Fragrances fill our noses when we enjoy summer’s warm weather, smells that remind us of outdoor adventures, trips to the beach, and playing on the lawn. Just one whiff can even make us nostalgic for youth’s carefree days. But what, exactly, is the science behind those aromas? Here are some of the season’s best perfumes explained.

1. AT THE SEASHORE

Bacteria are largely responsible for that distinctive seaside smell. As they chow down on phytoplankton—microscopic plants living in the ocean—the bacteria convert phytoplankton’s dimethylsulfoniopropionate to dimethyl sulfide, or DMS, which wafts into the air. Some seabirds can also recognize the compound's pungent odor before they even leave the nest.

2. IMPENDING TEMPEST

Before a storm, electrical charges break nitrogen and oxygen molecules into separate atoms, which can then recombine to form nitric oxide. That molecule reacts with other chemicals in the atmosphere to occasionally produce ozone. A storm’s downdrafts send those molecules that smell like chlorine into the air we breathe. 

3. AFTER THE STORM

After a rain, we nose a range of scents called petrichor that infuse the air. They result from molecules that accumulate on surfaces during dry periods (plants secrete oils that permeate soil and rocks, for example). When droplets come in contact with those molecules, the bouquet of smells (not all good) wafts into the atmosphere. A neologism coined by mineralogists in the 1960s, petrichor roughly means "the divine essence of stone" in Greek.

4. LAYERS OF THE LAKE

Many lakes emit a distinctive sulfurous smell as summer cools to fall. The fetor that can accompany the switch comes from warmer bottom layers of water swapping places with cooler top layers, burping up gas that accumulated near the lakebed. Over the summer months, bottom layers can become anoxic, or void of oxygen, so when algae falls to the lake bottom and decomposes, hydrogen sulfide gas (think rotten eggs) builds up below until the lake turns over.

5. CAMPFIRE COMBUSTION

Perhaps one of the most iconic smells of summer is the campfire. When you burn wood, a process called pyrolyzation, the carbon molecules lignin, cellulose, and hemicellulose—key parts of plant cell walls—break down into chemicals we find aromatic. For many people preparing to put food over that fire, the chemical composition starts mouths watering—and that's even before the toasted marshmallows get a chance to trigger the Maillard reaction.

6. WATERMELON SLICES

One fragrance undeniably linked to barbeques and sunburns is watermelon, a scent extremely difficult for scientists to replicate because of its complex flavor and aroma molecules. Aldehydes are the organic chemical compounds responsible for the fruit’s sweet smells. They're often found in perfumes. Because the aldehydes in watermelon are unstable, food chemists haven’t been able to turn them into an additive—yet.