down your window after the rain, take a moment to appreciate not just the fresh air, but also the small wonders of nature at work.
