How Does It Form

So plants and mud have a smell just like your ice-cream so when the water droplet touches the mud and makes it wet 

When lightning strikes,diatomic molecules of oxygen and nitrogen (containing 2atoms) are split, and rearrange to create nitric oxide (NO)and ozone, or O3. Ozonemolecules are carried down by droplets of rain to contribute to the scent. 

It is also helped by 

Actinomycetes, a type of bacteria found in soil, secrete a compound called geosmin , which is released from soil into the air by raindrops. Geosmin in the air can be detected by the human nose at less than 5 parts per trillion