I’m 5 years old, toddling behind my parents after a thunderstorm. They point at the sky: ‘There’s a rainbow!” I can’t see it. My brother grabs my head saying “Look, Kelly. It’s right there.” I still can’t see it. I smell car exhaust, violets, mulch and earth.

I’m 17 in a car with a boy who’s driving too slowly. Someone in this neighborhood just mowed their grass. When the fat drops start hitting my arm, I leave my window down. The air smells like watermelon, chlorine, cigarettes and earth.

I’m 27 and getting married. God, it’s hot. Everyone’s here, and our party clothes are getting sweat stains, but no one cares because we’re happy and drinking heavily. The toast goes one minute too long and the sky opens up. Our first dance smells like moonshine, sweat, basil and earth.

“If you drew a road map between the sense of smell and the memory and emotion centers in the brain, there would be an interstate highway between them,” says Charles Wysocki, a smell scientist at the Monell Chemical Senses Center in Philadelphia.

Our noses contain hundreds of receptors with millions of olfactory cells that recognize tiny pieces of the molecules that we inhale. But it’s our brains that draw up moments of the past.

The sense of smell brings on the strongest memories, Wysocki says.

He’s right; I can recall an old friend’s face, but it’s dim and fuzzy around the edges. The concert from last week has faded into a swirl of sounds in my mind. But one whiff of hot, sticky rain, and suddenly I’m stupid — I mean 14 — again, greeting the storm from the roof.

Before it hits the ground, rain is just water — it has little to no smell, much less any highly nostalgic ones. But after the drops hit the earth and interact with dirt, rocks and plants, the fragrance surges up.

This smell actually has a name. The sweet grassy scent is called petrichor, from the Greek words “petra” meaning stone, and “ichor” referring to the essence that flows in the veins of the gods. Literally: “essence derived from stone.”

The mechanism behind the smell isn’t terribly romantic, but it’s comforting, humble and natural.

“The earth is doing what it’s supposed to be doing,” Wysocki says. “Composting.”

The moisture excites groups of bacteria in the soil, which start breaking down organically rich substances like leaves and animal waste. Nutrients return to the soil, while the rain and wind scatter millions of bacteria spores into the air.

When these microbes die, they release a musty earth-smelling chemical called geosmin (it’s also the cause of the earthy taste in beets.)

One out of every 10 people, however, can’t smell geosmin, which is the main contributor to petrichor. But enough smells are churned up during a good rain storm that your nose will be plenty stimulated.

For instance, during a thunderstorm, lightning produces ozone, which has a sharp, bleachy smell, strangely like a photocopier that’s been running too long.

What you smell when the first drops of rain hit asphalt after a long dry spell isn’t necessarily geosmin either.

In urban areas where there’s not much open soil, you smell pockets of proteins containing miniature molecules of city-odor like particles of street food, sewage, gasoline and salt wafting off the tarmac.

During dry periods, tiny smell-laden molecules fold up on themselves. They lie curled up on the hot concrete, waiting for rain to unwrap and release them into the air.

The smell is stronger if the weather has been dry for a long time. “If you had rain yesterday, it’s already done its thing,” Wysocki says. The molecules have been unwrapped, the bacteria have quieted and the geosmin has scattered.

These bacteria live and die on every continent. The same sweet smell of rain triggers memories for people in the rainforest of South America, the deserts of Africa, the Australian outback and the cities of India. I can be in some strange city across the ocean, a thousand miles behind me, and still smell home.

I’m 28. I’m lying in bed in Carrboro, N.C., with the windows open. It’s been a long, hot day, but the wind begins to stir. It tosses in molecules smelling of the past and hinting of memories I have yet to make. The rain is coming.