Many times after a rain, there is a distinctive odor in the air — a sort of musky smell. This pleasant fragrance is most common in rains that follow a dry spell. If you are a gardener, you may find this smell similar to the smell you sense when you turn over your soil. Good organic soils contain bacteria. One bacterium that is abundant in damp warm soils is actinomycete.
Actinomycetes are a key ingredient in the decomposition of organic materials in the soil. These bacteria thrive when the soil is moist. When the soil dries out, the actinomycetes produce tiny spores. These spores are part of their reproduction cycle. Rain kicks up these spores when the raindrops hit the ground and make them airborne. Air movements then disperse the spores and carry some of the spores to our nose, where we detect a pleasant aroma. Actinomycetes are very common, which is why you experience the pleasing after-the-rain smell in many locations.
Winds can transport odors from areas upwind of you. We also can experience bad after-the-rain smells, especially in urban regions. Rainwater tends to be slightly acidic and can break apart organic materials on the ground. The rain can release minerals in the soil that can also react with any chemicals on roadways, such as gasoline and oil, leading to unpleasant smells. These chemical smells are more noticeable with rains that follow a dry spell, as the chemicals have not been diluted or washed away.