Where does the “smell of rain” come from?

You may have already smelled that characteristic smell that sometimes takes in the air after the rain. There is actually talk of “rain smell” (although this may also refer to some people's perception of a rain coming). Well, but how is this formed? Where do you come from? Is it just a cultural heritage? There are several answers to this phenomenon.

Several substances are believed to concur to form what common sense calls rain odor. First, there is the characteristic bond formed by three oxygen atoms - formation that takes the name of ozone.

It looks like chlorine, but it's ozone

First, there is what is commonly referred to as a certain "cleansing smell." While some people believe it is chlorine, what actually happens is that ongoing (or even forthcoming) rain promotes the formation of ozone particles in the air. And, well, it's also a fact that ozone brings with it an acrid odor very similar to chlorine.

Basically, even before a thunderstorm sets in, lightning can divide nitrogen and oxygen molecules found in the environment - which are therefore reduced to their fundamental atoms. It is from these parts that ozone is formed, also known as “trioxigen” - as it is formed by three oxygen atoms.

Such separation can also be promoted by ultraviolet light from the atmosphere. In any case, the result is always the same: free oxygen eventually finds an oxygen molecule (O2), forming ozone, which is then carried to the ground.

"But isn't ozone toxic?"

In fact, in high concentrations, ozone gas can cause severe damage to your lung cells. However, the amount released before a storm is so tiny that it is not at all risky.

It turns out that our perception of ozone is so sharp that it makes even a tiny amount detectable. To put it in numbers, the human nose is able to discern the "smell of ozone" even when the substance is in concentrations of only 10 PPB (parts per billion).

Smell of earth

In addition to ozone - and its “cleansing smell” - there is also another odor commonly reported by those who claim to notice the scent of post-rain. You may have heard someone talk about “earth smell” right? Usually, the more intense the stronger the precipitation. Yes, it does, though it is not the earth itself, but something that is in it.

The “earthy smell” is the result of a very common soil bacteria. In fact, certain microbes - particularly those in the Streptomyces group - release spores into the air during periods of drought. The longer the dry season, the more spores will be thrown into the air. But the answer is still further ahead.

Geosmine in the air

However, the smell does not come from spores, but rather from a chemical released by bacteria during spore production, which is called “geosmin”. And if you found human perception of ozone impressive, then be prepared: geosmin can be detected by human smell even at a concentration as low as 5 PPT (parts per trillion).

In fact, this is why the smell of ozone is usually perceived in the city. Not that it is not present in rural areas, of course. It turns out that when geosmin occupies the air, there is little room left in your nose - even for the acrid odor of ozone.

Plant oils

Finally, there is a third element that usually forms the feast of odors that comes after a particularly heavy rain. These are various oils formed by plants. Well, again, not the oils themselves, but some chemicals in them - which are usually released into the atmosphere during the rain (along with geosmin).

Although all of these elements are not known, one of the major contributors is believed to be “2-isopropyl-3-methoxypyrazine, ” as discovered by Nancy Gerber during the 1970s.

Petricor

However, even before the researcher, the duo Bear and Thomas had tried to trace the origins of this odor. And they even gave a name to the smell that "usually accompanies the rain after a drought": petricor - from the Greek, "stone" + "the substance attributed to the blood of the gods."

The researchers also argued that these oils are kept during drought periods to prevent the plant from producing seeds at particularly hostile times.