Have you ever noticed a distinctive smell before it has rained?

If you have, it is not the incoming rain that has the earthy scent but the moistening ground due to a smell called petrichor. It is fragrant compound made of oils from plants and actinobacteria which are tiny micro-organisms.

Actinobacteria help the decomposition of dead or decaying matter which then becomes nutrients for plants. The combination of plant and actinobacteria activity creates the compound geosmin which contributes to the petrichor smell.

With no rain, the productivity of the actinobacteria declines. Just before rainfall, the air becomes humid and actinobacteria productivity increases moistening the ground and creating geosmin.

When rain falls, the raindrops splatter on the ground ejecting tiny aerosols. The petrichor elements including geosmin which have dissolved in the raindrops are then released and carried by the wind, and this is what we smell.