The pre-rain smell is ozone. Lightning in the clouds splits nitrogen and oxygen gas into single atoms, which recombine into things like nitric acid and ozone. Downdrafts and the first drops of rain carry ozone to the ground, where we experience it as a sweet, lightly acidic smell. As the rain starts falling, drops of water strike plants and the ground and liberate organic compounds and aromatic oils, splashing them into the air as aerosols. Once the soil and dead leaves on its surface become wet, bacteria begin to produce geosmin, an alcohol that’s the signature musty-basement smell of decaying plant matter. In the countryside, these fragrant compounds come from decaying organic material that becomes attached to rock surfaces during dry spells and recombines with elements from the rock to form fatty acids, oils, and alcohols. When a raindrop lands on a porous surface, it flattens out, while air bubbles from the pores and compounds from the soil are drawn into the droplet. These tiny bubbles, like those you see in soda, burst out of the splashing drops as aerosol clouds, wafting their petrichor scent into the air.