As a ranger, I often encourage guests to use all of their senses whilst on safari. Not only is it important to admire the beauty of the bushveld through one’s eyes, but in my opinion, it’s important to engage more than just our sense of sight. Often after a long, dry winter, the arrival of the first summer rains bring much delight, not only to the animals but ourselves as well. The rainfall has brought more birdsong for us to listen to, it has filled our rivers, allowing us to take off our shoes and feel the water run past, and last but not least it has allowed us to smell a plethora of aromas.
Picture this, it’s a warm, dry day and we are in the early stages of the rainy season. There is an immense cloud build-up over the Drakensberg Mountain Range and you can see a blanket of rain coming towards you. You can smell it before you see it, can’t you? Most of you reading this article will know what I’m talking about when I describe that beautiful aroma that one smells when raindrops hit dry earth. That fresh, musky scent fills our nasal passages when raindrops make landfall and bring relief to barren soil or a hot tar road. Don’t let the title of this article mislead you, the rain itself has no scent, it is rather the moistening of the soil that speeds up the breaking down of dead or decaying matter by a group of bacteria, causing a certain compound to be released into the atmosphere that we smell.
Elaborating slightly further on the process without making it too complicated, the pleasant smell is attributed to a group of actinobacteria, more specifically Streptomyces, which is present all over the ground and in most soils. The earthy odour is known as ‘petrichor’ and it is in fact a result of us smelling an organic molecule, geosmin, which is released as the Streptomyces dies or is consumed by other bacteria.
But how does a molecule in the ground get into the air for us to smell after the rain?
In short, it becomes aerosolised. As the raindrops hit the ground, tiny little air bubbles trap the geosmin, then shoot up through the raindrop and pop out the top releasing the aerosols into the air. A similar example is watching the bubbles pop off the top of a freshly poured soda or Coca-cola, where you are able to then smell the coke. The wind then carries the aerosols across the land.
The word petrichor is derived from the Greek word, “Petros” meaning stone, and “ichor” meaning fluid that flows from the veins of the gods. An appropriate name and fairly apt description if you ask me. It was coined by two researchers in the 1960’s after they discovered that it was being captured as a fragrance and sold in Uttar Pradesh, India. The scent appeals to people because of its primal, primitive attributes and humans are particularly sensitive to it. In fact, human beings are more sensitive to the smell of geosmin than any other animal is, and this is a bold statement! Humans are more sensitive to the smell of geosmin than sharks are to the smell of blood. A shark can smell one part of blood per billion parts of water, whereas humans can smell geosmin at 5 parts per trillion. Sharks are designed to be able to smell blood to find food, humans must be designed to smell geosmin in order to find water.
The odour is especially evident after a prolonged period of dryness and it permeates the air in the early summer months of October and November. So the next time someone asks, “Don’t you just love that smell of the rain coming?” you’ll be able to tell them what it’s all about!