Petrichor: What is that earthy smell of rain hitting dry ground after weeks of sun?

What’s your favourite smell of summer?

Is it the lovely smell of suntan lotion on your skin, the smoky aroma of a sizzling BBQ, the fragrance of blooming roses in the garden, the chlorine of a holiday swimming pool – or that earthy smell of rain hitting dry ground after weeks of sunshine… petrichor?

The first smell of our summer holiday

Many of you will have noticed that distinct holiday smell when you step off a plane – not the jet fuel, but that earthy, warm scent in the air. It’s often a mix of baked-dry ground, unfamiliar pollens, and plants that we don’t typically encounter at home.

I find it especially noticeable when arriving on a late flight, after the heat of the day has eased and the cooler night air carries those scents more clearly. Every so often, we get a hint of that same smell here too – usually after a long, dry spell.

The smell of rain

Petrichor is that earthy smell that fills the air when rain finally returns after a long dry spell. Many of you will have experienced it – but interestingly, not everyone can detect it!

What makes the smell?

After a long spell of dry, sunny weather, the soil becomes baked and parched, creating the perfect conditions for bacteria and spores to build up. When rain finally arrives, the raindrops hit the ground with force, releasing tiny aerosols and spores into the air. One of the key ingredients behind that familiar earthy smell is a compound called geosmin.

Geosmin is produced by bacteria in the soil. Interestingly, while it helps keep fruit flies away from plants, it can attract other small insects and animals that unknowingly help spread it deeper through the soil.

That lovely scent isn’t just from the soil either. As raindrops strike leaves and plants, they release fragrant oils too, adding to the rich smell we associate with rain.

Insight

‘Wettest spell in months’ to hit parts of Scotland after days of sunshine

Sean Batty

Rain brings joy – yes, even in Scotland!

Human noses can detect the tiniest amount of geosmin in the air and it usually fills us with joy – so much so that many perfumiers add it to their fragrances to bring an earthy smell to their products.

But why does it have this effect on us? Well, it’s thought that this euphoric feeling comes from the simple fact that our ancient ancestors associated rain after a dry spell with something good – growing crops and a replenishment of water supplies.

Think about it – they were completely at the mercy of nature so it’s no surprise that after weeks of dry weather, limited water availability, drying soil and dying crops that rain would be looked upon as something joyful and that’s stuck with us.

Camels are also a dab hand when it comes to petrichor because finding areas of water is key to their survival in deserts.

Human v shark

The human nose is so incredibly sensitive to petrichor that we can detect it at just five parts per trillion.

To put that in perspective, think about how sharp a shark’s sense of smell is. In Jaws, the shark could pick up the faintest trace of blood from far away, and in real life, sharks can detect one part of blood in a billion parts of water. Impressive – but still not as finely tuned as our noses are to the smell of rain.

The best-smelling rain

It might seem strange, but too much rain isn’t actually good for petrichor. Heavy downpours soak the soil too quickly, making it harder for those earthy-smelling aerosols to escape into the air.

Lighter rain is much better. As each raindrop hits the ground, it allows tiny bubbles of gas from the soil to rise and burst – just like the bubbles in a fizzy drink. And with every pop on the surface of a raindrop, a little bit of nature’s perfume is released into the air.

How long does petrichor last?

Petrichor is more Eau de Toilette than Parfum with its short-lasting scent. The smell may only last a few hours until the soil has been saturated but may last longer if the rain remains light and there’s little wind around to mix it out of the air