The Smell of Rain

It's summer and it hasn't rained in a long time.  Then the first shower occurs and you notice THAT SMELL, particularly near concrete or roadways.   How do I describe it?  Sweet, musty?   A smell that is enhanced when the sidewalk is warm.   I really love that smell.

Well, it happened last week.  Some light showers came in after a long period (about a month) of no rain.  The welcome result: the rain smell.

I have wondered about this for a long time.  What is this smell?  What chemicals are released when rain hits dry pavement after a dry spell?  Let me tell you what I have found....and please note:  I am no expert on the chemistry of odors!

It turns out that others have asked this question and there is some research on the topic.

One of the earliest studies on the rain smell was published in 1964 by two Australian scientists, I. J. Bear and R. G. Thomas, in an article in Nature magazine titled "Nature of Agrillaceious Odor."   They termed a fancy name for the "rain on concrete" smell:  petrichor.  A wonderful name combining two Greek roots:  petra (stone) and ichor (the blood of the gods in Greek mythology).  Their work found that the main cause of the smell is a collection of oils produced by plants during dry periods.  The Austrailian scientists discovered that these oils inhibit seed germination, perhaps to limit competition during arid times. These compounds accumulate over time and are absorbed by concrete, rocks, and soil.  An initial rain mixes with these oils and release some of them into the air, producing the odor.

Bear and Thomas also found another source of the "rain smell" over moist, vegetated areas.  A soil-dwelling bacterium known as actinomycetes produces a chemical called geosmin when the bacteria produce spores as the environment dried out.  When rain begins it strikes the soil, it sends the spores and geosmin into the air, to be sensed by our noses.

And there is one other rain smell I should mention...the smell of ozone produced by lightning.   That ozone has a smell should not be surprising considering that its name came from the Greek word ozein, which means to smell.  Lightning can break up oxygen molecules, resulting in ozone generation (O3) that smells sort of like chlorine bleach/clean bed sheets.  Downdraft air from thunderstorms brings the ozone down to the surface.

Some perfume companies have even bottled the essences of the rain-on-the-ground smell.  And this is not a new scent.   In Lucknow, India the odor is collected from clay disks wetted during  the monsoon season.   Mixed with sandalwood oil it is marketed as matti ka attar – earth perfume.  In contrast, over the deserts of Western Australia, the Anangu people call it panti wiru, simply meaning ‘good smell’.  

In any case, such perfumes are guaranteed to drive any meteorologist crazy! 

Rain smell as a perfume?  For the girl that is tired of waiting?  For rain?  Or perhaps...?