I know you all have done this: On a hot and humid day you suddenly notice an earthy, dirt-like odor, and you turn to someone and say: “Smells like rain!” And sure enough a short time later here comes the thunder and lightning.
Rain does not smell because water has no odor, and yet there is a definite smell at times before a rain. Long ago people thought the smell came from rainbows, even smart guys like Aristotle. In the 1960s a group of Australian chemists figured out where it comes from. This gets a little crazy, so hang on.
On hot summer days mountains sometime take on a blue appearance, and there’s even a blue haze in the sky near them. The Great Smoky Mountains and the Blue Ridge Mountains are both named after this bluish haze. It’s the result of trees and plants releasing tons of complex chemicals (mostly fatty acids and oils) into the atmosphere through a process called transpiration. Think of it as a plant exhaling, only instead of carbon dioxide they breathe out oxygen (good for us), lots of water vapor, and fatty acids and oils. During periods of dry weather, the oils accumulate on the soil, which stores them and converts them into the familiar musty “rain smell.” When the humidity goes above 80 percent (enough moisture for rain to occur), the soil releases the smell and we often catch a whiff of it when conditions are right for it to rain. If it rains often, the rain smell will not develop because the oils are washed away.
Scientists named the smell “petrichor,” which comes from the Greek words for “stone-essence.” Some believe that petrichor might act as a growth promoter, because plants and mushrooms seem to spring up overnight when a dry spell ends with a good rain. To be scientifically correct, the next time you smell rain you need to say: “I think it’s going to rain, I can smell the one ten dimethyl-9-decanol”.
Steve Roark is the area forester in Tazewell, Tennessee, for the Tennessee Department of Agriculture, Forestry Division.