If you love freshly fallen snow like me, you definitely know that moment an hour or two before the first flakes fall, when we say we can smell it coming. It’s a clean, metallic smell in the air that promises those beautiful flakes.

But the truth is that you aren’t smelling anything at all.

Smell of snow is a mental thing

What you're actually sensing is your trigeminal nerve. It’s the same facial nerve that causes menthol to feel cold or hot pepper to feel like a burn in your nose and mouth.

(MORE: Why noises sound louder during the winter)

When cold, dry air rushes in ahead of a winter storm, along with the pressure shift that comes with it, your trigeminal nerve picks up on that cold-and-pressure combination and sends that “it’s about to snow” signal to your brain, even though no odor molecule is actually involved.

And because you've felt that specific sensation right before snow so many times, your brain has associated this feeling with snow coming.

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Cold air smells more muted

Cold, winter air is usually dry air, which is bad for actual smelling.

The moisture in your nose that helps trap and process odor molecules dries up in low humidity, which dulls your real sense of smell. At the same time, cold, still air holds far fewer volatile organic compounds, which are the molecules that noses detect.

(MORE: Winter weather prep: Keep these three things by your front door)

So part of what people are calling "the smell of snow" is really the absence of the usual smells of a warmer, more humid day.

Not to be confused with the smell of rain

Many of us are also familiar with the smell that comes during a rainstorm. That's a real smell, but from something completely different: a combination of ozone, plant oils and geosmin.

This distinct smell is called petrichor, and it's caused by a mix of chemical compounds released from the ground and air when water hits dry surfaces.

So the "smell of snow" and the "smell of a thunderstorm" are unrelated systems. One is triggered by the trigeminal nerve and the other is an actual smell, but both can lead to the sense of “I can smell the weather.”