It’s that distinctive earthy, musky scent that occurs after a good rain, and it has a name unbefitting its pleasant smell – “petrichor”. The petrichor odor is distinctive (and difficult to duplicate) because there are several sources/causes for the smell, including the force of the rain striking the soil and storm winds stirring everything up (notice that it smells a bit like dirt you’ve turned over in your garden). Soil contains bacteria called actinomycetes that thrive when the environment is damp and warm. When actinomycetes dry out, they produce spores in the soil. When it rains, the soil releases the actinomycetes spores into the air producing that pleasant “after it rains” aroma. The smell is especially pronounced if the soil has been allowed to dry out for a while before the rain.
There are a number of other reasons for the petrichor smell too. Plenty of ozone is generated during a rainstorm, especially if there is lightning, and ozone is a pleasant, clean smell for many people. Minerals in the soil are released in much the same way as the actinomycetes spores and contribute to the earthy fragrance. Trees and plant oils that have collected on the plant’s surface or on other surfaces such as rocks, are also released during a rain storm. All of these combined produce the unique sweet smell we experience after a rain shower.
Unfortunately, the slightly unpleasant aroma released after a good rain is due to organic debris or chemicals in the ground. These substances are more common in city environments and concentrate in the soil over time. The longer the dry period between rains, the more chemical solvents that will be released into the air when a good rain disturbs the soil. Of course the rain doesn’t smell quite so nice in the city.
All of these factors contribute to the smell that we experience after a rain shower but what about the odor we smell beforehand, as a rain shower is approaching us? The smell that occurs before the rain is caused by an organic compound called “geosmin”. Geosmin is produced by several types of bacteria (including the actinomycetes bacteria) and algae. The human nose is extraordinary sensitive to geosmin; we can detect is at a level of just ten parts in a trillion, and we can smell it from a very long distance away. When you smell rain as a storm threatens but has yet to produce any rainfall, you are smelling geosmin that was released from the soil by rain that is falling windward of your location (and most likely heading your way).