The Smell of Rain: before, during, and after.
Have you ever walked outside and been able to smell an approaching rain storm? You probably weren’t just imagining it. There are scientifically identifiable scents that come before, during, and after rain. Some of these beautiful and evocative smells even have their own names.
[Can you smell it? Photo by dcrock. Used with permission]
The first smell to arrive, before the storm has hit, is ozone. Molecules containing oxygen are split apart by the storm’s lightning, and the individual oxygen atoms recombine to form ozone (O3). This ozone is carried down to the ground by vertical winds (the ‘down-draft’) and pushed ahead of the storm. Ozone has a sharp smell, similar to chlorine. I just took a break from writing this article to go outside with dcrock and check out a passing storm – sure enough we got a blast of sweet, sweet ozone!
Ozone is always present in the atmosphere, where it is produced with the help of ultra-violet light from the sun. In the upper atmosphere the individual oxygen atoms come from molecular oxygen (O2) being split apart, whereas in the lower atmosphere (where lightning occurs) it is nitrogen dioxide (NO2) that is split. So you have the following reactions:
O2 + sunlight → 2O OR NO2 + (sunlight or lightning) → NO + O
and then: O + O2 → O3
Down and out
The process by which ozone is carried to the ground and pushed ahead of the storm requires a little explanation. When thunderstorms form, falling rain and hail can evaporate and rob the surrounding air of heat, causing it to get very dense and sink. This ‘down-draft’ can get quite fast, and when it hits the ground it has nowhere to go but outwards. Here is a radar image of a thunderstorm that demonstrates this beautifully:
[Image from the National Weather Service (USA)]
The colors indicate the amount of rain (or raindrop-sized things) reflecting the radar signal. The storm is at the top in red, and the cold air is spreading out ahead of it as a ‘gust front’. The front, which is mostly dry, is detected by the radar because it concentrates a lot of dust and bugs! Here is an animated version. It’s this gust front that carries the fresh-smelling ozone ahead of the rain storm.
“Petrichor” – written in the stone.
The smell during rain is more geological in nature. Rain on hot concrete is a particularly vivid smell for me, and it has to do with raindrops liberating organic molecules from the grooves and pore spaces in the rock. The same smell can come from all sorts of other rocks and from decaying organic matter exposed to the rain. This smell has been named ‘Petrichor’, a word deriving from the Greek ‘petri’ (stone) and ‘ichor’ (divine blood).
The word was coined by two Australian scientists, Bear and Thomas, in a very interesting paper from 1964. They set out to determine the origin of this lovely stench by studying rocks from all around Victoria. Having collected a variety of rocks that exhibit the smell (including volcanic scoria, bauxite, and kaolinized granite, for those of you who get off on that kind of thing), they heated the samples to 600°C in a furnace in order to remove any trace chemicals. By design, the rocks at this point had no scent, with or without water. But after leaving the rocks exposed to dry air for many months, they found that the rocks then gave off the petrichor smell when wet! Clearly the origin of the smell was coming from the air itself, biological contamination having been ruled out.
Bear and Thomas concluded that petrichor originates from substances in the atmosphere (mostly organic) that accumulate in the pore spaces of the rocks. After isolating the petrichor as a yellowish oil, chemical analysis revealed that its composition was remarkably consistent across all samples. They found that it included: elemental sulphur (from hydrogen sulphide in the air), cyclic bases such as pyridine and quinoline, nitrophenols, carboxylic acid, and other miscellaneous hydrocarbons.
[The chemical structure of pyridine, a component of petrichor]
In a follow up study, Bear and Thomas studied the effect of petrichor on the germination of seeds. Applying minute amounts of the oil to seeds from cress, mustard, and various grasses, they found that the seeds took a significantly longer time to germinate. So it would appear that plants release petrichor as a way of preventing their seeds from germinating during unfavourably dry conditions. When it rains, the petrichor blanket is lifted, and the seeds can begin their lives in healthy, wet weather.
Geosmin, the scent of the Earth.
Walking through a damp forest after rain has a very unique smell. This one comes from bacteria in the soil, and is known as ‘geosmin’ (literally “Earth smell”). Studies have identified geosmin as one of the metabolic by-products of a class of bacteria known as Actinobacteria. Geosmin is responsible for many earthy tastes in alcohols and foods such as fish and vegetables.
Why this smell is released during rain is unclear. It has been hypothesised that, by signalling the presence of water, the smell is a method of attracting thirsty animals which unknowingly help disperse the bacterial spores. Sneaky.
Links and stuff
Scientific American article on storm scents
I have to acknowledge that this article exists. It’s a great article, but I like mine more.
‘Nature of Agrillaceous Odour’
Original paper by Bear and Thomas (Paywalled, no abstract)
‘Petrichor and plant growth’
Follow-up paper from Bear and Thomas.