Exmoor Lockdown Diary 30 - Petrichor - The Word to Describe the Smell of Rain

There's an opportunity to sniff petrichor at the moment. What on earth is petrichor, you may ask?

I’ll admit I had never heard of it until those ever-helpful folk down at Exeter’s Met Office told me about the word.

Because petrichor is that curious aroma you can detect when rain has fallen on warm dry ground. You rarely sniff it in winter - but you almost always can smell it in spring and summer after a quick rain or thunderstorm puts an end to a period of sunshine and heat.  

The Met Office explains that the phrase was coined by two researchers at the Australian CSIRO science agency in a 1964 article for the journal Nature.

“In their research, rocks that had been exposed to warm, dry conditions were steam-distilled to reveal a yellow coloured oil that had become trapped in the rocks and soil, a substance they discovered was responsible for the smell,” explains the Met Office.

Apparently the source of this oil is a combination of oils secreted by plants during dry weather  and chemicals released by soil-dwelling bacteria.

“When a higher humidity is experienced as a precursor to rain, the pores of rocks and soil become trapped with moisture forcing some of the oils to be released into the air,” say the Met Office scientists. 

And you are most likely to smell it when rainfall arrives after a spell of warm dry weather. Dusty soils trap tiny air bubbles on the surface which then shoot upward when the raindrops hit. A bit like bubbles rising from a glass of champagne. The bubbles then burst, throwing aerosols of scent into the air where they are then distributed by wind.

So now you know… That curious, earthy, rather bitter, summer rain smell has its very own name. 

And it is, after all, the season for smells and aromas. Agreeable smells that are more apparent in the heat of late spring and summer than the wind-whipped chill of winter.

There are the lovely aromas of fresh mown grass for a start. It is most apparent at this time of year when the weather is muggy and there’s not a cooling breeze anywhere to be felt. And, as the sun burns down upon the earth it turns the grass to hay - and then we’re treated to another heady collection of sweet, earthy, almost savoury aromas that hint of the dairy and future cheeses.