It’s going to rain. Can you smell it?  

Being able to smell rain as it approaches isn’t something imagined. There really can be a distinctly heady aroma in the air before it rains. And it’s a smell that I’ve always found calming. In fact, there are several clean, earthy, strikingly pleasing, yet distinctly different smells that many of us associate with rainfall. They occur before, during, and after showers and storms. And all of them are scientifically identifiable.  

Ozone 

Before it rains; as the wind begins to pick up and the clouds thicken or roll in, you may become aware of a noticeably fresh scent in the air. That sharp, clear aroma is ozone; a molecule made up of three oxygen atoms bonded together (O₃) whose name comes from the Greek verb for smell; ozein. It’s the same gas we associate with the layer of our atmosphere that protects us from too much sunlight.

As unsettled weather approaches, ozone is carried to the ground in strong downdrafts that, once they hit the earth, move horizontally as strong, gusty winds that precede the arrival of the rain. That wind, or ‘gust front’, and the smell of ozone that’s carried in it, reaches your nose a short time before the approaching rain arrives. 

Petrichor  

The smell that you discern when it’s really humid, but not actually raining, is more geological and / or biological in nature. It involves organic molecules from pore spaces and grooves in stones, rocks, and decaying organic matter in soil being released into the air and occurs when high humidity or misty condensation develops prior to rain. Once the actual rain arrives and makes contact with the earth, the scent is dispersed by the wind. Since the decaying organic matter and resultant plant oils accumulate over time, the aroma tends to be more pronounced after lengthy periods without rainfall. 

The smell was first described scientifically by two Australian scientists, Dick Thomas and Isabel ‘Joy’ Bear, in a paper titled ‘The Nature of Argillaceous Odour’, which was published on March 7, 1964, in the journal Nature. (Isabel Bear passed away last month on April 8th.) To describe the noticeably earthy, yet somewhat ethereal smell that’s given off when rock and organic soil matter become damp, the researchers coined the word ‘petrichor’, which is derived from the Greek ‘petra’, meaning stone, and ‘ichor’, which translates as ‘divine blood’ or the ‘blood of the gods’, of Greek mythology. The word petrichor literally means ‘divine blood from a stone’ or ‘divine blood of the stone’.  

In 2015, scientists at the Massachusetts Institute of Technology, using high-speed cameras, were able to record super slow-motion video of raindrops releasing clouds of aerosols upon impact with a surface; the petrichor aerosolization process. That video can be viewed on YouTube. The aerosolization process also explains how some diseases caused by bacteria in the soil (e.g. E. coli) may be transmitted by rain. 

In laboratory settings, petrichor can be extracted from clay soils that have been baked by the sun, using steam distillation, and then purified and concentrated to make an essential oil, which can be worn as a fragrance, or used in diffusers or to make scented candles. It’s sometimes added to other essential ‘carrier’ oils, such as sandalwood and jojoba, to create novelty fragrance products.   

Geosmin 

One component of petrichor is a naturally occurring organic compound (chemical formula C₁₂H₂₂O) called geosmin; again from the Greek; ‘geo’ meaning earth, and ‘osme’, the noun for smell. Geosmin was scientifically identified more than 100 years ago and is produced by microorganisms living in soil. Or, more accurately, by microorganism dying in the soil. During periods of dry weather, Streptomyces, a common genus of bacteria that live in extensive fungi-like mycelial colonies in the soil (and are one of the most important sources of antibiotic, anti-fungal, and anti-cancer compounds known to science), produce spores (a means of reproduction), which contain geosmin. When Streptomyces die, they release the geosmin-containing spores, but the ‘earth smell’ of geosmin isn’t relinquished into the air until rain falls to the ground and disperses it or, in some cases, until the soil is otherwise disturbed. Humans are surprisingly sensitive to geosmin. It’s estimated that people can detect geosmin in the air at concentrations as low as 5 parts per trillion. 

Geosmin is also produced by blue-green algae and, while it may smell pleasant in the yard or garden, it can also be responsible for a muddy or pungent ‘off’ taste in drinking water. And it makes its way into a range of vegetables, fruits, and even fish; in some cases enhancing the flavor and, in others, making them less-palatable. If you relish eating beets, for example, you can thank geosmin for their earthy flavor. The same can be said for spinach, lettuce, and mushrooms. However, if you’ve ever eaten a muddy-tasting carp or catfish, the disappointing and somewhat disagreeable flavor was most-likely due to high concentrations of geosmin, can also occur in recirculating aquaculture systems used in home and commercial fish production. 

Photos: Fog often forms when relatively humidity is at 100%. Photo Credit: University Corporation for Atmospheric Research – UCAR

Photo Credit: Commonwealth Scientific and Industrial Research Organization (CSIRO -Australia’s National Science Agency)