MadSci Network: Chemistry

Re: What causes the distinctive odour of approaching rain?

Date: Fri Jun 5 10:36:05 1998
Posted By: Jason Goodman, Graduate Student, Massachusetts Institute of Technology
Area of science: Chemistry
ID: 896758257.Ch

The smell of rain is caused by ozone (O3). It is commonly produced through dissociation of molecular oxygen (02) into two unstable atomic oxygens (2 O), each of which then recombine with molecular oxygen to make ozone.

This dissociation can occur either by high-voltage electrical discharge or by bombardment with ultraviolet light. The high voltages which occur in thunderstorms create ozone within the cloud (even when lightning is not actively occuring), and this ozone is carried toward the ground by the downdraft in the thundercloud and blown out ahead of the storm, where you smell it and can tell rain is coming.

Ozone is also created high in the stratosphere by ultraviolet light: this is the "ozone layer" which protects us from harmful UV light.

A third place you're likely to find ozone is coming out of your photocopier or laser printer. These devices have high-voltage discharges inside them, and produce enough ozone to bother me while I'm sitting here next to one typing this.

Why would water on asphalt generate ozone? I don't think it would. But I've noticed the smell of water-on-pavement you're talking about, and I do associate it with rain. Your sense of smell is very evocative: it can bring very vivid memories associated with smells. My theory is that there are two different smells associated with rain: the ozone smell before it starts raining, and the "pavement" smell once the rain begins falling on the pavement. Either one will make you think of rain, and they probably get mixed in your mind.

So what is that smell produced by wet concrete? I honestly don't know, but It's similar to the smell of freshly-mixed concrete, and I've also smelled it when carrying out the reaction

  Ca(OH)2 + CO2 -> CaCO3 + H2O
Some rocks (limestone?) also smell like this when wet. I suspect the smell has something to do with calcium; either small particles are released into the air which enter your nose, or there's some sort of gaseous calcium-containing molecule produced by interaction between carbon dioxide, water, and a calcium compound (though that seems unlikely.)

If anyone with more of a background in chemistry knows about this, send e-mail to the moderators at

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