What Do We Actually Smell When It Rains?

What Do We Actually Smell When It Rains?

When those first fat drops of summer rain fall to the hot, dry ground, have you ever noticed a distinctive odor? Of course rain itself has no scent. But moments before a rain event, an “earthy” smell known as petrichor does permeate the air. People call it musky, fresh, generally pleasant.

Petrichor is the term coined by Australian scientists in 1964 to describe the unique, earthy smell associated with rain. It is caused by the water from the rain, along with certain compounds like ozone, geosmin, and plant oils.

Petrichor is a combination of fragrant chemical compounds. Some are from oils made by plants. The main contributor to petrichor are actinobacteria. These tiny microorganisms can be found in rural and urban areas as well as in marine environments. They decompose dead or decaying organic matter into simple chemical compounds which can then become nutrients for developing plants and other organisms.

A byproduct of their activity is an organic compound called geosmin which contributes to the petrichor scent. Geosmin is a type of alcohol, like rubbing alcohol. Alcohol molecules tend to have a strong scent, but the complex chemical structure of geosmin makes it especially noticeable to people even at extremely low levels. Our noses can detect just a few parts of geosmin per trillion of air molecules.

When raindrops fall on the ground, especially porous surfaces such as loose soil or rough concrete, they will splatter and eject tiny particles called aerosols. The geosmin and other petrichor compounds that may be present on the ground or dissolved within the raindrop are released in aerosol form and carried by the wind to surrounding areas. If the rainfall is heavy enough, the petrichor scent can travel rapidly downwind and alert people that rain is soon on the way.