Many people notice a distinctive smell after rain, especially when the first drops of summer rainfall on the hot, dry land. The fresh, 'earthy' scent permeates the air and gives a pleasant feeling.
Before hitting the ground, rain is just water with no smell. This scent, called petrichor, comes from the moistening of the soil.
What Causes Petrichor?
The term petrichor comes from the Greek words "petra," which means "stone," and "ichor," which refers to the fluid flowing like blood in the veins of the gods. This familiar smell after a rain was first characterized in 1964 by Australian scientists Isabel Joy Bear and Richard Grenfell Thomas.
Bear and Thomas investigated the earthy aroma that arises when rain falls on dry soil. They discovered the significant contributors to the petrichor scent and identified them as geosmin and 2-methylisoborneol, produced by certain soil bacteria.
To understand the physical mechanism of how the smell gets into the air, a study was conducted by Cullen Buie and Youngsoo Joung from the Massachusetts Institute of Technology in Cambridge. They filmed raindrops hitting 38 surfaces: 12 engineered materials and 16 soil samples.
The process of releasing petrichor into the air was observed with high-speed cameras. The researchers discovered that when a raindrop hits a porous surface, it traps tiny air bubbles at the point of contact, like bubbles in champagne. These bubbles then shoot upward and burst from the raindrop, creating a cloud of aerosols.
The cloud of hundreds of aerosol droplets is dispersed in as little as a few microseconds, depending on the properties of the soil and the speed of the droplet. This was seen more often during light and moderate rain, while fewer aerosols were released during heavy rain.
Experts have long observed that raindrops can trap and release aerosols upon falling on water. However, this is the first time that they have observed the process happening on soil.
Other Rain-Related Smells
The smell people associate with rain can be caused by several factors, including raindrops. One of the more pleasant smells often noticed in the woods is caused by bacteria. A filamentous microbe called actinomycetes grows in damp and warm soil. When the soil dries out, this bacteria produces spores in the soil.
The wetness and force of raindrops push these tiny spores into the air where after-rain moisture acts as an aerosol. These spores are carried to us easily by moist air, allowing us to breathe them in. Such spores have a distinctive, earthy smell we usually associate with rainfall.
The acidity of rain causes another distinctive scent. Chemicals in the atmosphere make rainwater somewhat acidic, especially in urban areas. When it comes in contact with chemicals in the ground or organic debris, it can lead to some aromatic reactions. As soil is broken apart, the minerals trapped inside are released and react with chemicals like gasoline, which give a stronger smell.
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