Northern Ireland

Rain: The science behind the sweet smell of a summer shower

Umbrellas at Royal Portrush Image copyright GLYN KIRK/afp/getty images
Image caption Golfers and spectators were caught in summer showers at The Open in Royal Portrush

Does rain have a smell?

It might seem like a strange question and, by itself, there is no scent from a spatter of rain.

But after a hot, dry spell, a pleasant earthy odour can often be detected in the air when rain arrives.

The name given to this scent is petrichor.

The smell is a combination of oils which come from plants during dry spells and bacteria which live in the soil.

Image copyright PAcemaker
Image caption A dog and its owner caught in a surprise shower

The combination is known as geosmin which contributes to the petrichor scent.

Sometimes it can be detected before rain arrives - humidity rises, moistening the soil and more geosmin is formed.

Petrichor: The term:

Coined by scientists Isabel Joy Bear and Richard Thomas in their 1964 article "Nature of Argillaceous Odour", published in the journal Nature.

The word was coined from Greek petros, meaning "stone", and ichor, meaning "the fluid that flows in the veins of the gods"

They capture tiny air bubbles containing the chemicals on the surface, which then bounce up into the air and burst releasing the geosmin and petrichor scent aerosols.

Image copyright CHUCK NEELY
Image caption A lightning strike in Belfast

Another occasion when you can smell rain is when lightning in a thunderstorm spits oxygen and nitrogen molecules to form nitric oxide.

When combined with other elements in the atmosphere they form ozone, which has more of a chemical smell.

In either situation, if the wind is coming from the direction of approaching wet weather, some people say they can smell that rain is coming before it arrives.

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