It’s a rainy day in Wisconsin. Here, the rain clouds often settle in and get comfortable, keeping a persistant drizzle and damp demeanor all day. Most of the leaves have fallen, and the crisp, bright autumn aroma from last week has faded from fresh into just the smell of damp leaves. It’s a constant smell, one I have to think about to sense, otherwise it fades into the background while I’m occupied with biking past puddles.

Clouds building over the Rocky Mountains.

In contrast, last weekend, I was attempting a hike in the foothills of the Rocky Mountains when a rainstorm began to blow in. The dark clouds had been hovering near the horizon all morning, pretending to mind their own business while the sun shone on us in town. Once we started up the trail, however, the sudden, startling smell of soon-to-be-raining hit me, and I knew we were going to get wet.

It smells not like water, but like earth. I love that smell, the just before rain aroma that comes on almost as fast as a summer storm, but I haven’t been noticing it as much here in Wisconsin as I used to in Nevada or other parts of the mountain west. As I started to research this particular smell, it became apparently why it’s more pronounced in arid environments.

Right before a big rain, you don’t smell the rain, you actually smell the earth. A friend once told me that as the barometric pressure decreases with an incoming storm, the air trapped in the soil oozes upward, and we smell the soil ecosystem. While pressure may play a small role, the increasing humidity in the air that precedes the storm enables us to smell the soil, because humid air carries smells better. The moist air carries up to our noses the aroma of the soil’s bacteria and fungi, the growth and decomposition constantly taking place underfoot.

Here in Wisconsin, the humidity is typically pretty high, so I’m probably always getting low levels of soil smells, a calm constant that my brain tunes out. In dry places, the humidity increase of an incoming rain hits dramatically, picking up aromas that have been trapped by the dry air for days or weeks. The smells of the earth, which have been there all along, are transformed by the humidity into something we can perceive clearly, a precise change as the humidity crosses a key threshold.

Then, it rains. Rain has its own smells, and in researching this story, I found a cool story from Scientific American about the smells that the active rain produces. You can check it out here. The water washing across all of the surfaces in our environments bring other dormant smells into the air and our perception. Just like a wet dog smells much more strongly then the same dog did moments before, when he was dry, moisture increases the power of smell.

When it rains all day, like today, it’s easy to tune out the increased aroma of the world. When a summer thunderstorm blows through, as the pouring rain slows and you step outside, the air always more, more fresh, more acidic, more complicated, depending on your environment. But it always smells more. Living in the desert, I learned to love rain, because it was a special occasion, but also because of how powerfully it changed the sensory environment. Today, I’ll try to appreciate the aromas in the air as I bike home in the drizzle.

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