The fresh pleasant smell of rain comes from a mixture of plant oils, bacterial spores, and ozone. Scientists believe we might have inherited our affection for rain smell from our ancestors.
It’s a hot summer day. The heat is making you feel sweaty and cranky. Suddenly the sky is covered with dense grey clouds. Big fat raindrops start plopping onto the ground.
then it slowly hits your that fresh, earthy rain smell. The smell of rain actually has its own name. It is called Petrichor. But rain is just water and water doesn’t have any smell. So what makes rain smell so nice? Well, it turns out there is more than one kind of rain smell and a number of factors that can cause them.
Once the smell comes before the rain even hits the ground. It’s slightly sweet but also a little sharp. That’s ozone, NOt the layer in the atmosphere, but the isn’t far off!
You see, before a thunderstorm rolls in, lightning can sometimes rip nitrogen and oxygen molecules in the environment to pieces which in turn can recombine into nitric oxide.
This substance interacts with other chemicals in the atmosphere to form ozone which has a sharp smell. The wind then carries down this ozone down to the ground level where it reaches your nose!
One of the more pleasant rain smells is caused by bacteria known as Actinomycetes. These bacteria grow in soil when conditions are damp and warm. When the soil dries out, the bacteria produce spores in the soil.
When the raindrops hit the ground, these spores are kicked up and they become airborne. The moist air easily carries the spores to us so we breathe them in. But the smell isn’t actually caused by the spores themselves. Rather it’s caused by a chemical excreted during the production of the spores known as geosmin. This chemical is responsible for the sweet smell after the rain.
The human nose is extremely sensitive to geosmin. Some people can detect geosmin at concentrations as low as 5 parts per trillion.
Since the bacteria thrives in moist soil but releases the spores once the soil dries out, the smell is most acute after a rain that follows a dry spell, although you’ll notice it to some degree after most rainstorms. Interestingly, more aerosol is released in light fain than during a heavy downpour.
Anther pleasant after-the-rain smell comes from volatile oils that various plants and trees release. These oils collect in the environment and, when it rains certain chemicals that make up the oils get released into the atmosphere (usually along with geosmin) causing a familiar and inviting scent.
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So while the science behind the sweet smell of petrichor isn’t all that romantic it is pretty cool! Who’d have guessed we’re actually smelling bacterial secretions and lightning.