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Bank Holiday Word of the Day

Petrichor - The smell of rain on dry ground.

It was named by two Australian researchers in an article in Nature in 1964, who discovered that the smell is an oily essence that comes from rocks or soil that are often (but not always) clay-based. The oil is a complicated set of at least fifty different compounds, rather like a perfume. It turned out that the oils are given off by vegetation during dry spells and are adsorbed on to the surface of rocks and soil particles, to be released into the air again by the next rains.

But that is not all - if you ask; "What causes the smell after rain?" - the smells people associate with rainstorms can be caused by a number of things. One of the more pleasant rain smells, the one we often notice in the woods, is actually caused by bacteria! Actinomycetes, a type of filamentous bacteria, grow in soil when conditions are damp and warm. When the soil dries out, the bacteria produces spores in the soil. The wetness and force of rainfall kick these tiny spores up into the air where the moisture after a rain acts as an aerosol (just like an aerosol air freshener). The moist air easily carries the spores to us so we breathe them in. These spores have a distinctive, earthy smell we often associate with rainfall. The bacteria is extremely common and can be found in areas all over the world, which accounts for the universality of this sweet "after-the-rain" smell. Since the bacteria thrives in moist soil but releases the spores once the soil dries out, the smell is most acute after a rain that follows a dry spell, although you'll notice it to some degree after most rainstorms.

The word comes from Greek petros, a stone, plus ichor, from the Greek word for the fluid that flows like blood in the veins of the gods. So the word means something like “essence of rock”

Comments

So that would be the word that your fellow Wiltshire resident, Terry Pratchett, was looking for at the end of LAst Continent. At last I know what it is.

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