1 . When raindrops fall to the ground in summer, a familiar scent always follows. Many people call this the “smell of rain”. However, rain is made up of odorless (无味的) water. So, where does this smell come from?

    1     But it actually comes from wet soil. This special odor even has its own name: petrichor (潮土油), which comes from a pair of chemical reactions.

The pleasant smell has long been an interest of scientists since Australian scientists first documented the formation of petrichor in 1964. According to scientists, some plants produce oils during dry periods.     2    

Yet, it’s the second reaction that creates the most petrichor. It occurs when chemicals produced by bacteria in soil are released.     3     These bacteria break down and change into simple chemical compounds, providing nutrition for plants. Meanwhile the process also produces geosmin (土臭素), a compound with a distinct earthy aroma (香味).

Lots of animals are sensitive to geosmin but human beings are extremely sensitive to it. When it rains after a long period of dryness, drops of water hit the ground. Geosmin is then released from the surfaces.     4     And it finally gives raindrops their familiar scent. If it rains heavily enough, geosmin will be spread downwind, warning others of coming rain.

    5     And the next time someone mentions how much they love the smell of rain, you can prove your intelligence by explaining where that smell comes from.