My garden welcomed the recent rains after almost a week of dry weather. Along with the rain came that wonderful, earthy smell known as “petrichor.” This scent is noticeable when rain falls on dry soil. The smell comes from a combination of factors.
Texas A&M University described the main source of the odor thus: “The main contributor to petrichor are actinobacteria. These tiny microorganisms can be found in rural and urban areas as well as in marine environments. They decompose dead or decaying organic matter into simple chemical compounds which can then become nutrients for developing plants and other organisms. A byproduct of their activity is an organic compound called geosmin which contributes to the petrichor scent. Geosmin is a type of alcohol, like rubbing alcohol. Alcohol molecules tend to have a strong scent, but the complex chemical structure of geosmin makes it especially noticeable to people even at extremely low levels. Our noses can detect just a few parts of geosmin per trillion of air molecules.”
Plant oils are another contributor to the smell. Plant leaves secrete these oils almost like they are producing their own sunscreen, but the goal is to reduce moisture loss through transpiration. Rainfall rinses those oils into the soil and into the surrounding air. In a 2015 study, scientists used high speed photography to capture raindrops causing tiny particles to be ejected into the air. The technical term for this is “frenetic bubble generation.” MIT describes the process as similar to bubbles rising in a glass of champagne.
The scent of rain can also be enhanced by ozone resulting from lightning that occurs during summer thunderstorms.
The different factors combine to create a smell that people can detect – those of us with good sniffers, that is. My grandfather could always predict a rain before it started without the benefit of doppler data. Now, I settle into a comfortable porch chair when the rain starts, sip a glass of iced tea, and enjoy that sweet smell, Mother Nature’s perfume.
