The Petrichor effect.
Are you one of those people who loves the smell of rain? I am. It is such a distinctive, fresh aroma. But have you ever wondered why you can sometimes smell rain long before it arrives?
It is known as the Petrichor effect.
In 2015, researchers using high speed cameras discovered that drops of water release tiny clouds of particles when they hit surfaces like leaves, sand, and soil. When a raindrop hits an uneven surface, it flattens, trapping tiny bubbles of air found in the surface’s pores. These air bubbles shoot upwards and can burst out of the top of the water droplet.
If there is also a breeze blowing across the surface, these bubbles, which contain small amounts of aromatic particles of oil and dust, can be blown for miles by the winds ahead of the rainstorm. This allows you to smell the rain before it arrives.
Petrichor potpourri can come from vegetation and organic matter, such as decomposing plants or animals, but also fatty acids, alcohol, and hydrocarbons. Once a storm has passed, however, you are likely to smell geosmin — a metabolic byproduct of bacteria — which leaves an earthy, or musty, scent.
This effect occurs most often in the summer, especially after a long, dry spell when lots of dust and oils have accumulated on plants and on the ground. You are more likely to smell the rain if it is a light to moderate shower. When the rain is light, more of these bubbles become aerosols (airborne), producing a cloud of tiny, aromatic droplets within a few microseconds.
Heavy rain, on the other hand, which is typically accompanied by thunderstorms, does not allow enough time to make bubbles inside the raindrop. What you are likely to smell with an approaching thunderstorm is ozone as the storm’s downdrafts carry O3 from higher altitudes to the surface.
In 1964, scientists in Australia were the first to coin the term “Petrichor” for the smell of approaching rain.
Take a look at what happens to raindrops when they impact a surface during light, moderate, and heavy rainfall. It clearly shows why the Petrichor effect can’t happen with stronger storms.