One of the most pleasant sensations of spring is the smell we all associate with rain. Rain is essentially water, which is odorless. Yet rainfall does undoubtedly come with an odor; a pleasant, slightly musky scent. The smell which comes with the rain actually has a complicated variety of sources.
One of the most common causes of the rain smell comes from bacteria which are released from the soil by raindrops. Observant gardeners are familiar with this smell because digging in the soil releases the same bacteria. The great variety of bacteria in the soils leads to a great complexity to the smell, making it a difficult scent to reproduce in factories despite great efforts by the manufacturers of shampoos and fabric softeners.
Complicating the smell even further is the scent provided by small amounts of ozone released whenever lightning strikes. So the smell of rain, though easily recognizable, is actually the result of some pretty complicated natural chemistry.