Thursday, August 8, 2013

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...?

16 comments:

Oldfart said...

Great insights Dr. Mass. I would like to name the first rain in a long time, Refresh. That's what it does for me.

Marypat Meuli said...

I recently learned about actinomycetes from this comic, http://birdandmoon.com/intheair.html

Unknown said...

JI love that smell, too! I've called it the "wet rock" smell since I was a child. (We used to get that all the time after rain in Arizona.) And yes, I smelled it here last week. Thanks for telling me what it really is.

Bob said...

Always suspected the rain itself was not a direct factor in the actual smell, as I have repeatedly noted it on a hot, dry day while walking near an area where sprinklers have just come on. Obviously, just needs the large drops to kick up the oils.

Thanks for the post!

Michael said...

Cliff

I didn't know you were a Doctor Who fan...Amy Pond was tried of waiting too.

Lindsey said...

Maybe I'm one of the few, but I've never liked that smell.

Molly said...

As a perfume lover AND a rain lover, I love this post and most definitely agree about that smell. I always cite it as one of my favorites in the world. Will share this post with my fragrance friends!

Charles Vigneron said...

Great post!

I've a question off topic, however?

If the air contains 5% more moisture doesn't that increase the mass (power?) of high winds?

Are the hurricane and tornado scales to be affected?

If 5% isn't enough, what point would it become noticeable?

(I'm listening to Bill McKibben on Speakers' Forum as I write)

Kind regards,
Chuck
Walla Walla

sciencefirst said...

Fascinating post! Makes me think of the California desert smells, and particularly creosote, after a downpour.

LewisLucanBooks said...

I always thought that smell would make a great men's cologne. Call it "Wet Asphalt." :-) .

Jazzzzzzzzz said...

The smell of first rain is one of my favorites. Please tell me fall is around the corner for the PNW!

weatherfan said...

Verry interesting. We've a certain spot on our graveled drive that my wife remarks she is reminded of being near Salton Sea Ca. We only notice it on dry days and not where the drive crosses a creek, but farther on. Maybe not the rain smell but moisture in the air?

Mike said...

It may have been Cliff that hit his topic (sort of) a whjle ago. Air with water vapor in it is actually LIGHTER than dry air, because the molecular weight of H2O is 18 and N2&O2 is ~28. The same volume of the same total number of molecules (i.e. at the same pressure) will weigh less. Therefore when a "dry line" front comes across Texas from the west here in Houston, its kind of like a cold front and scoops under the humid air near the coast and lifts it, even though it is definitely not cooler!

Charles Vigneron said...

Thank you Mike!

He did explain the Walla Walla Valley heating phenomena with winter storms, too.

So many posts...

Seamus Lowe said...

They really have a rain perfume. Wow. That is awesome. Rain is one of mine and my wife's favorite things in the world and if they have a rain perfume, I'll have to get it for her.

BOOGA said...


Pat answered 2 mins ago
I have been having a rain issue for 10+ years now. About 3 years ago was the first time I had the burning sensation in my eyes.... it had just begun to rain when I got out of my car. The rain smelled terrible and actually felt like it was taking my breath away, but I think the smell was kind of chemical vapor in nature. My eyes burned for about 20 min. Since then, I noticed that before it starts to rain, there is the same smell which now prompts me to close my windows until the rain has stopped because the smell bothers my sinuses I think and burns my nose slightly.

I live in Florida in the winter and when I first moved into the Miami area, I was continually sick, especially after it rained or even when it was overcast sometimes. I live in Ohio in the summer and about 3 years ago was the first time I was caught in the rain and had the burning sensation, now I notice the same thing in Ohio on a regular basis. I get a kind of raspy sore throat as a reaction.

I never go out in the rain because of this smell and the burning anymore. There is also a weird taste to the rain. I am ultra sensitive I guess, but the rain used to be pleasant smelling and I loved when it rained. Now my sinuses react, I sometimes even get a slight sore throat, like I am allergic to the rain. Any clues? Anyone else having these issues? I am searching for answers, any help appreciated.

I miss enjoying the rain :_(