The scent of rain on the desert floor after a long period of dryness is a pleasure. It's even sensuous. In fact, it has a scientific name, 'petrichor', coined by two Australian researchers back in 1964. The origins of the term are rooted in Greek and its mythology: petros means stone and ichor, the fluid that flows in the veins of the gods.
The odor is said to be a concoction of a variety of compounds. One source is plant chemicals that are trapped in the earth, or in plant leaf and bark cuticles. Rain water releases them into the air. Some plants exude oils during dry periods, which are then absorbed by clay-based soils and rocks. Water from rain will often signal plants to release oils from their cuticles, and droplets displace oils that are trapped on soil surfaces.
Many silicious and clay (argillaceous) materials are known to have odors that are released when wet. During a rainstorm and especially after a drought, compounds which accumulate over time in dry rocks and soil are mixed and released into the atmosphere. Falling water disturbs and displaces odoriferous molecules on surfaces, particularly on dry ones, and then carries them into the air.
In wet areas, a metabolic by-product of soil-dwelling bacteria (Actinobacteria) is secreted when they produce spores during dry conditions. These compounds attach to soil particles. The force of rain landing on the ground sends these spores and the odiferous compounds up into the air. The result is a distinctive scent, called 'geosmin'. The smell of geosmin is often the strongest during the first rain after a dry spell because the largest supply of spores has collected in the soil.
Another contribution to the scent of rain may be ozone, especially if there is lightning. A lightning bolt’s electrical charge will split oxygen and nitrogen molecules in the atmosphere. These molecules often recombine into nitric oxide (NO), which then interacts with other chemicals in the atmosphere to produce ozone. And this can be blown on the wind for miles.
But why are we so attracted to that smell of rain? Why is it like a siren that calls us and makes us smile? Anthropologist Diana Young of the University of Queensland, Australia, explored the connection between color and the smell of rain. While studying the culture of Western Australia’s Pitjantjatjara people, she observed that they associate the smell of rain with the color green. This suggests that there is a deep-seated link between a season’s first rain and the expectation of growth and associated game animals. Both are critical as a food source. She calls this 'cultural synesthesia', the blending of different sensory experiences on a society-wide scale due to evolutionary history.
For us in the desert and other arid-environments, the smell is also associated with a rebirth and awakening of life.
I can smell the drenched desert floor. It’s not the old heavy odor of wet wood, soggy black mud and swollen green grass of the north country. Nor is it the overwhelmingly sharp salty fish and frothy seaweed smell of the ocean coast. Instead it is an acrid aroma of hidden and dead ancient sea creatures whose shells remain to tell stories, a spicy phenolic scent released from plants that awaken only in the presence of moisture. A strong musky odor from eons of baked, blown and accumulated dust, sand, gravel and rocks encapsulates all these other smells into an undulating mixture that wafts into nasal cavities of warm and cold-blooded creatures. If you open your mouth, you can even taste it.
From those sensory channels, these odors are translated into mixed signals and stories in the brain. It is a discourse that knows no words, only ticklings deep inside that elicit subconscious memories and innate responses. - Excerpt from an essay by Elzi Volk (me), My Desert Smells Like Rain