What causes that earthy smell when it rains?
You might have even heard people saying they know its going to rain when they smell it. Theres actually a name for this unique smell and its called Petrichor! Its a term coined by Australian scientists in the 1960s used to describe the earthy smell associated with rain. The answer may surprise you. Its a mix of several different things: plant oils, ozone and lightning, and geosmin. Geosmin is a compound that is secreted from a bacteria called Actinomycetes commonly found in soil. Geosmin is also found in beets! What makes geosmin extra peculiar is that humans are actually quite sensitive to it and the human nose can detect it in the air at less than 5ppt (parts per trillion)! Well, when lightning strikes, oxygen molecules in close vicinity are split and rearranged to create ozone, or O3. Those ozone molecules are then carried down to the ground by droplets of rain which contribute to the scent. Lastly, we have plant oils and compounds which are secreted by plants during dry weather. These oils accumulate in between rocks and in the soil until it rains and they are released into the air. Those oils paired with ozone from lightning strikes and geosmin from Actinomycetes create that earthy smell of Petrichor. For more information, visit the ACS (American Chemical Society).