The “clean” smell is thought to be due to ozone, which rainstorm downdrafts can pull down to earth. Another part of the smell comes from geosmin, a chemical compound that actinomycete bacteria produce as they form spores in soil.
“Rain liberates the spores and geosmin, creating the familiar ‘first‑rain‑after‑drought’ smell, most noticeable in warm seasons,” says Stevenson. This might explain why humans are so sensitive to it – more sensitive than sharks are to blood. Scientists hypothesize that we evolved to understand that petrichor signifies the renewed abundance of fresh water, which likely helped our ancestors feel safe and calm.
These smells have been shown to yield distinct changes in alpha and beta wave activity in the brain that are linked to a calmer, more relaxed state.
And thanks to its unique fragrance and how dramatically it changes the environment, rain can also be a powerful nostalgia trigger. My camp thunderstorm experience happened over 20 years ago, yet every time it rains, my mind paints the picture of that day with remarkable clarity.
“A sensory experience like the smell of approaching rain or the aftermath can become the backdrop or context that becomes attached to our memories of many different places or emotions,” says Dalton, who has extensively studied the psychological significance of smell.
Any smell, she says, can activate the amygdala, the part of the brain responsible for processing emotions and emotionally significant memories. That tie to our emotional epicentre is why memories associated with smells tend to stick in the brain and remain vivid. So whether you perceive a smell like rain as good or bad doesn’t matter; the context within which you experience that smell is what makes it evocative.
So take a whiff the next time it rains from an open window or during a post-rain stroll and see what details of long-past moments surface.
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