How rain gets its smell

There is something unmistakable about the damp, earthy smell that lingers in a forest after rainfall. Yet until now, there has been some mystery as to how it is created.

Researchers at MIT #link:http://bit.ly/1Bk2fxC:have deciphered# the mechanism that leads to the release of this loamy, woody scent. And they’ve already given it a name. #link:http://www.oxforddictionaries.com/definition/english/petrichor:Petrichor# is the official term used to describe the smell that results from rain hitting dry ground following a long spell of warm weather.

With the help of high speed cameras, the researchers were able to see what happens when a raindrop makes its landing. Observing in slow motion, they noted how it starts to flatten while simultaneously trapping tiny air bubbles from the surface. These bubbles rush through the raindrop, just like those in a sparkling glass of champagne - and finally burst out into the air.

These aerosols, which can contain aromatic components found in the soil, are then distributed by the wind.

But not all rainfall has the same effect. The scientists observed that heavy rainfall led to the release of fewer aerosols, while more were set free during light or moderate showers.

Would you like to see for yourself how the smell gets into the air? The scientists captured their experiments in a time-lapse video:

Although it is interesting to know what gives the rain its characteristic smell, it was not a quest for that knowledge that prompted the original research.

Besides the aromatic elements released from the soil as a result of rainfall, bacteria and viruses held in the ground could also find their way into the air.

The research team is currently carrying out further experiments on surfaces contaminated with gut-bacteria such as E. coli, to see whether these pathogens are dispersed via the same mechanism as petrichor. They hope their results might give an insight into how soil-born diseases spread.