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Rain is a symbol of growth and cleansing. No wonder we enjoy the earthy smell it brings! But why does rain water smell like that?

Picture it: You’re back in primitive times and the water supply is running short. The stream that you always relied on for water has dried up. Now you have to run further to find some. Even the vegetation has started turning yellow. Then, a few hours later, clouds roll in and a cool breeze sweeps over the landscape. You feel relieved just by the smell that the breeze brings. What is it? It’s the smell of rain!

 

One of the reasons why rain smells so good to us today is that way back when (when indoor plumbing wasn’t even thought of- don’t take it for granted!), our ancestor’s survival relied heavily on rainfall. Without it, eating and drinking got tricky. In fact, studies have shown that tribal regions of the world, mainly in Africa and Australia, link the color green with rain. Green symbolizes growth, something that is vital for these tribes to continue surviving. This blend of an input from the senses and a part of how a society functions is called Cultural Synthesesia and it’s why even today in a first world country, we find the smell of rain to be attractive.

The smell of rain is made from a series of chemical reactions. Rain itself is just water. There is no smell. But when you combine water and rocks, or air and lightning, certain aromas are produced.

The most common of these smells is Petrichor which was discovered on March 7, 1964 by two Australian scientists, Joy Bear and Richard Thomas.  Rocks have microscopic holes in them that fill with a yellowish oil, which is known as the “blood of the rock,” (Greek: Petra – stone, and ichor-heavenly blood). When it rains, even very lightly, the moisture gets into those holes and forces the oil out of the rock. When there is a hard rain, the force of the water causes the oil to fly into the air, almost like an aerosol! The breeze then catches the oil where it hits the receptors in our nose.

Another distinctive scent is Geosmin. This is caused by bacteria that live in the soil, called actinomycetes, when they release their spores into the ground. The bacteria only release spores during dry spells to ensure their survival, so when it finally rains after a long dry spell, the smell of the Geosmin becomes more pronounced as it builds up over time. But it doesn’t have to be raining for us to smell this. Every time we dig around in the garden we smell this. It is simply the actinomycetes going about their life.

When it does, air trapped in the soil gets forced out by the water and spread into the air that we breathe. This can explain why soil microbes have been found high in the atmosphere; the rain has forced the spores of the bacteria out of the ground and into the air during rain, where they are light enough to be carried far away and high up by the wind!

Not inherently caused by the rain is the smell of Ozone (O3). While we associate this with rain, it is really caused by lightning splitting oxygen and nitrogen molecules in the air. When the molecules recombine, they often form Nitric Oxide (NO) which interacts with other chemicals that naturally occur in the air to form the sharp smell of ozone. This smell can be carried by the wind from far off storms, hence being able to smell a storm before it comes.

Next time it rains, try to catch a whiff of these smells on the breeze and compare it to the smells in your garden!

Here are some interesting links that explain more of the science behind the chemical reactions that make these smells.

Actinomycetes and Prokaryotes

Soil Microbes in the Atmosphere

Actinomycetes are bacteria, yet they have a function in our soil!

The Splitting of Oxygen and Nitrogen to form Ozone

BCAS Fun Facts – Explaining Ozone formation with pictures

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