Scientists have come up with a theory that explains why rainfall smells the way it does. A team of researchers from the Massachusetts Institute of Technology believe they have found what causes the fresh, earthly smell during and after a rainfall.
The scientists studied the raindrops as they fell on the ground, using special cameras that filmed in a high-speed mode. They researchers observed that when the drops of rain fell and hit the earth, the cameras captured what seemed to be small air bubbles that were trapped under the drops of rain. These bubbles would rise up through the water drops, similar to the bubbles that rise up in soda drinks.
When the tiny air bubbles burst through the top of the rain drops, it released aerosols into the air. The scientists discovered that the speed at which the rain drops hit the ground, as well as the structure of the surface the drops land on, play an important role in how many aerosols are released into the air during this process.
The scientists did the experiment in more than 600 runs, using 28 different surfaces, including 16 types of soil and 12 materials that were man-made. The researchers measured how permeable the types of soil were by putting samples of it in tubes and putting water underneath the soil. Then they recorded how long it took for the water to rise to the top of the tube.
The aerosols which are released from rain drops can have different bacteria and microbes that lived in the soil previously.
One of the scientists involved in the study, Youngsoo Joung, explained that until this recent experiment, no one knew that aerosols can be generated by raindrops falling on the ground.
The scientists observed that more aerosols were released when there are light and moderate rainfalls. During heavier rains the number of aerosols released into the air was smaller. This is because during a heavy rains water drops hit the ground hard and do not allow the air bubbles to form, before the liquid disperses.
A team of researchers from Australia named the smell of rain Petrichor.
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