Petrichor is the earthy fragrance produced when rain falls on soil or stone after a long period of warm, dry weather. It comes from the Greek “petra” meaning stone, and “ichor”, the fluid that flows in the veins of the gods in Greek mythology.
Petrichor was named by Australian researchers in 1964, who described it as a combination of plant oils and the chemical compound geosmin which are released from the soil when it rains. Geosmin is a bicyclic alcohol with the formula C12H22O, a derivative of decalin.
It was also romanticised by Shakespeare in a sonnet:
Love comforteth like sunshine after rain,
But Lust’s effect is tempest after sun.
Love’s gentle spring doth always fresh remain;
Lust’s winter comes ere summer half be done.
Love surfeits not, Lust like a glutton dies;
Love is all truth, Lust full of forged lies.
In 2015, scientists from Massachusetts Institute of Technology (MIT), USA, used high-speed cameras to show how the smell of rain gets into the air. Raindrops trap tiny air bubbles as they hit the ground. The bubbles then shoot upwards through the raindrop and erupt into a fizz, producing extremely fine liquid droplets that stay suspended in the air as aerosols of scent.
The smell of rain is strongest when light rain falls on sandy or clay soils. The speed of the raindrops during heavy rain makes it harder to trap the air bubbles that produce petrichor. Some scientists believe that humans developed a liking for the smell of rain as our ancestors depended on rainy weather for survival.
The MIT high-speed cameras, below, show raindrops landing and the emergence of petrichor aerosols as tiny white flecks.