Photo by DunkDriver, Wikimedia Commons
(Click on this link to listen to an audio version of this blog … Scent of rain
One of the hardest things to get used to about living in Northern Colorado is the astounding infrequency of rain. You can look at numbers (an average of 14 inches of a rain a year) and intellectually extrapolate that rainfall will not be a very common occurrence, but until you have lived here and watched rain cloud after rain cloud go racing past with no action on the ground, you just don’t know what it feels like.
Back in Western Pennsylvania we got 40+ inches of rain a year. The rain was spread out incredibly evenly throughout the year with each month getting its 3.5 or so inches of moisture. Sometimes there were big thunderstorms, sometimes there were big snow storms, but mostly it was all quite even and felt well controlled. Gentle, nighttime rains and afternoon showers were the norm.
When it does rain here in Colorado, Deborah and I get very excited. One afternoon late last week, I was working on my computer when I heard the not-so-distant rumble of thunder. I unplugged my computer and went out front to pick up some piles of weeds that I had pulled that morning. Some of them were in the street gutter, and I didn’t want the rain water to get backed up behind them or carry them off to one of my neighbors’ yards. I was very confident that it was really going to rain! I felt a raindrop here and there as I forked up the weeds into the wheelbarrow. I got back to the garage, though, without hardly getting wet at all,
Photo by Picrepo
That night we sat out at the table in our breezeway and painfully watched thunderhead after thunderhead dodge around the city limits of Greeley. There is a myth about Greeley that the city, mysteriously generates a local high pressure cell that repels most rain storms. I have yet to see any science about this, but observing storm after storm racing toward us from the west splitting neatly into north and south portions around the city does make one wonder!
We sat out in the breezeway for a while longer and then went inside, had dinner and then went to the living room to watch some TV. While we were watching, finally, some storm clouds slipped through the Greeley blockade, and it finally cut loose and rained. We heard the rain drops thumping on our living room skylight and immediately went out to the front porch to watch the rain fall. According to the weather service, almost 0.7 inches of rain fell in less than an hour.
Standing out under the overhang of the eaves Deborah and I felt the cool air blowing down the street and enjoyed the side-spray of the falling water and the incredible scent of rain on dry soil and concrete. This aroma is called “petrichor,” and it comes from a complex array of organic chemicals that have been secreted by plants and soil bacteria onto dry, outside surfaces. The rain stirs up and aerosolizes these chemicals and generates the aesthetically satisfying atmospheric. It is a smell of a cleansing power, a smell of renewal and change. It is a smell of life.
Photo by HDWallpaper
Petrichor had been discussed by scientists since the late 19th Century but was first precisely described (and named) by two Australian researchers, I. J. Bear and R. G. Thomas in 1964. They later demonstrated that as these complex chemicals build up in dry soils act to inhibit the germination of seeds. When the rains come and the petrichor chemicals are released from the soil this chemical check on seed germination is released. The aroma we smell is the signal for long suppressed plants to begin the growth phase of their life cycles. Maybe we’ll get another burst of weeds out in our front yard prairie!
Another chemical that plays a role in the “after the rain” scent is geosmin. Geosmin (also known as dimethyl-9-decalol) is the familiar “earth” smell that we perceive during spring soil thaws or after plowing or tilling. Geosmin is synthesized by several types of soil bacteria, but it is especially the product of a genus of actinobacteria called Streptomyces. Streptomyces are very important in breaking down complex and resistant chemicals in the soil and play vital roles in biological decomposition and in the recycling of nutrients in our ecosystems. Streptomyces also synthesize an incredible array of diverse chemicals that collectively are referred to as “secondary metabolites.” Secondary metabolites often play complex roles in the chemical ecology of an ecosystem, and it is thought that some of the Streptomyces secondary metabolites are extremely important mediators in symbiotic relationships among soil organisms. Some of these secondary metabolites act as chemical defense agents against other bacteria, and a number of these Streptomyces “antibiotics” (including streptomycin, actinomycin, and neomycin) were isolated and identified by Selman Waksman and his students in the 1940’s. These discoveries lead to Waksman’s Nobel Prize in 1952.
Public Domain Photo
Geosmin is concentrated in the spore covers of the Streptomyces bacteria. Human olfactory systems are incredibly sensitive to geosmin and can detect concentrations as low as five parts per trillion in the air. Further, other organisms are also quite sensitive to geosmin. Earthworms and many other soil dwelling invertebrates are attracted to loci of concentrated geosmin. This attraction may act to recruit soil biota into areas of intense biological decomposition, and, thus, further accelerate and facilitate this vital biogeochemical process. The recruited soil invertebrates may also act to transport and disperse the relatively immobile Streptomyces throughout the soil volume thus increasing the presence and impact of these important bacteria.
Standing outside watching the rain, we noticed that none of our neighbors were out reveling in it! Maybe they didn’t even notice that it was happening! Also, standing there getting slightly wet, we actually felt cold! This is the first time since May that “cold” has been part of our vocabulary! Maybe this hot, dry summer will come to an end after all! Bring on the short autumn season and even bring on winter! We’re ready!