Before it rains, there are usually some signs that appear, such as dark clouds gathering in the sky, an increase in humidity in the air, but some people can smell the rain before it falls, knowing that it is about to rain. Why do people have this kind of predictive ability?
According to a report by “Scientific American,” storms and other rainy weather can bring ozone from the upper atmosphere to the level of the nose. This phenomenon occurs before the rain actually reaches the ground.
Ozone is an allotrope of oxygen, and its name comes from ancient Greek, meaning “to smell.” This gas emits a fresh, clean odor that can linger for hours after the rain stops.
The “Thames Newspaper” reported that ozone is produced by lightning, and it may flow towards the ground before the thunderstorm clouds arrive. Some people can even detect ozone in concentrations as low as ten parts per billion (10 ppb) in the air.
Additionally, moisture in the air often arrives at the ground before the rain. At this time, spores produced by actinomycetes in the soil during dry periods are dispersed into the air, and people with a keen sense of smell can detect the earthy scent produced by geosmin, a smell that appears before rainfall.
After the rain, when the ground is wet, this moldy smell becomes particularly strong because a large number of spores are dispersed into the air at that time. In fact, geosmin is a strong signal of rain. Some cactus flowers even emit this smell to simulate rain, deceiving bees into thinking they have found a water source and enticing them to visit.
After the rain, if the ground has been dry for a long time, it often emits various smells. These smells are collectively known as petrichor, originating from the aforementioned ozone and geosmin, as well as from volatile substances released into the air.
Australian scientists were the first to describe this phenomenon. They found that volatile substances released by plants and trees adhere to the ground and are released during rainfall.
According to the website “Space Daily,” early humans may have viewed geosmin as a signal of a water source. The smell of damp soil drifts with the wind, and for people in arid plains, having a keen sense of smell to detect geosmin helps in finding water sources.
This sense of smell once helped thirsty ancestors find moist riverbeds. Today, it can also help shoppers determine whether beetroot is spoiled.
Paradoxically, geosmin also serves as a warning signal. When the concentration of geosmin in drinking water is high, it may indicate the presence of microbial activity. Therefore, water supply systems often use filtration methods to remove compounds with these earthy or musty flavors before the water flows to the tap.