When it rains, it smells. Researchers have discovered what causes the familiar scent that pervades their air when it rains.

Massachusetts Institute of Technology researchers claim they've identified the process that forms this earthy smell. Their study is the first that shows the effect of aerosols on soil.

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Researchers used high-speed cameras to take videos of raindrops falling and hitting the ground. The images revealed that tiny air bubbles became trapped underneath the raindrops and then rose up through the water, similar to soda bubbles. These tiny pockets of air burst when they reached the top of the raindrop, releasing aerosols that caused that distinct smell.

Factors such as the velocity of the raindrops as it struck the surface of the Earth and how the surface is structured were considered important factors in the amount of aerosols released during rains.

The experiment was conducted 600 times on 28 different types of surfaces, including 16 soil samples and 12 materials. When rain droplets emit these aerosols, they also contain bacteria and other microbes that originate from the soil.

This is the first evidence that proves aerosols can be generated from the soil, spreading the microbes and chemicals that exist underground, said to Youngsoo Joung, a postdoctoral student at MIT. This also provides new insight into how aerosols travel in the environment and to humans.

During light to moderate rains, more amounts of aerosols are released as opposed to heavier rains, according to the study. Water from heavy rains hit the ground harder, preventing the air bubbles from rising-up and being released into the air.

Australian scientists first coined the term petrichor to describe the earthy mixture of smells from bacteria and plant oils that produced the distinct rain aroma.

This study was published in the journal, Nature Communications.

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