How Can We Smell Rain?
You must have noticed it at some point. You’re sitting in a classroom on a hot day feeling sticky, lazy and cranky and then suddenly you get this strangely fresh and comforting smell coming in through the windows and you know that it’s about to rain. Soon enough clouds gather and it begins to rain and then this lovely earthly smell is in the air.
But what are these smells, and where do they come from?
Let’s find out.
What does it mean when somebody says they can smell the rain coming?
The smell that helps you predict rain is the scent of ozone. During a thunderstorm, the electric charge from lightning can split oxygen and nitrogen molecules in the atmosphere into separate atoms. And they can occasionally recombine into nitric oxide. This substance interacts with other chemicals in the atmosphere to produce a molecule made up of three oxygen atoms — ozone, which has a sharp smell and that is why you can sometimes smell rain coming.
Then what is the smell we get when rain hits dry ground?
This earthly smell is called petrichor. The term signifies a pair of chemical reactions. Let’s take a look at them.
One responsible element is the oils that plants secrete during dry periods. When rain finally comes into contact with the plants, these oils are released into the air. The second reaction occurs when an organic compound known as geosmin is released. Geosmin is produced by the bacteria Streptomyces and various cyanobacteria, and is released when these micro-organisms die. It has also been isolated from several soil dwelling bacteria called actinomycetes. And when it rains, the rains drops kick up geosmin-containing compounds in aerosol form.
So why do we only get this smell with fresh rain?
You may ask, why don’t long days of downpour smell the same as fresh rain after a dry spell? The culprit is dryness. Dryness slows down the geosmin-producing bacteria responsible for breaking down the organic matter in the soil. But as soon as a few drops of rain are added to the equation the bacteria kick into high gear and produce more geosmin.
Did you know?
The human nose is especially sensitive to geosmin, it is able to detect it at concentrations as low as 5 parts per trillion! Some scientists happen to believe that humans appreciate the rain scent because our ancestors may have relied on rainy weather for survival.