Petrichor: That Rain Smell Explained

You’re not crazy. There’s definitely a distinct smell- sometimes- after rainfall. Sometimes you could even swear you smelled it before the rain came tumbling down. It’s autumn, and inevitably the rain will be dogging us more stubbornly with its persistent gray sheets. We’re grateful for the rain, but we’re a little weirded out, too. Why is it that we can smell rain, but only occasionally? More importantly, what in the world is that smell? Science has the answer. That answer is petrichor.

“Petrichor” is the name for the odor released when rain falls on dry soil. We can thank Australian researchers Isabel Joy Bear and Richard G. Thomas for identifying and subsequently entitling that mysterious rain smell, back in 1964. The two scientists determined that the distinct smell is caused by a blend of oils secreted by certain plants.

During a period of drought, oils from certain plants begin to accumulate in dry rocks and soil. When the rain does finally come, these oils get mixed with other nearby compounds, such as geosmin. Geosmin is a chemical made by bacteria that live in soil. Ozone molecules are often also a contributing factor, especially when there’s been a thunderstorm. Ozone is commonly produced by a lightning bolt’s electrical charge, and has an odor reminiscent of chlorine.

A raindrop landing on a porous surface causes air from the pores to form small bubbles. These bubbles make their way to the surface and release aerosols, which carry the scent of the plant oils and the geosmin into the air. That rain smell is essentially a concoction of plant oils and bacteria chemicals that have made their way into your nostrils, heralding the triumphant return of rain to a once-parched landscape.

In short? You smell rain after long periods of dry weather because plant oils and soil chemicals have had a chance to collect and mix. When rain comes, that mixture makes its way into the air, and, voilà- petrichor.

On an intriguing side-note, the plant oils have been found to inhibit seed germination. Scientists Thomas and Bear have speculated that this helps to limit competition for the precious resource of water during times of arid weather.

Well played, plants. Well played.