Mass: Oh, the Smell of Rainfall on Hot Concrete after a Dry Spell!

Aug 9, 2013

Cliff Mass has a soft spot for the smell of rain, more specifically rain showering down on hot concrete after a long dry spell.

“This is something I’ve noticed for years,” said Mass, KPLU weather expert and professor of atmospheric sciences at the University of Washington. “After a dry period, you have the first rain and there’s this smell. I kind of like the smell myself.”

The technical name for the smell is “petrichor,” which Mass describes as “sweet, musty.”

And this distinct smell comes around generally during the summer, usually following a weeks-long dry streak.

So what causes this smell? Mass did some research, and learned several studies have been conducted on just this topic.  

“And it appears that most of this is caused by oils that are put off by plants during arid periods. Actually, these oils prevent other seeds from germinating, so I guess it reduces competition,” said Mass.

Concrete and the earth tend to absorb these chemicals, says Mass, then when the first rain falls, some of these chemicals get released, creating that distinct smell, which, by the way, has nothing to do with bacteria spores.  

“It turns out there are some other related smells that happens when rain falls on earth, on newly-wetted earth, and that has to do with spores from bacteria, but that’s something else,” he said.

If you haven’t experienced this smell before and are curious to give it a sniff, you don’t have to wait for Mother Nature.

“You can duplicate the smell by sprinkling water on hot concrete with your sprinkler or something,” said Mass.

And you might even find it at the department store.

“There are perfumes that are sold on the market right now that supposedly have this smell,” said Mass.

'Happy' Smell, 'Dusty' Smell

We asked some of you to describe the the rain-on-hot-concrete smell, and your answers ran the gamut. 

Lamanda Joy called it "a happy smell." Annabelle Asher described it as "almost earthy."

"It’s not a natural smell," said Glen Hylton. "It’s not like smelling dew or fragrance of roses; just some sort of acrid type of aroma."

"Always a little dusty and moldy, like all those smells that have been sitting on the concrete just sort of come up," said Mike Naler. 

"I suppose it smells fresh," said Chris Graesser. 

"It just smells like fresh air ... and something that’s been renewed," said Kenya Fredie.

How would you describe the smell? Tell us in the comments below. 

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The weekly KPLU feature "Weather with Cliff Mass" airs every Friday at 9 a.m. immediately following BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KPLU Environment Reporter Bellamy Pailthorp. Cliff Mass is a University of Washington Professor of Atmospheric Sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to a podcast of “Weather with Cliff Mass” shows.