Can you smell these photos? Raindrops cover flowers in Ketchikan, Alaska on August 27, 2017 (L) and a tourist visits the Mendenhall Glacier on a rainy day (R, date unknown).
When a summer thunderstorm arrives and rain falls on the hot, dry ground, why does it smell so good outside? Farmers have said that they can “smell rain” before a storm arrives. Some describe the scent before a rain storm a musky, earthy smell called petrichor. But, we know that rain itself has no scent. The process, which was first studied in 1964 by Australian scientists, comes from a moistened ground.
Petrichor is made up of oils of plants and bacteria that live in the ground. Tiny microorganisms, called actinobacteria, are found in rural, urban, and marine environments and help decompose decaying matter into nutrients for plants and organisms. Their byproducts, called "geosmin", are similar to a type of alcohol. The chemical structure of geosmin makes it detectable, even in trillion parts per air molecule.
During periods of prolonged dryness or drought, the actinobacteria slow down their decomposition activity. As a storm approaches and the air becomes humid, the ground begins to moisten. This helps the actinobacteria to speed up their activity, forming geosmin. As raindrops fall, porous surfaces such as loose soil, splatter and eject aerosols, similar to a ring on a lake after throwing a rock in. These tiny particles can be carried by the wind, alerting people downwind that rain will soon be on the way. The scent goes away as the storm passes and the ground dries up once again, leaving the actinobacteria in waiting of when it might rain again, the sound of thunder rumbling in the distance.
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©2022 Meteorologist Sharon Sullivan