Life's Little Mysteries

Why Does Rain Smell Good?

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A certain chemical called petrichor may largely account for its pleasing scent.

Nothing smells quite like Earth after it's been through the rinse cycle.  In fact, rain is such a pleasant smell that perfume-makers, soap makers and scent-purveyors the world over try to recreate it. 

But what makes rain smell so nice? It turns out there are several scents associated with rainfall that people find pleasing.

One of these odors, called "petrichor," lingers when rain falls after a prolonged dry spell. The word is derived from a combination of the prefix "petr" referring to stone, and "ichor," referring to a tenuous essence, and was first coined in 1964 in the journal Nature by two Australian scientists studying the smells of wet weather. The team conducted their research in India and found it specifically referred to the smell emitted when the baked clay soil in arid regions meets a splash of water. The second reaction that creates petrichor occurs when chemicals produced by soil-dwelling bacteria known as actinomycetes are released. These aromatic compounds combine to create the pleasant petrichor scent when rain hits the ground.

Another source of rain's pleasing scent may be the flora in a region. Some plants secrete oils during dry periods, and when it rains, these oils are released into the air. These volatile fatty acids, including stearic acid and palmitic acid, add a pleasing earthy note to the smell of rain.

Another scent associated with rain is ozone. During a thunderstorm, lightning can split oxygen and nitrogen molecules in the atmosphere, and they in turn can recombine into nitric oxide. This substance interacts with other chemicals in the atmosphere to form ozone, which has a sharp smell faintly reminiscent of chlorine.

When someone says they can smell rain coming, it may be that wind from an approaching storm has carried ozone down from the clouds and into the person's nostrils.

Elizabeth Peterson
Live Science