Environment

What Causes the Earthy Smell When Rain Falls?

Petrichor is the term used to describe the earthy scent produced during a rainfall on dry soil.

After a heavy rainfall, the surrounding atmosphere seems to give off a fresh, earthy smell. This unique smell is often associated with either springtime or summertime for many people. In general, the smell is considered pleasant and one that many individuals look forward to with the onset of a rainstorm. What exactly causes this earthy smell after a rainfall? The phenomenon is known as petrichor.

What Is Petrichor?

Petrichor is the scientific word used to describe the smell in the air after a rainfall. The term dates back to 1964, when researchers in Australia published an article with the intent of providing an explanation for the cause of the smell. According to the researchers, plants secrete a number of natural oils during extended dry spells, and these oils then make their way into the surrounding soil, where they are stored until the next rainfall. When rain mixes with the dry dirt, these oils are released into the atmosphere. At the same time, geosmin, a chemical compound found in soil, is also stirred up by falling rain. Other researchers have attributed the smell to actinomycetes, a bacteria that grows in warm and moist soil. During dry spells, this bacteria produces spores for reproduction and when it rains, the spores are disturbed into the air.

These are the environmental factors that work together during and after a rainstorm to create the petrichor smell. The term petrichor is a combination of two Greek words, which refer to stones and the substance believed to flow through the veins of the gods.

How Does Petrichor Get Into the Air?

A basic understanding of petrichor does not fully explain how this smell makes its way into the air. The way in which the compounds that cause the petrichor smell get into the air was explained by researcher from the Massachusetts Institute of Technology (MIT) in 2015. According to the researchers, when a drop of rain hits the ground (or any other porous material), the water creates small bubbles of air. These air pockets collectively move upward toward the top of the raindrop, and this movement causes the water droplet to burst open, spraying the inner components throughout the atmosphere. These components are largely made up of aerosols, which are micro-sized particles. As these aerosols are released into the air, they carry the petrichor smell.

Several other factors influence how widely these aerosols can be spread, including the velocity and strength of the rainfall and the makeup of the ground soil. The researchers reported that hundreds of aerosols may be dispersed from a single water droplet in less than a second. Interestingly, the harder and heavier the rainfall, the smaller the number of aerosols that are released into the air. The study indicated that light and medium rain showers produce a stronger petrichor smell in the surrounding atmosphere. These research results, which were obtained by using high-speed cameras, are significant because it was the first time that the aerosol process was documented from soil. This information may also be used to explain how some pollutants, contaminants, viruses, and bacteria can be spread through the air across large areas.

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