Smelling Rain

I have recently discovered that I can smell approaching rain, when I am outside, sometimes to the point of being able to work out how far away it is. This comes as a great surprise to me not least because my sense of smell is at best undeveloped, and frequently non-existent.

Thanks to my nose and lung problems over the years, I am frequently unable to even breathe through my nose. I can often smell in reverse by taking a deep breath and blowing out through my nose, or blowing my nose after drinking a good wine or eating chocolate, but smelling things through my nose is pretty variable. If I need to check the milk is okay, I get a second opinion! However maybe thanks to the work I have been doing with rain, I have discovered I can actually smell it before it arrives. So I have started to try and understand this process.

There are three possible things I could be smelling. One is from the Air above us, Ozone, particularly associated with thunderstorms. This is because lightning can split oxygen and nitrogen molecules to form small amounts of nitric oxide – which then re-reacts to form ozone. These ozone molecules, smelling similar to chlorine, may then be blown on the winds that precede a storm. That assumes a storm is present however, which I’m not sure is always the case!

The second and third scents come from the Earth and are Petrichor, the oils released from rocks formed from a variety of plant and animal sources, plus the compound geosmin, which is created by soil-dwelling bacteria. Most investigations have focused on the fact that these scents are released when rain drops hit the soil or rocks, sending spores and oils into the air and giving that fresh, earthy smell so many people love after it has rained. But this doesn’t explain why I should smell anything before it rains – especially as I have never smelt anything particular afterwards, just noticed the change in the feel of the air. A more recent theory says that as the humidity increases, the petrichor is released from pores in the rocks or soil, thus preceding the rainfall. This may be possible, and it has been suggested it is the cause for cattle becoming restless before rain. However, I don’t get the ‘smell’ of rain on a humid day! I only get the ‘smell’ shortly before actual rain.

Then last year researchers from MIT managed to film water drops landing on different surfaces and replay it in slow motion – and found that the Petrichor effervesces like champagne from porous surfaces, when the rain is light. These aerosols can then be carried on the winds in front of the rain for long distances. When you consider that a front is not just the mile or so wide it seems to us on the ground where the weather is, but often hundreds of miles wide all the way up through the atmosphere, then it makes sense to be smelling approaching rain before we can see it. It would also explain how other bacteria become airborne and cause certain diseases to spread.

However, I still can’t help wondering, given how poor my sense of smell actually is, whether there is some intuition or other sense at work rather than simply smelling. Many animals are aware of approaching rain – and from an evolutionary perspective this is a useful thing as following the smell back would lead to where it had rained and food will be growing. (Camels finding oases is one suggested example.) It helps us know to find shelter, or to put out pots for harvesting rainwater. In my own life, living in a modern world with waterproof clothes, houses, and taps I can turn on and off at will, I don’t often need these skills – and yet as the Earth becomes more unbalanced these may become the skills of the future.

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