We often think about spring showers and summer thunderstorms. But rain is also important in the fall. It helps prepare the ground for seasonal freezing. And it helps prepare the plants and trees for hibernation. Also–the brilliance and timing of the annual, colorful leaf season depends not only the amount of sun but also on the amount of rain.
But regardless of the season, you’ve probably noticed a distinct odor in the air before and after it rains. Water is colorless and odorless. So from where does this smell come?
Well–the smell actually occurs when the rain water “activates” nutrients in the plants and soil. (Similarly, water can “activate” compounds in water-soluble herb-vitamin mixes.)
In the natural world, this chemical process actually begins in the atmosphere before the rainfall.
You see, in the atmosphere, oxygen and nitrogen are usually bound together in a two-atom oxygen molecule for stability. In stormy weather, electricity in the air can split apart molecules into single “free radical” atoms of oxygen. Then, the free oxygen atoms combine with other oxygen molecules to form ozone. (These are the same kind of free radicals responsible for “oxidation,” when we talk about “anti-oxidants,” such as vitamins and other plant constituents.)
Of course, the ozone layer in the upper atmosphere filters out some dangerous solar radiation. But during a rain storm, winds also drive ozone down toward the surface of the Earth. This part of the process gives the air that characteristic smell when it’s about to rain.
Then, when the rain actually begins to fall on plants, the aroma in the air changes again. The rain water releases oils the plant produces during dry, dormant periods. Plants actually produce these chemicals during dry spells to inhibit growth. The chemicals reduce the plant’s need to compete for limited moisture. And they prevent the plant from using up available water in the soil.
The pleasant smell in the air after it begins to rain actually has a name. Scientists call this mix of plant oils released by new rain “petrichor.”
Then, when the rain seeps into the soil, it affects bacteria in the soil and produces another well-recognized smell.
When soil gets wet, it releases small amounts of a chemical called geosmin into the air. Humans are extremely sensitive to the aroma of this chemical. In fact, we can detect its presence at levels of only five parts per million. (That’s about one teaspoon in 200 Olympic-sized swimming pools.)
When the chemical is present in water, it gives the water a muddy flavor. Geosmin is also present in beet root and it gives the plant its earthy flavor.
As a side note, beets are highly nutritious and contain a number of unique biologically active compounds. In fact, beets contain the cellular powerhouse betaine, aptly named since it was first isolated from beets.
I will tell you about more about the importance of betaine in the upcoming November 2015 issue of my Insiders’ Cures newsletter. If you’re not yet a subscriber, now is the perfect time to get started.