First Rain Storm of the Season
That wonderful smell of rain on wet asphalt, is actually a chemical reaction. The air smells good after a rain, and much of the reason is a thing called “petrichor.”
“Petrichor,” is constructed from Greek petra, meaning “stone”, and ichor, the fluid that flows in the veins of the gods in Greek mythology (Blood of the Stone). The smell itself comes about when increased humidity – a pre-cursor to rain – fills the pores of stones (rocks, soil, etc.) with tiny amounts of water. While it’s only a minuscule amount, it is enough to flush the oil from the stone and release petrichor into the air.
Some plants secrete oils during dry periods that accumulate in dirt and rocks, and when it rains, these oils are released into the air. As the water activates the oils it invariably hits you: the sweet, fresh, powerfully evocative smell of fresh rain. The second reaction that creates petrichor occurs when chemicals produced by soil-dwelling bacteria known as actinomycetes are released. These aromatic compounds combine to create the pleasant petrichor scent when rain hits the ground. Studies have revealed that the human nose is extremely sensitive to geosmin in particular—some people can detect it at concentrations as low as 5 parts per trillion. You may also smell this if you are turning soil over in your garden.
Another scent associated with rain is ozone (O3). Some people say they can smell a storm coming. This can occur if wind pushes the ozone ahead of the storm. During a thunderstorm, lightning can split oxygen and nitrogen molecules in the atmosphere, and they in turn can recombine into nitric oxide (NO). This substance interacts with other chemicals in the atmosphere to form ozone, which has a sharp smell faintly reminiscent of chlorine and it can carry over far distances from high altitudues.