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May. 12th, 2014

For the longest time, I have found it fascinating that the smell of far-off rain is actually the smell of the earth. Geosmin, the Straight Dope (and other sources) tell us - and we humans are extremely sensitive to it.

"The human nose, not normally considered a particularly acute instrument, is extraordinarily sensitive to geosmin; we can detect it at a level of just ten parts in a trillion. [...]

Geosmin is produced by several types of bacteria and algae, which manufacture a volatile compound that can be kicked up when soil is disturbed, such as by gardening, plowing, or a hard rain. When a storm threatens and a few molecules of geosmin waft your way, that signifies rain is falling to windward, and in the fullness of time will fall on you."

This is followed by a lovely speculation:

"Long ago we were mainly nomads wandering in arid regions. It’s easy to imagine a parched band trudging mapless in the desert looking for the next watering hole. Then the breeze picks up, and what do they detect? Had they lacked the appropriate olfactory adaptation, nothing, with possibly disastrous consequences. As it was, if they were fortunate, they might smell the faint odor of moist earth, and with it the promise that they'd live another day."

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