Why does heavy rainfall after a long dry period leave such a distinctive smell

A.� This rather pleasant smell is actually the result of actinomycetes, a bacteria, being stirred up by the rain.

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Q.� Why can't we smell the bacteria all the time

A.� The bacteria grow in the soil during warm damp conditions. When conditions become dry the bacteria produces spores that stay in the soil. It's only after a heavy rainfall, when the drops are sufficient and forceful enough to stir up these spores into the air, that the aroma is released. The moisture acts like an aerosol carrying the spores to our noses.

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Q.� How come I can smell the same smell regardless of whether I'm in the countryside or the city

A.� Actinomycetes are very common bacteria, found all over the world. It can be found in the soil that collects in the cracks of pavements as well as that in the woods.

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Q.� I often notice a less pleasant smell after heavy rain in the city, is this different type of bacteria

A.� It's probably more likely that acid rain is at the root of this less evocative smell. Chemicals in the atmosphere, particularly numerous in urban environments, cause rain to be acidic.

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When this rainwater breaks up soil in the pavement cracks it reacts with chemical and organic substances within. Chemicals such as petrol are given a much more pungent smell by this reaction and the overall effect is much less pleasant that that produced by actinomycetes and rainwater.

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Again, the smell is particularly noticeable after a long dry spell, which allows the chemicals to reach a higher, and therefore more potent, concentration.

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by Lisa Cardy