Smelling the Rains Down in Africa
on Jan 30, 2024The summer rains have rolled in at Sabi Sabi, which is a welcome relief from the blazing heat for guests, guides, and animals alike. Due to our proximity to the East Coast of South Africa and Mozambique, rains can be rather unpredictable when they decide to fall in Africa. When they do, however, it is a magical time, culminated by the flowing of the Msuthlu River which is always a beautiful sight as it winds its way through the reserve. The senses are enthralled by the sight of falling rain, sounds of water flowing and raindrops splashing in puddles. There is usually one unanimous thing that guests and guides note when the rain falls, the smell. The smell of rain is distinctly unique, but the question then arises, “How? Water has no smell so how can rain have an odour? What are we actually smelling? Why is it so strong and how can we smell a storm coming from miles away?”
Petrichor is a word you would most likely never have heard of, but it is the name of the scent that is produced when rain falls on dry earth. The rain itself does not carry the smell, rather it is made up of three phenomena occurring at the same time and is carried on the wind that is regularly associated with a storm. These phenomena are ozone, bacteria and plants, which all provide a different element, that when combined creates the immediately distinct smell of rain.
Ozone
This gas, most closely associated with protecting the earth from the sun’s harsh ultraviolet rays as the ‘Ozone Layer’, in fact, has an extremely strong smell, but is not from the aforementioned layer but rather is created. Lightning strikes provide an enormous amount of energy, enough to split molecules, namely oxygen and nitrogen, each molecule is made up of two atoms. These will quickly reform into stable molecules but not always as oxygen and nitrogen molecules, some will reform into ozone, which is three oxygen atoms and nitric oxide, a nitrogen and oxygen atom that is a toxic gas. Not a vast amount of ozone is made in this way but is enough to clean the air of other smells, which then allows other smells to have an even greater effect.
Bacteria
These make up most of the smell of Petrichor. A microscopic bacteria known as Streptomyces, found abundantly in soil, produces a chemical compound called geosmin. This natural oil is secreted by the bacteria within the soil. When water enters the micropores of the soil, small bubbles are formed. The oil and water mix and gasses are released as an aerosol into the atmosphere. The smell of geosmin is strong to all animals, but humans are particularly sensitive perceiving it as a strong earthy odour. Humans are so sensitive to it that we can detect five parts of geosmin in over a trillion parts of air. To understand how sensitive that is, it would be the same as taking a single-car garage, filling it with beach sand and only five grains of black sand, and instantly being able to identify the five black grains. It is the reason geosmin is being used in the fragrance industry.
Plants
Geosmin is not only due to microscopic bacteria, but also produced in small quantities by plants, and is the source of the smell of many plants. The compound is found in the tiny hairs on the leaves. When water droplets land on the leaf, the hairs are damaged or broken, this then releases the scent into the air.
The smell of rain is then much more complex than one would have first thought, even the term, “the smell of rain” is not entirely correct. It is the sequence of events due to the rain that gives off the beautifully fragrant earthy smell we all enjoy so much. While the smell is very pleasant, the taste however is not as pleasant and that is why people do not enjoy the taste of beetroot. The reason we are so acutely sensitive to geosmin however is still a mystery, with no real definitive answer. So, should you find yourself mid-storm in the African bush or downwind of an impending storm, take a moment to appreciate the petrichor, and all that is happening to produce the wonderous odour.
Blog by Devon Jansen (Selati Camp Ranger)
