Asked by: David Pugh, Newmarket
The smell is called petrichor, or sometimes geosmin, and is caused by the chemical Dimethyl-9-decalol. This substance is found in the spore coat of certain species of Actinomyces and Streptomyces of certain bacteria. The bacteria are ubiquitous in soils all round the world and in moist conditions they form filaments. When the soil dries out, they produce spores and the next time it rains, the spores are bounced high enough into the air by the force of the rain’s impact for you to breath them in. the spores we don’t breath in fall to the moist ground and become filaments.