Sweet with the evening rain

There are few scents quite so evocative of summer as that of rain upon warm earth-but why does it smell so good? Martin Fone explores petrichor

-  Martin Fone

SCENT of earth, sweet with the evening rain,' wrote Edith and Saretta Nesbit in All Round The Year (1888). One summer, a spell of fine, dry weather was suddenly punctuated by a short, sharp burst of rain. As I walked in the garden, I was conscious of an intensely earthy, fresh and almost sweet aroma, as if the shower had awoken the earth and the plants and they were rejoicing by giving off this distinctive fragrance. It is one of the most evocative and invigorating smells of summer-you can even buy it in a bottle.

For several generations, perfumiers in Kannauj in India's Uttar Pradesh have captured and absorbed the scent in sandalwood oil, in a process that takes some 15 days to complete. Having baked clay in a kiln, they immerse it in water held in copper cauldrons called degs, sealed with earth. A cow-dung fire is lit under the cauldron and the resultant vapour travels through bamboo pipes to condense in receivers, over a base of oil, to form what they call matti attar or earth perfume', an essence released by the interaction between earth and water.

Although James Joyce didn't seem the sort of chap who would splash a bit of perfume behind his ears, he, too, recognised that the fresh smell after a shower of rain was due to some reaction with the earth. In 1916, he wrote, in A Portrait of the Artist as a Young Man, how 'the trees in Stephen's Green were fragrant of rain and the rain-sodden earth gave forth its mortal odour, a faint incense rising upward through the mould from many hearts'. Curiously, it took scientists until 1964 to understand quite what was going on.