petrichor

Petrichor is the pleasant earthy scent that often accompanies the first rain after a dry spell. The term was coined in 1964 by Australian researchers Isabel Joy Bear and Richard G. Thomas to describe this phenomenon. It derives from Greek, with petros meaning stone and ichor referring to the mythic fluid that flows in the gods’ veins.

The odor originates from two main sources: oils secreted by plants during dry periods and allowed to accumulate on soil and rocks, and geosmin, a scent-producing compound produced by soil-dwelling bacteria such as actinomycetes. When rain falls, these compounds are released into the air as aerosols, producing the familiar earthy aroma.

Geosmin and related compounds such as 2-methylisoborneol contribute the characteristic scent, which can be perceived at very low concentrations by humans, often at a few parts per trillion. The relative contribution of plant oils and microbial compounds varies with soil type, humidity, and weather conditions.

The phenomenon is observed worldwide, most noticeably after extended dry periods followed by light to moderate rainfall. Petrichor is distinct from other storm-associated smells, such as ozone produced by lightning, though all contribute to the sensory experience of rain.