Soil releases the ‘after rain’ smell
Streptomyces bacteria that grow in wet earth play an important part in breaking down organic matter in the soil.
During a dry spell, the Streptomyces produce tiny spores and when the air turns humid before rain, the spores pop off into the air and are easily breathed in.
People with a sensitive-enough nose can smell the spores and give a credible forecast of showers on the way. But the really powerful scent is released when rain hits the dry ground and kicks up clouds of spores in a fine spray.
The fragrance comes from substances called geosmins, which are thought to attract earthworms and springtails to the bacteria to pick up and spread the spores.
The Times, 20 June 2010