[CPT ArkAngel]

I woud say that it is simply some soil fine particles going in the air because of the water hitting the ground or some fine particles stick to the evaporating water. The smell is stronger where the surface is warmer like on a street after a sunny afternoon (strong evaporation)...

[Sam]

2 Years Later... If it helps (I'm interested because I am trying to find a candle with the actual smell of rain), there is an oil that supposedly only appears during and after a rain called Petrichor. I'm guessing that this oil is secreted during a rain for a number of reasons. It could be to spread pores, to wick the plants so that their leaves could withstand a volatile beat down from the oncoming rain or to simply make excess water drip off easier.