The Smell of Rain

Many people claim they can "smell a storm coming" (meaning they can sense when bad luck is headed their way), but did you know that this weather expression also has literal meaning?

It's true, there are some kinds of weather that actually do produce a unique smell, and we're not just talking the smell of flowers in spring. Based on personal accounts, here are some of the weather's recurring aromas, plus, the scientific reason behind them. 

When Rainstorms Wet Dry Earth

Rainfall is one of nature's most soothing sounds, but it's also behind one of the weather's most pleasing smells. Described as an "earthy" scent, petrichor is the aroma that arises when raindrops fall onto the dry soil. But, contrary to belief, it isn't the rainwater that you're smelling.

During dry spells, certain plants secrete oils that become attached to the soil, rocks, and pavement surfaces. When it rains, the falling water disturbs these molecules and the oils are released into the air along with another soil inhabitant; a naturally occurring chemical called geosmin that's produced by fungi-like bacteria. 

Had a recent rainstorm, but didn't have the lingering petrichor afterward? How strong the scent will depend on several things, including how long it's been since the last rainfall and rainfall intensity. The longer the geosmin and plant oils are allowed to accumulate during periods of dry weather, the stronger the scent will be. Also, the lighter the rain shower, the stronger the petrichor scent, since lighter rains allow more time for the ground's scent-carrying aerosols to float. (Heavier rains keep them from rising up as much into the air, which means less smell.)    

Chlorinated Clashes of Lightning

If you've ever experienced a lightning strike that's too-close-for-comfort or stood outdoors just before or after a thunderstorm, you may have caught a whiff of another rain-related scent; ozone (O3).

The word "ozone" comes from the Greek ozein meaning "to smell," and is a nod to ozone's strong odor, which is described as a cross between chlorine and burning chemicals. The smell doesn't come from the thunderstorm itself, but rather, the storm's lightning. As a bolt of lightning travels through the atmosphere, its electrical charge splits air's nitrogen (N2) and oxygen (O2) molecules apart into separate atoms. Some of the lone nitrogen and oxygen atoms recombine to form nitrous oxide (N2O), while the leftover oxygen atom combines with an oxygen molecule in the surrounding air to produce ozone. Once created, a storm's downdrafts can carry the ozone from higher altitudes to nose level, which is why you'll sometimes experience this smell before it starts storming or after the storm has passed.  

Unscented Snow

Despite some people's claims that they can smell snow, scientists aren't entirely convinced.

According to olfactory scientists like Pamela Dalton of Philadelphia's Monell Chemical Senses Center, the "smell of cold and snow" isn't so much about a particular smell, it's more about the absence of smells, as well as the nose's ability to sense that air is cold and moist enough for the weather to possibly turn snowy.

"We're not as sensitive to odors in winter... and odors aren't as available to be smelled," Dalton says.

Not only do smells not waft as easily when air is cold, but our noses don't work as well. The "smelling" receptors within our noses bury themselves more deeply within our nose, likely as a protective response against the colder, drier air. However, when cold air becomes more humid (as it does before a snowstorm), the sense of smell would sharpen ever so slightly. It's possible that we humans link this small change in smell to an oncoming snowstorm and hence, why we say we can "smell" snow.