Have you ever gone outside on a cloudy day and smelled that it was going to rain? Or, after a rainstorm, noticed that there was a certain smell in the air?
That pleasant scent is called petrichor, and it’s pronounced petra-kor. It was coined by CSIRO mineralogists Isabel Joy Bear and Richard Thomas in 1964. It’s from the Greek petros meaning stone and ichor, meaning the fluid that flows in the veins of the gods.
Petrichor is the scientific name for the smell of rain, but how is that smell created? This article breaks down the chemical processes behind the smell of rain.
Even without checking the Bureau of Meteorology website or app, you can often tell it’s going to rain by going outside and smelling the air. That sweet, zingy and pungent smell that’s hanging in the air is ozone (chemical formula O3).
Ozone is a naturally occurring gas in the atmosphere. Its name is derived from the Greek word, ozein, which means smell. Electrical charges, such as those from lightning or man-made sources, split the atmospheric oxygen molecules into separate atoms. These free oxygen atoms combine quickly with other oxygen molecules in the air to form ozone. Ozone molecules are carried down from higher altitudes to nose level by a storm’s downdraft.
Petrichor comes from plant oils that have accumulated over dry periods—primarily in plant leaves. Secreted oils settle into the pavement or soil and are released when disturbed by the rain. The smell of petrichor is stronger after long periods of drought as the amount of oils have built up over long periods of time.
The smell of petrichor has been described as earthy and musky. Before the Australian scientists identified the smell, it was being used in an Indian perfumery. They called the scent matti ka attar or earth perfume and used it in sandalwood oil.
When it stops raining, a damp smell lingers in the air. This smell is sharp and strong but also earthy. This smell comes from organisms called geosmin.
Gram positive bacteria called actinomycetes live in the soil. When it’s not raining, airborne molecules of decomposing animal or plant matter attach themselves to mineral or clay surfaces within the soil. Actinomycetes get to work on the decomposing matter and produce geosmin, which is released into the air when drops of water hit the ground.
The odour of geosmin can be detected by animals as well as humans. In fact, lots of animals are sensitive to the molecule, but humans are especially sensitive and can smell it even when there are only tiny amounts of it in the air. The human nose is so sensitive that it can detect airborne geosmin at less than five parts per billion (ppb).
However, despite humans being drawn to the smell of geosmin, they generally dislike the taste. We perceive the presence of geosmin in foods as a slightly off-flavour. It is commonly known to cause a damp taste in wine and it gives beetroots their potent, earthy taste.
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