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When It Smells Like Rain

It smells like rain, but how? Find out in today's A Moment of Science.

If you’ve been outside before a rainfall in the spring, you might have noticed a particularly fresh or sweet smell that seeps into the air a few minutes before the first drops begin to fall. If you’re familiar with this smell, it’s a great way to predict when the rain is about to start. What, exactly, makes the air smell like rain?

A lot of stories and folk tales have arisen to explain this odor. Long ago, people used to believe that rain clouds picked up sweet smells from Heaven, and the rain carried these angelic odors to Earth. A more up-to-date–but equally incorrect–version of this folktale is that the smell comes from ozone, carried from the upper atmosphere by the falling rain.

In fact, that fresh smell isn’t coming down from the sky at all. It’s coming up, from the ground beneath your feet.

As a spring rain approaches, the humidity level at the ground tends to increase. Moist air is much better than dry air at transmitting smells–this is why you might use warm, steaming water to carry the smell from dried potpourri into the air of your house. As the humidity rises, the moist air carries the fresh smell of oils up to your nose. It also carries the odors of ground dwelling bacteria and fungi.

These smells are always in the air immediately above the ground, as you can test for yourself by sticking your nose into your lawn and breathing deeply. When a rainstorm approaches and the humidity climbs, the odor rises up from its earthly embrace and you say–It smells like rain!

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