We all know the scent, that earthy fresh aroma that fills the air during those first few minutes of rain. Had someone asked me two days ago what causes the phenomenon, I would have had no idea. After all, rain is just water, and water has no odor, right? So, what's making the smell?

Thankfully, the scientists at the Massachusetts Institute of Technology (MIT) conducted roughly 600 experiments and found out why they believe rain brings forth that lovely perfume. By using high-speed cameras to observe raindrops as they hit various porous surfaces, they discovered that small air bubbles become trapped under the drops upon impact, rise up to the surface, and then escape into the surrounding air. It's in the released air that we'll find the root of the scent.

The researchers determined that soil bacteria and microbes are swept up into the air within the drops when the water hits the ground, and those same bacteria and microbes are then released into the air. Think of the little pockets of air as a delivery service to make the bacteria and microbes airborne.

The scientists at MIT suspect that in natural environments, petrichor, the scent that we associate with rain, also gets swept up into the droplets along with the bacteria and viruses one normally finds in soil. Once the rain releases its air, the contents can then be spread through gusts of wind, allowing us to smell the rain.

Cullen R. Buie, an assistant professor of mechanical engineering at MIT, said about the findings, "Rain happens every day — it's raining now, somewhere in the world. It's a very common phenomenon, and it was intriguing to us that no one had observed this mechanism before."

Single drops of rain were tested on 28 surfaces, some man-made and other natural, simulating various types of rainfall. Water released from shorter distances mimicked lighter rainfall and water released from higher up acted like more of a downpour.

Not all types of rain are created equal when it comes to delivering aerosols into the air. MIT found that light and moderate rains were best suited for the task, and that, the harder the rain hits the ground, the less likely air would be to rise up to the surface of the drops.

This study is presenting the scientific community with new information, and not just about the smell of rain. It was already known that aerosols can be released when rain lands on water, but this finding, that aerosols can be released when rain lands on soil, is brand new. MIT believes that the information will help them understand exactly how substances, such as bacteria and chemicals, move throughout our environment and even spread some soil-based diseases to people.

To see those little bubbles of air that contain the smell as well as bacteria, chemicals and microbes, watch MIT's short video (above) that slows down the process with those impressive high-speed cameras.

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