Question of the Week

What is the smell of rain?

Sat, 11th Sep 2010

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Question

Julia, Edinburgh asked:

What makes the distinctive smell immediately after it's rained?

Answer

We put this question to Beryl Zaitlin, a hydrogeologist from the University of Calgary in Alberta, Canada...

Beryl -   Well, I looked it up and the actual answer is that we really don’t know!  Odourrainstorm is something that has to be a volatile chemical of some sort; it’s a compound that goes to your nose and the receptors in your nose absorb that chemical.  So it has to be some sort of chemical that’s in the air.  In the 1960s, two researchers – Bear and Thomas in Australia – extracted some oil from clay.  They thought that oil had a biological origin and they thought it smelled like the smell of rain.  That was all the research that was done on it for ten years.  And then in the 1970s, Nancy Gerber, who was one of the very prominent researchers in taste and odour, isolated three chemicals from actinomycetes.  

Diana -   Actinomycetes are bacteria that live in soil and break down plant material like leaves and twigs, making compost.

Beryl -   One of those chemicals was 2-isopropyl-3methoxypyrazine which is what she thought Bear and Thomas had originally isolated.  It was a chemical that smelled like rain.  She published that she thought this might be the chemical that made the smell of rain and she thought it might have been the chemical that Bear and Thomas had isolated.  And that was it, nobody’s done anything since as far as I can tell.  It might be that the smell of rain comes from Actinomycetes and it might not.  At the moment, nobody really knows as far as I can tell.    

Diana -   So it could be that these bacteria are producing this smell but the odour jury is still out on this one.  We had some suggestions on the forum, including Tay who said it could be plants emitting lots of gases when they get wet and their pores open.  And our “Forumer of the Week” award goes to Variola who identified the smell as geosmin, an organic compound which bacteria – the actinomycetes – produce when they die, usually after the soil has been disturbed or it’s rained following a dry period.  Geosmin is one of the other chemicals that Nancy Gerber, whom our expert answerer mentioned, isolated when tracking down the earthy smell. 

 

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I recall that TNS answered this question a while back (I think it might have been before QOTW started).  The likely explanation was that the smell was of bacterial spores being kicked up by the rain.

http://www.thenakedscientists.com/HTML/content/latest-questions/question/2336/  jpetruccelli, Mon, 2nd Aug 2010

Hint: it is a form of oxygen... Voltaire, Thu, 5th Aug 2010

No, I don't think so...or are you thinking of the slightly different smell created when there's a thunderstorm? chris, Sun, 8th Aug 2010

Yes. I was thinking of ozone Voltaire, Sun, 8th Aug 2010

Is thunder/lightning a condition for that particular smell as far as the original question is concerned?
So what do you think you are smelling in a rainy day without lightning?
If anything, shouldn't it smell LESS since the rain washed away all the particulates that would otherwise go into our nose and create the sensation of smell?

(I get a lot of mud and compose smells, but I'm pretty sure that's just local phenomenon.) ccheric, Sun, 8th Aug 2010

I would say that the smell your nose detects when it rains is the cumulative smell of many (all?) plants in the rainy area that open the pores on their leaves to respire. When it rains, perhaps the plants 'know' that it's time to start sucking up water and in-taking CO2 in order to conduct photosynthesis, and so they open the pores and expel oxygen (that smells like the plants) and intake CO2.
The reason why you smell it more when it rains is because normally plants keep the pores more or less closed to retain as much water and other essential things as possible.

This is merely my best guess, so I can't wait until September to hear the correct answer!

Cheers,

Tay Tay, Tue, 10th Aug 2010

Tay, I agree with you. I lived in Japan for a year and when I returned to Britain I couldn't believe how strong the plants smelt after a rain storm. British gardens in the suburbs smelt so good after living in the centre of Tokyo with no plants around. Make it Lady, Thu, 12th Aug 2010

I bet it was a nice treat!
It's a good point about the dense city because typically there isn't the same rainy smell when there aren't as many plants around; leading me to believe that they're linked.

When I water my tomatoes in the back garden, they smell super pungent when I douse the leaves and they don't smell really at all when I just water the roots. People always told me to only water the roots so as not to damage the foliage, but I like to mimic nature when I interact with my garden, and I'm pretty sure rain doesn't discriminate. lol

I'd love to see a leaf under a microscope as it was dampened. I bet it would look really cool.

Tay Tay, Sat, 14th Aug 2010

This is an interesting question, one to which I would like to know the answer. I must confess, I do not know the answer to this one, although I suspect that Tay may be on the right track.

Just for the record, plants don't respire, they transpire. CreativeEnergy, Mon, 16th Aug 2010

Creative, plants both transpire and respire.  transpiration is the evaporation of water from the stomata on the surface of leaves.  respiration is limited to cellular respiration as there is no physical gas transport system. respiration is (in super simplified terms) the conversion of oxygen and food into energy.  both o2 in and co2 out during the night (when cellular respiration dominates) and co2 in and o2 out (during daylight hours with photosynthesis) occur mainly through straight diffusion via the same stomata that allow/cause the plant to lose water.  i think they can do it their roots as well.  Matthew imatfaal, Mon, 16th Aug 2010



Hi Matthew,

I checked and sure enough you are correct. You must excuse me. It's been several decades since I studied this stuff, so I am a little rusty. I stand corrected.

Eric CreativeEnergy, Mon, 16th Aug 2010

It's Geosmin.


I know that, my lab reeks of it... 

EDIT

There ya go http://en.wikipedia.org/wiki/Geosmin  Variola, Tue, 17th Aug 2010

I always thought it was ozone as well henryhippo, Tue, 24th Aug 2010

I shall invite people round for a lab-fridge sniffing party....  Variola, Tue, 24th Aug 2010

I think the smell is from the dust and molecules on the ground and in the sky. Smarties, Sat, 28th Aug 2010

got that smell when I grew an isolate on Actinomycete Isolation Agar. The colony was extremely difficult to perform a gram stain on, they had a creamy off white, glossy, irregular circular colonies, and when viewed under a wet mount had a filamentous appearance with bacilli shape, and no motility.  ironmunya, Mon, 6th Sep 2010

I liked Tay's answer best :) Rather poetic in a way and the smell surely is that from living green cells :) Ahem, sort of? yor_on, Wed, 8th Sep 2010

following my previous post, a logical conclusion after careful analysis of the ‘Differentiation of Aerobic Actinomycetes’ table from ‘Medically important fungi – a guide to identification by Davise H. Larone’ (Larone 1993, 51).. Streptomyces was selected in this case because of the match of colony morphology in the table and the AIA isolate. Without further investigation, I agree with the user Variola. It's Geosmin, probably produced by many species of the genera Actinomyces. I performed a lactophenol blue stain on the isolate. ironmunya, Thu, 9th Sep 2010

I woud say that it is simply some soil fine particles going in the air because of the water hitting the ground or some fine particles stick to the evaporating water. The smell is stronger where the surface is warmer like on a street after a sunny afternoon (strong evaporation)... CPT ArkAngel, Sun, 19th Sep 2010

2 Years Later... If it helps (I'm interested because I am trying to find a candle with the actual smell of rain), there is an oil that supposedly only appears during and after a rain called Petrichor. I'm guessing that this oil is secreted during a rain for a number of reasons. It could be to spread pores, to wick the plants so that their leaves could withstand a volatile beat down from the oncoming rain or to simply make excess water drip off easier. Sam, Wed, 12th Dec 2012

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