I don’t know if this is a world wide phenomona or just local to Chicago, but when it rains here in the Chicago area when the weather is nicer (say 55 degrees or more) there is a particular smell that permeates the air when it first starts raining. As a child, I decided it must be the worms I was smelling. I know it is not, but my husband says it is nitrogen being released from the soil. I don’t believe him either. Anyone know for sure?
I believe Tattva’s cite refers to the smell after the rain. Chrisk72’s question was about the smell immediately after the rain starts. That smell depends on where you are. A storm pushes ahead of itself a cloud of dust kicked up by the leading edge drops. That has an earthy smell. If you are near roads or parking lots, the dust will also carry the tang of oil and gasoline. If you are in a stadium, you get the smell of sweat and wet clothes.
I always thought it was ozone and/or ionized air.
Ozone smells a lot like chlorine. About the only place you smell ozone is after operating a Tesla Coil, or in a closed room with a defective negative ion generator. Sometimes you can smell ozone AFTER a thunderstorm, but it smells nothing like that first gust of wind before the rain hits.
Regarding ozone, part of the usual ozone smell might be from oxides of nitrogen. Sparks do some plasma chemistry, and they screw with the nitrogen as well as generating O3.
Ionized air has no distinct smell that I can detect, but ozone sure does.