Another factor is the oils produced by plants, particularly when there is a relatively long spell of dry weather. Studies have suggested that plants produce a particular mix of oils during dry weather, designed to inhibit growth and minimise competition for water. These oils collect in the soil and in rocks; rain then causes a range of smaller, volatile compounds within them to be released into the air. Their combination with geosmin in the air causes ‘petrichor’ – the name given by scientists in 1964 to the smell after rain.