Streptomyces Colony

The smell of rain

Ever wondered why theres a distinct smell to rain?

That musky, dirt smell after a quick spring rainstorm..
This is..

Why rain smells like rain:

Most people will notice that after a rainstorm, the air smells musky, some may like this smell, some may hate it. This smell is so common that it has been attributed to rain, as most people actually call it the smell of rain. I’m here to explain why this smell occurs.

Theres a special name for this mixture of smells, called petrichor (coined in 1964 by two Australian researchers) . Several things cause this kind of smell. Here are the 4 most common culprits:

Culprit #1: Actinobacteria

Actinobacteria makes up for a high percentage of life within the topsoil. These bacteria grow rapidly during moist soil conditions, and release spores into the soil when its dry. Rainfall disturbs the soil, distributing some of these spores into the air, the spores contain the distinct, rainy smell. It has been shown that certain soil bacteria do have positive psychoactive benefits when handled. Mycobacterium vaccae, may increase our serotonin when we handle soil.
This is the ‘good feeling’ you get after it rains, serotonin levels is shown to dictate mood in humans.

Culprit #2: Geosmin

Literal translation of geosmin, an organic molecule is earth smell. It is produced by cyanobacteria and actinobacteria, and is released when these microbes die. The ground is filled with this chemical and the human nose is extremely sensitive to it. Rain disturbs the soil enough to release this compound, the same effect is achieved by spraying tap water at the ground.
Chemical structure- Source wikipedia
Culprit #3: Ozone
Not generally associated with rain, but after a heavy thunderstorm, the air sometimes smells slightly metallic and/or electric. That is ozone, produced by lightning, it lingers within the lower atmosphere after a thunderstorm. Ozone is generated naturally by short-wave solar ultraviolet radiation and appears in our upper atmosphere, and can also be generated by a electrical discharge through oxygen molecules, dissociating an oxygen molecule and having the ions attach each to molecular oxygen, forming O3 (g).


Culprit #4: Various oils from vegetation. 

These oils are absorbed by neighbouring surfaces and later released when coming in contact with water, it only makes up a small part of the rainy smell. Many plants, including grasses secret certain kinds of aromatic oils.

 Culprit 5: Acidity of rain and limestone
Ever smelled a wet rock? Especially a sedimentary rock, it has a smell similar to the smell of rain. This is partly because of geosmin on the surface of the rock but also because of different chemical reactions of slightly acidic precipitation and the mineral contents of the rocks themselves.


Author of  Scientific Madness Blog