Somehow, the rain brought out this smell, but the exact process was yet  unknown until two Massachusetts Institute of Technology (MIT) scientists captured it in high-speed video.

They found that at the right velocity on the right kind of soil, a falling water drop can trap tiny air bubbles under it. Those bubbles capture molecules in the soil. As the water drop deforms, the bubbles scoot up through the drop and jet out into the air, like champagne bubbles or spray from a crashing wave.

The researchers found if the drop falls too slowly, it is absorbed; too fast, and it splatters without the bubbles emerging. “The sweet spot has to do with the velocity of the droplet and the qualities of the soil,” said Cullen R Buie, an assistant professor of mechanical engineering.

Although the weight of a raindrop wouldn’t affect the speed at which it falls in a vacuum, it does in the atmosphere. Moderate or light rain on dry soil is the recipe for petrichor. “It’s not from the rain itself,” Dr Buie said, “it’s from the earth.”

Using high-speed cameras, the MIT researchers observed that when a raindrop hits a porous surface, it traps tiny air bubbles at the point of contact. As in a glass of champagne, the bubbles then shoot upward, ultimately bursting from the drop in a fizz of aerosols.

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