A warm summer day in the swimming pool, or a barbecue... and suddenly a downpour or thunderstorm. Sometimes it's over right away, and yet a certain smell rises to the nose. Everybody knows this special smell of rain, which falls on dry earth. For this smell there is also a name, and that again only few people know.
...is the name of this natural perfume. Yet rain itself does not smell, because the raindrops consist of water, and that is basically odorless in pure form. What is it that you smell? It is what the rain does on and with the ground! Thereby the smell varies depending on the surface – in the forest and on a meadow it smells differently, than in the city and on asphalted surfaces. And yet the causes are the same in various degrees.
Petrichor is a combination of words from the ancient Greek pétrÄ (rock) and Ä«chá¹r (liquid that flows in the veins of the gods – thus also blood of the gods). This term comes from the two Australian researchers Isabel J. Bear and Richard G. Thomas, they were studying the interaction of rain and substances in the soil and published an article in the well-known journal Nature in 1964.
The main components of the aroma are geosmin and certain plant oils; in the case of thunderstorms, ozone can also contribute (formed by the lightning discharges). Geosmin is an alcohol and is produced by bacteria in the soil. It smells earthy-musty and is perceived by humans even in very low concentrations. Among other things, this substance is also present in certain vegetables, for example beetroot.
When a drop of water hits the ground, it reacts with the cocktail of substances on the surface. As in a champagne glass, the finest air bubbles form in the mini-puddle of water, which transport the substances upwards and then release them as aerosols into the surrounding air. These are then distributed further with the wind, which is why you can sometimes actually smell an approaching rain shower even before it starts.
Fig. 1: Aerosol formation in a raindrop; Source: Young Soo Joung
Petrichor is most effective after dry phases and at higher temperatures – in summer. And the smell is most intense when the rain is short-lived and the ground is not completely soaked. In continuous rain, the odor-forming substances are gradually washed out.
On the today's day only an attempt offers itself via garden hose, but already tomorrow Friday are again completely natural rain showers and thunderstorms on the program. Perhaps one perceives this smell now still a little more consciously, and knows then that it is called Petrichor.