Ah, the sweet smell of rain …

By Ellie Highwood

Despite all the rain of the past winter, there is something about rain that I have missed – its perfume. As we head towards spring, with daffodils all around us already, I am looking forward to the first time I get enveloped in the unmistakeable strong sweet fragrance that says ”it’s just rained here”. This smell has a name – “petrichor” from the Greek “petros” meaning  stone  and “ichor” meaning  the fluid that flows in the veins of the gods in Greek mythology. It turns out to be inextricably linked to my research area.

In my research concerning atmospheric particles (aerosols) I care about rainfall primarily because it washes aerosol out of the atmosphere, for example occasionally leaving red Saharan dust films on cars in the UK. This “wet deposition” is a challenge to represent in climate models because you need to get the rainfall pattern right as well as the details of the aerosol since different particles respond differently to passing through rain. When this isn’t right in climate models it can mean that aerosol either stays in the air too long and is transported too far away from source, or doesn’t get transported far enough. In the case of petrichor however, the rainfall is responsible for the emission of aerosol upwards into the atmosphere, allowing it to become airborne and therefore reach our noses – essentially carrying the smell of rain. Petrichor was first described in a Nature paper in 1964 by Bear and Thomas as being airborne molecules from decomposing plant or animal matter that during dry weather become attached to rocks or soil and then are disturbed when the raindrops hit the ground. In fact there are lots of smelly molecules on the Earth’s surface that can be disturbed by raindrops and carried into the air as aerosols. Volatile oils from plants and trees that have collected on rocks can also be vaporised by rainfall – some of these are the inspiration for room fragrances, washing powders and perfumes. The slightly musty, earthy tones of petrichor are provided by soil based bacteria called actinomycetes. These common filamentary bacteria spread their tentacles through damp warm soil pretty much the world over. When the soil and its resident bacteria dry out, spores and a compound called geosmin are produced. The moisture and simple force of rainfall releases spores into the air so that they can drift towards our noses. This is why the “it’s just rained” smell is often strongest after the first rain following a long dry spell, and also more noticeable in spring and summer. Incidentally, the geosmin smells pleasant in the context of earth and gardening, but the same substance when present in drinking water or indeed wine is not so desirable, and is an acquired taste when present in beetroot!

2016 02 25 Ellie Highwood Fig 1 petrichor

Petrichor is the name for the smell after rain has fallen. However, there is also sometimes a different but equally distinctive sharp smell that precedes rainfall. This is not produced by aerosol as such, but by gas phase atmospheric chemistry, specifically ozone production. It’s particularly noticeable ahead of thunderstorms in which electrical charges in the atmosphere split nitrogen and oxygen molecules into individual atoms. Atmospheric chemistry manages to combine some of the oxygen molecules into trios – the ozone molecule. The strong downdrafts associated ahead of thunderstorms can transport this ozone down to nose-level even when it is generated up in the clouds.

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The ‘wilderness path’ between two meteorology buildings on the University of Reading campus – drying nicely in the early spring sunshine, but always ready to resume its normal existence as a sea of sticky mud whenever it rains!

One of the fascinating things about weather and climate is the universality of some of the weather related experiences. The “smell of rain” occurs in poetry and song from across the world, thanks to the global presence of soil bacteria and their role in generating smelly aerosols.

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