There’s a really interesting article on the BBC News site discussing why rain smells so good.

The smell of rain hitting the ground releases a smell that has been dubbed ‘petrichor’. It was named by Australian researchers in the 1960s, and the smell is produced by bacteria in the earth.

But here’s the twist that will interest wine lovers: Petrichor is the same as Geosmin, which is a wine fault. The same bacteria that produce it in the soil also invade grape clusters during wet weather and can produce this off-flavour. While we like it when the rain falls on dry ground, but it’s a problem when it’s in our wine.

Geosmin smells a bit musty and earthy, like beetroot and freshly turned earth.

It was first identified geosmin as a wine fault back in 2000. Darriet and colleagues studied a set of wines with doors they described as smelling of freshly tilled earth and damp cellars in red and white wines of different origins. They used gas chromatography−mass spectrometry to identify the chemical culprit as geosmin (trans-1,10-dimethyl-trans-9-decalol), which is a sesquiterpene. Penicillium expansum was the main microbial culprit in geosmin production, but on its own it can’t produce geosmin in grapes: it needs to have the complementary action of Botrytis in order to do this.

(For more, see Darriet P, Pons M, Lamy S, Dubourdieu D 2000 Identification and Quantification of Geosmin, an Earthy Odorant Contaminating Wines. J Agric Food Chem 48:4835–4838 and La Guerche S, Chamont S, Blancard D, Dubourdieu D, Darriet P 2005 Origin of (-)-geosmin on grapes: on the complementary action of two fungi, botrytis cinerea and penicillium expansum. Antonie Van Leeuwenhoek 88:131–139)

But things are never simple. What can smell nice in one context, smells bad in another. And geosmin is being used now as a component in some fragrances.